Modern C++
Programming
22. Performance Optimization II
Code Optimization
Federico Busato
2024-11-05
I/O Operations
I/O Operations are orders of magnitude slower than
memory accesses
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I/O Streams
In general, input/output operations are one of the most expensive
• Use endl for ostream only when it is strictly necessary (prefer \n )
• Disable synchronization with printf/scanf :
std::ios base::sync with stdio(false)
• Disable IO flushing when mixing istream/ostream calls:
<istream obj>.tie(nullptr);
• Increase IO buffer size:
file.rdbuf()->pubsetbuf(buffer var, buffer size);
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I/O Streams - Example
#include <iostream>
int main() {
std::ifstream fin;
// --------------------------------------------------------
std::ios_base::sync_with_stdio(false); // sync disable
fin.tie(nullptr); // flush disable
// buffer increase
const int BUFFER_SIZE = 1024 * 1024; // 1 MB
char buffer[BUFFER_SIZE];
fin.rdbuf()->pubsetbuf(buffer, BUFFER_SIZE);
// --------------------------------------------------------
fin.open(filename); // Note: open() after optimizations
// IO operations
fin.close();
}
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printf
• printf is faster than ostream (see speed test link)
• A printf call with a simple format string ending with \n is converted to a
puts() call
printf("Hello World\n");
printf("%s\n", string);
• No optimization if the string is not ending with \n or one or more % are
detected in the format string
www.ciselant.de/projects/gcc_printf/gcc_printf.html
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Memory Mapped I/O
A memory-mapped file is a segment of virtual memory that has been assigned a
direct byte-for-byte correlation with some portion of a file
Benefits:
• Orders of magnitude faster than system calls
• Input can be “cached” in RAM memory (page/file cache)
• A file requires disk access only when a new page boundary is crossed
• Memory-mapping may bypass the page/swap file completely
• Load and store raw data (no parsing/conversion)
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Memory Mapped I/O - Example 1/2
#if !defined(__linux__)
#error It works only on linux
#endif
#include <fcntl.h> //::open
#include <sys/mman.h> //::mmap
#include <sys/stat.h> //::open
#include <sys/types.h> //::open
#include <unistd.h> //::lseek
// usage: ./exec <file> <byte_size> <mode>
int main(int argc, char* argv[]) {
size_t file_size = std::stoll(argv[2]);
auto is_read = std::string(argv[3]) == "READ";
int fd = is_read ? ::open(argv[1], O_RDONLY) :
::open(argv[1], O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
if (fd == -1)
ERROR("::open") // try to get the last byte
if (::lseek(fd, static_cast<off_t>(file_size - 1), SEEK_SET) == -1)
ERROR("::lseek")
if (!is_read && ::write(fd, "", 1) != 1) // try to write
ERROR("::write")
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Memory Mapped I/O Example 2/2
auto mm_mode = (is_read) ? PROT_READ : PROT_WRITE;
// Open Memory Mapped file
auto mmap_ptr = static_cast<char*>(
::mmap(nullptr, file_size, mm_mode, MAP_SHARED, fd, 0) );
if (mmap_ptr == MAP_FAILED)
ERROR("::mmap");
// Advise sequential access
if (::madvise(mmap_ptr, file_size, MADV_SEQUENTIAL) == -1)
ERROR("::madvise");
// MemoryMapped Operations
// read from/write to "mmap_ptr" as a normal array: mmap_ptr[i]
// Close Memory Mapped file
if (::munmap(mmap_ptr, file_size) == -1)
ERROR("::munmap");
if (::close(fd) == -1)
ERROR("::close");
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Low-Level Parsing 1/2
Consider using optimized (low-level) numeric conversion routines:
template<int N, unsigned MUL, int INDEX = 0>
struct fastStringToIntStr;
inline unsigned fastStringToUnsigned(const char* str, int length) {
switch(length) {
case 10: return fastStringToIntStr<10, 1000000000>::aux(str);
case 9: return fastStringToIntStr< 9, 100000000>::aux(str);
case 8: return fastStringToIntStr< 8, 10000000>::aux(str);
case 7: return fastStringToIntStr< 7, 1000000>::aux(str);
case 6: return fastStringToIntStr< 6, 100000>::aux(str);
case 5: return fastStringToIntStr< 5, 10000>::aux(str);
case 4: return fastStringToIntStr< 4, 1000>::aux(str);
case 3: return fastStringToIntStr< 3, 100>::aux(str);
case 2: return fastStringToIntStr< 2, 10>::aux(str);
case 1: return fastStringToIntStr< 1, 1>::aux(str);
default: return 0;
}
}
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Low-Level Parsing 2/2
template<int N, unsigned MUL, int INDEX>
struct fastStringToIntStr {
static inline unsigned aux(const char* str) {
return static_cast<unsigned>(str[INDEX] - '0') * MUL +
fastStringToIntStr<N - 1, MUL / 10, INDEX + 1>::aux(str);
}
};
template<unsigned MUL, int INDEX>
struct fastStringToIntStr<1, MUL, INDEX> {
static inline unsigned aux(const char* str) {
return static_cast<unsigned>(str[INDEX] - '0');
}
};
Faster parsing: lemire.me/blog/tag/simd-swar-parsing
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Speed Up Raw Data Loading 1/2
• Hard disk is orders of magnitude slower than RAM
• Parsing is faster than data reading
• Parsing can be avoided by using binary storage and mmap
• Decreasing the number of hard disk accesses improves the performance →
compression
LZ4 is lossless compression algorithm providing extremely fast decompression up to
35% of memcpy and good compression ratio
github.com/lz4/lz4
Another alternative is Facebook zstd
github.com/facebook/zstd
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Speed Up Raw Data Loading 2/2
Performance comparison of different methods for a file of 4.8 GB of integer values
Load Method Exec. Time Speedup
ifstream 102 667 ms 1.0x
memory mapped + parsing (first run) 30 235 ms 3.4x
memory mapped + parsing (second run) 22 509 ms 4.5x
memory mapped + lz4 (first run) 3 914 ms 26.2x
memory mapped + lz4 (second run) 1 261 ms 81.4x
NOTE: the size of the Lz4 compressed file is 1,8 GB
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Heap Memory
• Dynamic heap allocation is expensive: implementation dependent and interact
with the operating system
• Many small heap allocations are more expensive than one large memory allocation
The default page size on Linux is 4 KB. For smaller/multiple sizes, C++ uses a
sub-allocator
• Allocations within the page size is faster than larger allocations (sub-allocator)
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Stack Memory
• Stack memory is faster than heap memory. The stack memory provides high
locality, it is small (cache fit), and its size is known at compile-time
•
static stack allocations produce better code. It avoids filling the stack each
time the function is reached
• constexpr arrays with dynamic indexing produces very inefficient code with
GCC. Use static constexpr instead
void f(int x) {
// bad performance with GCC
// constexpr int array[] = {1,2,3,4,5,6,7,8,9};
static constexpr int array[] = {1,2,3,4,5,6,7,8,9};
return array[x];
}
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Cache Utilization
Maximize cache utilization:
• Maximize spatial and temporal locality (see next examples)
• Prefer small data types
• Prefer std::vector<bool> over array of bool
• Prefer std::bitset<N> over std::vector<bool> if the data size is known in
advance or bounded
• Prefer stack data structures instead of head data structures, e.g. std::vector
vs. static vector
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Spatial Locality Example 1/2
A, B, C matrices of size N × N
C = A * B
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
int sum = 0;
for (int k = 0; k < N; k++)
sum += A[i][k] * B[k][j]; // row × column
C[i][j] = sum;
}
}
C = A * B
T
for (int i = 0; i < N; i++) {
for (int j = 0; j < N; j++) {
int sum = 0;
for (int k = 0; k < N; k++)
sum += A[i][k] * B[j][k]; // row × row
C[i][j] = sum;
}
}
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Spatial Locality Example 2/2
Benchmark:
N 64 128 256 512 1024
A * B < 1 ms 5 ms 29 ms 141 ms 1,030 ms
A * B
T
< 1 ms 2 ms 6 ms 48 ms 385 ms
Speedup / 2.5x 4.8x 2.9x 2.7x
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Temporal-Locality Example
Speeding up a random-access function
for (int i = 0; i < N; i++) // V1
out_array[i] = in_array[hash(i)];
for (int K = 0; K < N; K += CACHE) { // V2
for (int i = 0; i < N; i++) {
auto x = hash(i);
if (x >= K && x < K + CACHE)
out_array[i] = in_array[x];
}
}
V1 : 436 ms, V2 : 336 ms → 1.3x speedup (temporal locality improvement)
.. but it needs a careful evaluation of CACHE , and it can even decrease the performance for
other sizes
pre-sorted hash(i) : 135 ms → 3.2x speedup (spatial locality improvement)
lemire.me/blog/2019/04/27
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Memory Alignment
Memory alignment refers to placing data in memory at addresses that conform to
certain boundaries, typically powers of two (e.g., 1, 2, 4, 8, 16 bytes, etc.)
Note: For multidimensional data, alignment only means that the start address of the data is
aligned, not that all start offsets for all dimensions are aligned., e.g. for a 2D matrix, if
row[0][0] is aligned doesn’t imply that row[0][1] has the same property. Also the strides
between rows need to be multiple of the alignment
Data alignment is classified in:
• Internal alignment for struct/class layout optimization → reducing memory
footprint, optimizing memory bandwidth, and minimizing cache-line misses
• External alignment across several elements of the same type → minimizing
cache-line misses, vectorization (SIMD instructions)
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Internal Structure Alignment
struct A1 {
char x1; // offset 0
double y1; // offset 8!! (not 1)
char x2; // offset 16
double y2; // offset 24
char x3; // offset 32
double y3; // offset 40
char x4; // offset 48
double y4; // offset 56
char x5; // offset 64 (65 bytes)
}
struct A2 { // internal alignment
char x1; // offset 0
char x2; // offset 1
char x3; // offset 2
char x4; // offset 3
char x5; // offset 4
double y1; // offset 8
double y2; // offset 16
double y3; // offset 24
double y4; // offset 32 (40 bytes)
}
(1) We are wasting 40% of memory for ( A1 )
(2) Considering an array of structures (AoS) and a cache line of 64 bytes (x64
processors), every access to A1 involves two cache line operations (∼2x slower)
see also #pragma pack(1)
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External Structure Alignment 1/3
In addiction to internal layout problems, even the structure A2 introduces overhead if
organized in an array. Loads lead to one or two cache line operations depending on the
alignment at a specific index, e.g.
index 0 → one cache line load
index 1 → two cache line loads
It is possible to fix the structure alignment in two ways:
• Memory padding refers to manually introducing extra bytes at the end of the
data structure to enforce memory alignment.
e.g. add a char array of size 24 to the structure A2
• Align keyword or attribute allows specifying the alignment requirement of a
type or an object (next slide)
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External Structure Alignment 2/3
• Explicit alignment/padding for variable / struct declaration → affects
sizeof(T)
C++11 : alignas(N)
GCC/Clang : attribute ((aligned(N)))
MSVC : declspec(align(N))
• Explicit alignment for pointers
C++20 : std::assume aligned<N>(ptr) ( <memory> )
C++17 : aligned new or std::aligned alloc(align, size)
GCC/Clang : builtin assume aligned(ptr, N)
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External Structure Alignment 3/3
struct alignas(16) S1 { // C++11
int x, y;
};
struct __attribute__((aligned(16))) S2 { // compiler-specific attribute
int x, y;
};
constexpr auto MaxAlign = __STDCPP_DEFAULT_NEW_ALIGNMENT__;
S1 s; // 16B alignment
alignas(16) int var[3]; // 16B alignment
auto ptr1 = new S1[10]; // Warning! no aligment guarantee
auto ptr2 = new int[100]; // alignment: max(4B, MaxAlign)
auto ptr3 = std::aligned_alloc(8, 4); // C++17, alignment: max(8B, MaxAlign)
auto ptr4 = __builtin_assume_aligned(ptr2, 16); // compiler-specific attribute
auto ptr5 = std::assume_aligned<16>(ptr2); // C++20
auto ptr = new (std::align_val_t(4)) int[2]}; // C++17, max(16B, MaxAlign)
::operator delete[] (ptr, std::align_val_t{4});
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Memory Prefetch
builtin prefetch is used to minimize cache-miss latency by moving data into a
cache before it is accessed. It can be used not only for improving spatial locality, but
also temporal locality
for (int i = 0; i < size; i++) {
auto data = array[i];
__builtin_prefetch(array + i + 1, 0, 1); // 2nd argument, '0' means read-only
// 3th argument, '1' means
// temporal locality=1, default=3
// do some computation on 'data', e.g. CRC
}
Alternatively, -fprefetch-loop-arrays can be used to emit prefetching
instructions
The pros and cons of explicit software prefetching
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Multi-Threading and Caches
The CPU/threads affinity controls how a process is mapped and executed over
multiple cores (including sockets). It affects the process performance due to
core-to-core communication and cache line invalidation overhead
Maximizing threads “clustering” on a single core can potentially lead to higher cache
hits rate and faster communication. On the other hand, if the threads work
independently/almost independently, namely they show high locality on their working
set, mapping them to different cores can improve the performance
C++11 threads, affinity and hyper-threading
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Hardware Notes
• Instruction throughput greatly depends on processor model and characteristics,
e.g., there is no hardware support for integer division on GPUs. This operation is
translated to 100 instructions for 64-bit operands
• Modern processors provide separated units for floating-point computation (FPU)
• Addition, subtraction, and bitwise operations are computed by the ALU, and they
have very similar throughput
• In modern processors, multiplication and addition are computed by the same
hardware component for decreasing circuit area → multiplication and addition can
be fused in a single operation fma (floating-point) and mad (integer)
uops.info: Latency, Throughput, and Port Usage Information
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Data Types
• 32-bit integral vs. floating-point: in general, integral types are faster, but it
depends on the processor characteristics
• 32-bit types are faster than 64-bit types
• 64-bit integral types are slightly slower than 32-bit integral types. Modern processors
widely support native 64-bit instructions for most operations, otherwise they require
multiple operations
• Single precision floating-points are up to three times faster than double precision
floating-points
• Small integral types are slower than 32-bit integer, but they require less
memory → cache/memory efficiency
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Arithmetic Operations 1/3
• Arithmetic increment/decrement x++ / x-- has the same performance of
x + 1 / x - 1
• Arithmetic compound operators ( a *= b ) has the same performance of
assignment + operation ( a = a * b ) *
• Prefer prefix increment/decrement ( ++var ) instead of the postfix operator
( var++ ) *
* the compiler automatically applies such optimization whenever possible. This is not ensured for
object types
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Arithmetic Operations 2/3
• Keep near constant values/variables → the compiler can merge their values
• Some operations on unsigned types are faster than on signed types because
they don’t have to deal with negative numbers, e.g. x / 2 → x >> 1
• Some operations on signed types are faster than on unsigned types because
they can exploit undefined behavior, see next slide
• Prefer logic operations || to bitwise operations | to take advantage of
short-circuiting
Is if(A | B) always faster than if(A || B)?
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Arithmetic Operations 3/3
bool mainGuT(uint32_t i1, uint32_t i2, // if i1, i2 are int32 t, the code
uint8_t *block) { // uses half of the instructions!!
uint8_t c1, c2;
// 1 // why? if i1, i2 are uint32 t the compiler
c1 = block[i1], c2 = block[i2]; // must copy them into 32-bit registers to
if (c1 != c2) return (c1 > c2); // ensure wrap-around behavior before passing
i1++, i2++; // them to the subscript operator (size_t)
// 2 // On the other hand, int32 t overflow is
c1 = block[i1], c2 = block[i2]; // undefined behavior and the compiler can
if (c1 != c2) return (c1 > c2); // assume it never happens
i1++, i2++;
// ... continue repeating the // the code is also optimal with size t on 64-bit
} // code multiple times // arch because block cannot be larger than it
Garbage In, Garbage Out: Arguing about Undefined Behavior with Nasal Daemons,
Chandler Carruth, CppCon 2016
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Arithmetic Operations - Integer Multiplication
Integer multiplication requires double the number of bits of the operands
// 32-bit platforms
int f1(int x, int y) {
return x * y; // efficient but can overflow
}
int64_t f2(int64_t x, int64_t y) { // same for f2(int x, int64_t y)
return x * y; // always correct but slow
}
int64_t f3(int x, int y) {
return x * static_cast<int64_t>(y); // correct and efficient!!
}
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Arithmetic Operations - Power-of-Two Multiplication/Division/Modulo
• Prefer shift for power-of-two multiplications ( a ≪ b ) and divisions
( a ≫ b ) only for run-time values *
• Prefer bitwise and ( a % b → a & (b - 1) ) for power-of-two modulo
operations only for run-time values *
• Constant multiplication and division can be heavily optimized by the compiler,
even for non-trivial values
* the compiler automatically applies such optimizations if b is known at compile-time. Bitwise
operations make the code harder to read
Ideal divisors: when a division compiles down to just a multiplication
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Conversion
From To Cost
Signed Unsigned no cost, bit representation is the same
Unsigned Larger Unsigned no cost, register extended
Signed Larger Signed 1 clock-cycle, register + sign extended
Integer Floating-point
4-16 clock-cycles
Signed → Floating-point is faster than
Unsigned → Floating-point (except AVX512
instruction set is enabled)
Floating-point Integer fast if SSE2, slow otherwise (50-100 clo ck-cycles)
Optimizing software in C++, Agner Fog
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Floating-Point Division
Multiplication is much faster than division*
not optimized:
// "value" is floating-point (dynamic)
for (int i = 0; i < N; i++)
A[i] = B[i] / value;
optimized:
div = 1.0 / value; // div is floating-point
for (int i = 0; i < N; i++)
A[i] = B[i] * div;
* Multiplying by the inverse is not the same as the division
see lemire.me/blog/2019/03/12
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Floating-Point FMA
Modern processors allow performing a * b + c in a single operation, called fused
multiply-add ( std::fma in C++11). This implies better performance and accuracy
CPU processors perform computations with a larger register size than the original data
type (e.g. 48-bit for 32-bit floating-point) for performing this operation
Compiler behavior:
• GCC 9 and ICC 19 produce a single instruction for std::fma and for a * b + c with
-O3 -march=native
• Clang 9 and MSVC 19.* produce a single instruction for std::fma but not for
a * b + c
FMA: solve quadratic equation
FMA: extended precision addition and multiplication by constant
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Compiler Intrinsic Functions 1/5
Compiler intrinsics are highly optimized functions directly provided by the compiler
instead of external libraries
Advantages:
• Directly mapped to hardware functionalities if available
• Inline expansion
• Do not inhibit high-level optimizations, and they are portable contrary to asm code
Drawbacks:
• Portability is limited to a specific compiler
• Some intrinsics do not work on all platforms
• The same instricics can be mapped to a non-optimal instruction sequence
depending on the compiler
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Compiler Intrinsic Functions 2/5
Most compilers provide intrinsics bit-manipulation functions for SSE4.2 or ABM
(Advanced Bit Manipulation) instruction sets for Intel and AMD processors
GCC examples:
builtin popcount(x) count the number of one bits
builtin clz(x) (count leading zeros) counts the number of zero bits following the
most significant one bit
builtin ctz(x) (count trailing zeros) counts the number of zero bits preceding
the least significant one bit
builtin ffs(x) (find first set) index of the least significant one bit
gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
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Compiler Intrinsic Functions 3/5
• Compute integer log2
inline unsigned log2(unsigned x) {
return 31 - __builtin_clz(x);
}
• Check if a number is a power of 2
inline bool is_power2(unsigned x) {
return __builtin_popcount(x) == 1;
}
• Bit search and clear
inline int bit_search_clear(unsigned x) {
int pos = __builtin_ffs(x); // range [0, 31]
x &= ∼(1u << pos);
return pos;
}
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Compiler Intrinsic Functions 4/5
Example of intrinsic portability issue:
builtin popcount() GCC produces popcountdi2 instruction while Intel
Compiler (ICC) produces 13 instructions
mm popcnt u32 GCC and ICC produce popcnt instruction, but it is available only
for processor with support for SSE4.2 instruction set
More advanced usage
• Compute CRC: mm crc32 u32
• AES cryptography: mm256 aesenclast epi128
• Hash function: mm sha256msg1 epu32
software.intel.com/sites/landingpage/IntrinsicsGuide/
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Compiler Intrinsic Functions 5/5
Using intrinsic instructions is extremely dangerous if the target processor does not
natively support such instructions
Example:
“If you run code that uses the intrinsic on hardware that doesn’t support the lzcnt
instruction, the results are unpredictable” - MSVC
on the contrary, GNU and clang builtin * instructions are always well-defined.
The instruction is translated to a non-optimal operation sequence in the worst case
The instruction set support should be checked at run-time (e.g. with cpuid
function on MSVC), or, when available, by using compiler-time macro (e.g. AVX )
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Automatic Compiler Function Transformation
std::abs can be recognized by the compiler and transformed to a hardware
instruction
In a similar way, C++20 provides a portable and efficient way to express bit operations
<bit>
rotate left : std::rotl
rotate right : std::rotr
count leading zero : std::countl zero
count leading one : std::countl one
count trailing zero : std::countr zero
count trailing one : std::countr one
population count : std::popcount
Why is the standard "abs" function faster than mine?
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Value in a Range
Checking if a non-negative value x is within a range [A, B] can be optimized if
B > A (useful when the condition is repeated multiple times)
if (x >= A && x <= B)
// STEP 1: subtract A
if (x - A >= A - A && x - A <= B - A)
// -->
if (x - A >= 0 && x - A <= B - A) // B - A is precomputed
// STEP 2
// - convert "x - A >= 0" --> (unsigned) (x - A)
// - "B - A" is always positive
if ((unsigned) (x - A) <= (unsigned) (B - A))
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Value in a Range Examples
Check if a value is an uppercase letter:
uint8_t x = ...
if (x >= 'A' && x <= 'Z')
...
→
uint8_t x = ...
if (x - 'A' <= 'Z')
...
A more general case:
int x = ...
if (x >= -10 && x <= 30)
...
→
int x = ...
if ((unsigned) (x + 10) <= 40)
...
The compiler applies this optimization only in some cases
(tested with GCC/Clang 9 -O3)
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Lookup Table
Lookup table (LUT) is a memoization technique which allows replacing runtime
computation with precomputed values
Example: a function that computes the logarithm base 10 of a number in the range [1-100]
template<int SIZE, typename Lambda>
constexpr std::array<float, SIZE> build(Lambda lambda) {
std::array<float, SIZE> array{};
for (int i = 0; i < SIZE; i++)
array[i] = lambda(i);
return array;
}
float log10(int value) {
constexpr auto lamba = [](int i) { return std::log10f((float) i); };
static constexpr auto table = build<100>(lambda);
return table[value];
}
Make your lookup table do more
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Control Flow
Computation is faster than decision
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Branches 1/2
Pipelines are an essential element in modern processors. Some processors have up to
20 pipeline stages (14/16 typically)
The downside to long pipelines includes the danger of pipeline stalls that waste CPU
time, and the time it takes to reload the pipeline on conditional branch operations
( if , while , for )
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Branches 2/2
• Prefer switch statements to multiple if
- If the compiler does not use a jump-table, the cases are evaluated in order of
appearance → the most frequent cases should be placed before
- Some compilers (e.g. clang) are able to translate a sequence of if into a switch
• In general, a branch has negligible effect on performance if it is not taken
• Not all control flow instructions (or branches) are translated into jump
instructions. If the code in the branch is small, the compiler could optimize it in a
conditional instruction, e.g. ccmovl
Small code section can be optimized in different ways 2 (see next slides)
Branch predictor: How many ‘if’s are too many?
2 Is this a branch?
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Minimize Branch Overhead
• Branch prediction: technique to guess which way a branch takes. It requires
hardware support, and it is generically based on dynamic history of code executing
• Branch predication: a conditional branch is substituted by a sequence of
instructions from both paths of the branch. Only the instructions associated to a
predicate (boolean value), that represents the direction of the branch, are actually
executed
int x = (condition) ? A[i] : B[i];
P = (condition) // P: predicate
@P x = A[i];
@!P x = B[i];
• Speculative execution: execute both sides of the conditional branch to better
utilize the computer resources and commit the results associated to the branch
taken
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Branch Hints - [[likely]] / [[unlikely]]
C++20 [[likely]] and [[unlikely]] provide a hint to the compiler to optimize
a conditional statement, such as while , for , if
for (i = 0; i < 300; i++) {
[[unlikely]] if (rand() < 10)
return false;
}
switch (value) {
[[likely]] case 'A': return 2;
[[unlikely]] case 'B': return 4;
}
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Signed/Unsigned Integers
• Prefer signed integer for loop indexing. The compiler optimizes more
aggressively such loops because integer overflow is not defined. Unsigned loop
indexing generates complex intermediate expressions, especially for nested loops,
that the compiler could not solve
• Prefer 32-bit signed integer or 64-bit integer for any operation that is
translated to 64-bit. The most common is array indexing. The subscript
operator implicitly defines its parameter as size t . Any indexing operation with
32-bit unsigned integer requires the compiler to enforce wrap-around behavior,
e.g. by moving the variable to a 32-bit register
unsigned v = ...;
// some operations on v
array[v];
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Loops
• Prefer square brackets syntax [] over pointer arithmetic operations for array
access to facilitate compiler loop optimizations (e.g. polyhedral loop
transformations)
• Range-based loop could provide minor performance improvements for small loops
that iterate over a container 1
• On the other hand, range-based loops and iterators could inhibit many
optimizations such as loop unrolling and vectorization
1 The Little Things: Everyday efficiencies
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Loop Hoisting
Loop Hoisting, also called loop-invariant code motion, consists of moving statements
or expressions outside the body of a loop without affecting the semantics of the
program
Base case:
for (int i = 0; i < 100; i++)
a[i] = x + y;
Better:
v = x + y;
for (int i = 0; i < 100; i++)
a[i] = v;
Loop hoisting is also important in the evaluation of loop conditions
Base case:
// "x" never changes
for (int i = 0; i < f(x); i++)
a[i] = y;
Better:
int limit = f(x);
for (int i = 0; i < limit; i++)
a[i] = y;
In the worst case, f(x) is evaluated at every iteration (especially when it belongs to
another translation unit)
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Loop Unrolling 1/2
Loop unrolling (or unwinding) is a loop transformation technique which optimizes
the code by removing (or reducing) loop iterations
The optimization produces better code at the expense of binary size
Example:
for (int i = 0; i < N; i++)
sum += A[i];
can be rewritten as:
for (int i = 0; i < N; i += 8) {
sum += A[i];
sum += A[i + 1];
sum += A[i + 2];
sum += A[i + 3];
...
} // we suppose N is a multiple of 8
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Loop Unrolling 2/2
Loop unrolling can make your code better/faster:
+ Improve instruction-level parallelism (ILP)
+ Allow vector (SIMD) instructions
+ Reduce control instructions and branches
Loop unrolling can make your code worse/slower:
- Increase compile-time/binary size
- Require more instruction decoding
- Use more memory and instruction cache
Unroll directive The Intel, IBM, Arm, Nvidia, clang, and GCC compilers provide the
preprocessing directive #pragma unroll ( #pragma GCC unroll for GCC) to insert above
the loop to force loop unrolling. The compiler already applies the optimization in most cases
Why are unrolled loops faster?
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Assertions
Some compilers (e.g. clang) use assertions for optimization purposes: most likely
code path, not possible values, etc. 3
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Compiler Hints - [[assume]]/std::unreachable()
C++23 allows defining an assumption in the code that is always true
int x = ...;
[[assume(x > 0)]]; // the compiler assume that 'x' is positive
int y = x / 2; // the operation is translated in a single shift as for
// the unsigned case
C++23 also provides std::unreachable() ( <utility> ) for marking unreachable
code
Compilers provide non-portable instructions for previous C++ standards: builtin assume()
(clang), builtin unreachable() (gcc), assume() (msvc)
Note: sometimes user-provided information leads to worse optimization, see
@llvm.assume blocks optimization and Refined Input, Degraded Output:
The Counterintuitive World of Compiler Behavior
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Recursion 1/2
Avoid run-time recursion (very expensive). Prefer iterative algorithms instead
Recursion cost: The program must store all variables (snapshot) at each recursion
iteration on the stack, and remove them when the control return to the caller instance
The tail recursion optimization avoids maintaining caller stack and pass the control to
the next iteration. The optimization is possible only if all computation can be executed
before the recursive call
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Function Call Cost
Function call methods:
Direct Function address is known at compile-time
Indirect Function address is known only at run-time
Inline The function code is fused in the caller code (same translation unit or
Link-time-optimization)
Direct/Indirect function call cost:
• The caller pushes the arguments on the stack in reverse order
• Jump to function address
• The caller clears (pop) the stack
• The function pushes the return value on the stack
• Jump to the caller address
The True Cost of Calls
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Argument Passing 1/3
The optimal way to pass and return arguments (by-value) to/from functions is in
registers. It also avoid the pointer aliasing performance issue. The following conditions
must be satisfied:
• The object is trivially copyable: No user-provided copy/move/default constructors,
destructor, and copy/move assignment operators, no virtual functions, apply recursively to
base classes and non-static data members
• Linux/Unix (SystemV x86-64 ABI): data types ≤ 16 bytes (8B × 2), max 6
arguments
• Windows (x64 ABI): data types ≤ 8 bytes, max 4 arguments
• when are structs/classes passed and returned in registers?
• System V ABI - X86-64 Calling Convention
• x64 calling convention - Parameter Passing
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Argument Passing - Active Objects 2/3
• If the previous conditions are not satisfied, the object is passed by-reference. In
addition, objects that are not trivially-copyable could be expensive to pass
by-value (copied).
• Pass by-reference and by-pointer introduce one level of indirection
• Pass by-reference is more efficient than pass by-pointer because it facilitates
variable elimination by the compiler, and the function code does not require
checking for NULL pointer
Three reasons to pass std::string view by value
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Argument Passing - const Parameters 3/3
const modifier applied to values, pointers, references does not produce better code
in most cases, but it is useful for ensuring read-only accesses
In some cases, pass by-const is beneficial for performance because const member
function overloading could be cheaper than their counterparts
GoTW#81: Constant Optimization?
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inline Function Declaration 1/2
inline
inline specifier for optimization purposes is just a hint for the compiler that
increases the heuristic threshold for inlining, namely copying the function body
where it is called
inline void f() { ... }
• the compiler can ignore the hint
• inlined functions increase the binary size because they are expanded in-place for
every function call
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inline Function Declaration 2/2
Compilers have different heuristics for function inlining
• Number of lines (even comments: How new-lines affect the Linux kernel
performance)
• Number of assembly instructions
• Inlining depth (recursive)
GCC/Clang extensions allow to force inline/non-inline functions:
[[gnu::always inline]] void f() { ... }
[[gnu::noinline]] void f() { ... }
[[msvc::forceinline]] void f() { ... }
• An Inline Function is As Fast As a Macro
• Inlining Decisions in Visual Studio
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Inlining and Linkage
The compiler can inline a function only if it is independent from external references
• A function with internal linkage is not visible outside the current translation unit,
so it can be aggressively inlined
• On the other hand, external linkage doesn’t prevent function inlining if the
function body is visible in a translation unit. In this situation, the compiler can
duplicate the function code if it determines that there are no external references
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Symbol Visibility
All compilers, except MSVC, export all function symbols → the symbols can be used in
other translation units and this can prevent inlining
Alternatives:
• Use static functions
• Use anonymous namespace (functions and classes)
• Use GNU extension (also clang) attribute ((visibility("hidden")))
gcc.gnu.org/wiki/Visibility
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Function Attributes
Some compilers, including Clang, GCC, provide additional attributes to optimize
function calls:
• attribute ((pure)) / [[gnu::pure]] no side effects on its parameters
and no external global references (program state)
→ subject to data flow analysis and might be eliminated
• attribute ((const)) / [[gnu::const]] depends only on its parameters,
no read from global references
→ subject to common sub-expression elimination and loop optimizations
note: the compiler is able to deduce such properties in most cases
Implications of pure and constant functions
attribute ((pure)) function attribute
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Pointers Aliasing 1/4
Consider the following example:
// suppose f() is not inline
void f(int* input, int size, int* output) {
for (int i = 0; i < size; i++)
output[i] = input[i];
}
• The compiler cannot unroll the loop (sequential execution, no ILP) because
output and input pointers can be aliased, e.g. output = input + 1
• The aliasing problem is even worse for more complex code and inhibits all kinds of
optimization including code re-ordering, vectorization, common sub-expression
elimination, etc.
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Pointers Aliasing 2/4
Most compilers (included GCC/Clang/MSVC) provide restricted pointers
( restrict ) so that the programmer asserts that the pointers are not aliased
void f(int* __restrict input,
int size,
int* __restrict output) {
for (int i = 0; i < size; i++)
output[i] = input[i];
}
Potential benefits:
• Instruction-level parallelism
• Less instructions executed
• Merge common sub-expressions
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Pointers Aliasing 3/4
Benchmarking matrix multiplication
void matrix_mul_v1(const int* A,
const int* B,
int N,
int* C) {
void matrix_mul_v2(const int* __restrict A,
const int* __restrict B,
int N,
int* __restrict C) {
Optimization -O1 -O2 -O3
v1 1,030 ms 777 ms 777 ms
v2 513 ms 510 ms 761 ms
Speedup 2.0x 1.5x 1.02x
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Variable/Object Scope
Declare local variable in the innermost scope
• the compiler can more likely fit them into registers instead of stack
• it improves readability
Wrong:
int i, x;
for (i = 0; i < N; i++) {
x = value * 5;
sum += x;
}
Correct:
for (int i = 0; i < N; i++) {
int x = value * 5;
sum += x;
}
• C++17 allows local variable initialization in if and while statements, while
C++20 introduces them for in range-based loops
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Variable/Object Scope
Exception! Built-in type variables and passive structures should be placed in the
innermost loop, while objects with constructors should be placed outside loops
for (int i = 0; i < N; i++) {
std::string str("prefix_");
std::cout << str + value[i];
} // str call CTOR/DTOR N times
std::string str("prefix_");
for (int i = 0; i < N; i++) {
std::cout << str + value[i];
}
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Object Optimizations
• Prefer direct initialization and full object constructor instead of two-step
initialization (also for variables)
• Prefer move semantic instead of copy constructor. Mark copy constructor as
=delete (sometimes it is hard to see, e.g. implicit)
• Use static for all members that do not use instance member (avoid passing
this pointer)
• If the object semantic is trivially copyable, ensure defaulted = default
default/copy constructors and assignment operators to enable vectorization
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Object Dynamic Behavior Optimizations
• Virtual calls are slower than standard functions
- Virtual calls prevent any kind of optimizations as function lookup is at
runtime (loop transformation, vectorization, etc.)
- Virtual call overhead is up to 20%-50% for function that can be inlined
• Mark final all virtual functions that are not overridden
• Avoid dynamic operations, e.g. dynamic cast
- The Hidden Performance Price of Virtual Functions
- Investigating the Performance Overhead of C++ Exceptions
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Object Operation Optimizations
• Minimize multiple + operations between objects to avoid temporary storage
• Prefer x += obj , instead of x = x + obj → avoid object copy and temporary
storage
• Prefer ++obj / --obj (return &obj ), instead of obj++ , obj-- (copy and
return old obj )
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Object Implicit Conversion
struct A { // big object
int array[10000];
};
struct B {
int array[10000];
B() = default;
B(const A& a) { // user-defined constructor
std::copy(a.array, a.array + 10000, array);
}
};
//----------------------------------------------------------------------
void f(const B& b) {}
A a;
B b;
f(b); // no cost
f(a); // very costly!! implicit conversion
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From C to C++
• Avoid old C library routines such as qsort , bsearch , etc. Prefer std::sort ,
std::binary search instead
- std::sort is based on a hybrid sorting algorithm. Quick-sort / head-sort
(introsort), merge-sort / insertion, etc. depending on the std implementation
- Prefer std::find() for small array, std::lower bound ,
std::upper bound , std::binary search for large sorted array
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Function Optimizations
• std::fill applies memset and std::copy applies memcpy if the
input/output are continuous in memory
• Use the same type for initialization in functions like std::accumulate() ,
std::fill
auto array = new int[size];
...
auto sum = std::accumulate(array, array + size, 0u);
// 0u != 0 → conversion at each step
std::fill(array, array + size, 0u);
// it is not translated into memset
The Hunt for the Fastest Zero
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Containers
• Use std container member functions (e.g. obj.find() ) instead of external
ones (e.g. std::find() ). Example: std::set O(log(n)) vs. O(n)
• Be aware of container properties, e.g. vector.push vector(v) , instead of
vector.insert(vector.begin(), value) → entire copy of all vector elements
• Set std::vector size during the object construction (or use the reserve()
method) if the number of elements to insert is known in advance → every implicit
resize is equivalent to a copy of all vector elements
• Consider unordered containers instead of the standard one, e.g. unorder map
vs. map
• Prefer std::array instead of dynamic heap allocation
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Critics to Standard Template Library (STL)
• Platform/Compiler-dependent implementation
• Execution order and results across platforms
• Debugging is hard
• Complex interaction with custom memory allocators
• Error handling based on exceptions is non-transparent
• Binary bloat
• Compile time (see C++ Compile Health Watchdog, and STL Explorer)
STL isn’t for *anyone*
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Other Language Aspects
• Prefer lambda expression (or function object ) instead of std::function
or function pointers
• Avoid dynamic operations: exceptions (and use noexcept ), smart pointer
(e.g. std::unique ptr )
• Use noexcept decorator → program is aborted if an error occurred instead of
raising an exception. see
Bitcoin: 9% less memory: make SaltedOutpointHasher noexcept
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