[Bug c++/108585] New: Using std::byte instead of char results in different (worse?) code in array zero initialization

2023-01-28 Thread technicallyanonymous at proton dot me via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108585

Bug ID: 108585
   Summary: Using std::byte instead of char results in different
(worse?) code in array zero initialization
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: technicallyanonymous at proton dot me
  Target Milestone: ---

This code, compiled with GCC 12.2 (-O2 -std=c++20)

#include 

int foo(std::byte *arr);

int square(int num) {
std::byte arr[96] = {};
return foo(arr);
}

results in this:

square(int):
sub rsp, 104
xor eax, eax
mov ecx, 12
mov rdi, rsp
rep stosq
mov rdi, rsp
callfoo(std::byte*)
add rsp, 104
ret

After swapping "std::byte" with "char" the result is:

square(int):
sub rsp, 104
pxorxmm0, xmm0
mov rdi, rsp
movaps  XMMWORD PTR [rsp], xmm0
movaps  XMMWORD PTR [rsp+16], xmm0
movaps  XMMWORD PTR [rsp+32], xmm0
movaps  XMMWORD PTR [rsp+48], xmm0
movaps  XMMWORD PTR [rsp+64], xmm0
movaps  XMMWORD PTR [rsp+80], xmm0
callfoo(char*)
add rsp, 104
ret

As you can see, SSE instructions are emitted only with an char array. BTW, in
clang, both versions are identical and use movaps.

[Bug c++/108586] New: Using std::array instead of a plain array results in different (worse?) code in array zero initialization

2023-01-28 Thread technicallyanonymous at proton dot me via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108586

Bug ID: 108586
   Summary: Using std::array instead of a plain array results in
different (worse?) code in array zero initialization
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: technicallyanonymous at proton dot me
  Target Milestone: ---

This code, compiled with GCC 12.2 (-O2 -std=c++20):

#include 

int foo(char *arr);

int square(int num) {
std::array arr = {};
return foo(arr.data());
}

results in this:

square(int):
sub rsp, 104
xor eax, eax
mov ecx, 12
mov rdi, rsp
rep stosq
mov rdi, rsp
callfoo(char*)
add rsp, 104
ret

However, this very similar code:

int foo(char *arr);

int square(int num) {
char arr[96] = {};
return foo(arr);
}

is compiled as:

square(int):
sub rsp, 104
pxorxmm0, xmm0
mov rdi, rsp
movaps  XMMWORD PTR [rsp], xmm0
movaps  XMMWORD PTR [rsp+16], xmm0
movaps  XMMWORD PTR [rsp+32], xmm0
movaps  XMMWORD PTR [rsp+48], xmm0
movaps  XMMWORD PTR [rsp+64], xmm0
movaps  XMMWORD PTR [rsp+80], xmm0
callfoo(char*)
add rsp, 104
ret

As you can see, SSE instructions are emitted only with a plain array. I also
tested clang and in both cases it emits movaps instructions.