All Questions
Tagged with gcc optimization
827
questions
0
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50
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GCC instruction scheduling: how do the algorithms differ?
GCC offers a number of options related to instruction scheduling in the compiler flags. An overview of what this means is on the GCC Wiki for "Instruction Scheduling", but this is well out ...
- 612
15
votes
1
answer
572
views
NRVO vs early return for types not benefitting from move semantics (GCC 14 -Wnrvo)
GCC 14 introduced a new -Wnrvo flag:
New -Wnrvo warning, to warn if the named return value optimization is not performed although it is allowed by [class.copy.elision]. See the manual for more info.
...
- 92.4k
0
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0
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86
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compiler generated assembler
A question about compiler generated assembler:
My to-be-optimized main loop includes two memory accesses instead of register.
loop:
mov xmm, mem // pre-calculated value pushed on the stack
pxor xmm, ...
0
votes
0
answers
55
views
Optimizing declaration of symbols implemented in stdlib
I'm creating a static compiled application for an embedded device.
The device has implementation for common std functions like memset/strlen.
In order to reduce the size of the binary I'm compiling ...
- 40
1
vote
1
answer
55
views
Unexpected generated infinite loop where there is no side effect [duplicate]
When a program built with gcc 8.4.3 and -O2, following loop was turned into an assembly code of infinite one
for(int i=0; i < PCU_MAX; i++)
{
auto loc = getApp()->Loc[i + 1 + PCU_LOCATION]...
- 14.2k
2
votes
1
answer
98
views
Constant function pointer optimization
I am trying to implement an abstract interface in C using function pointers inside a struct.
Something like the following
typedef int (*fn_t)(int);
typedef struct
{
int x;
const fn_t fnp;
}...
- 23
0
votes
0
answers
101
views
GCC: Tell the compiler that access outside array limits is allowed?
We have the following problem:
There are often structures that look like this:
struct {
someType foo;
/* The actual length of bar[] depends on the
* size passed to malloc() */
...
- 18.3k
1
vote
0
answers
65
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How to force GCC change the generated assembly order of a branch?
My problem is deterministically arranging the position of different branches' code in the compiled binary/assembly. Note that my primary goal is NOT to optimize performance, but to be sure to have a ...
0
votes
2
answers
68
views
Program hangs if GCC optimizations are enabled
I'm doing some kind of "homework" on a platform (training.olinfo.it) where, given a certain problem, you have to create a program that solves it, and it must respect memory and time ...
0
votes
1
answer
95
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Strange unnecessary stack usage by GCC and Clang [duplicate]
I was using Compiler Explorer and noticed GCC and Clang would emit seemingly unnecessary instructions related to the stack when compiling this simple function (Compiler Explorer).
void bar(void);
int ...
- 8,561
8
votes
2
answers
173
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Why does GCC fail to reduce a loop that increments two locations of the same buffer?
Here is a bounded loop that increments two locations of the same buffer.
unsigned int getid();
void foo(unsigned int *counter, unsigned int n) {
unsigned int A = getid();
unsigned int ...
- 99
0
votes
1
answer
51
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Are these codes(concurrent C codes) necessary to use memory barrier?
I found these codes in project libuv 1.3.0. But I can't understand that why memory barriers(for compiler) are need.
static int uv__async_make_pending(int* pending) {
/* Do a cheap read first. */
...
- 3
0
votes
0
answers
51
views
how to fine tune gcc optimizations for space and speed
I am compiling some embedded software with speed and space constraints,
using a pic32 port of gcc. Due to the space constraints, I need to optimize
for size (-Os) or the image won't fit (e.g. with -O2)...
- 11
0
votes
2
answers
144
views
How to chain avx2 intrinsics efficiently to perform chain of arithmetic operations?
I wrote a large program to simulate molecular system. I ran it on a desktop computer whose processor is a Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz. Most of the time (75%) is used to calculate a Lennard ...
- 2,273
7
votes
0
answers
180
views
Why can the compiler not optimize away the access to this const parameter?
I have already searched for some answers on Google and Stack Overflow, and I am aware that compilers cannot assume that functions won't modify arguments passed through const references, as these ...
- 1,032