[Bug tree-optimization/32605] Missing byte swap optimizations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32605

--- Comment #6 from Andrew Pinski  ---
(In reply to Jed Brown from comment #5)
> The missed optimization even exists for code such as this, which should
> compile to a simple load on LE architectures.

That is because GCC decided to only optimize this code at -O2 and above. 
Rather than at -O1. IIRC this is because in some cases it can introduce some
bigger decrease compile time.

[Bug tree-optimization/32605] Missing byte swap optimizations

[jed at 59A2 dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32605

Jed Brown  changed:

   What|Removed |Added

 CC||jed at 59A2 dot org

--- Comment #5 from Jed Brown  ---
The missed optimization even exists for code such as this, which should compile
to a simple load on LE architectures.

unsigned read_u32_le(const unsigned char arr[]) {
  return (arr[0] << 0)
| (arr[1] << 8)
| (arr[2] << 16)
| (arr[3] << 24);
}

gcc-8.3/trunk -O:

read_u32_le:
  movzx eax, BYTE PTR [rdi+1]
  sal eax, 8
  movzx edx, BYTE PTR [rdi+2]
  sal edx, 16
  or eax, edx
  movzx edx, BYTE PTR [rdi]
  or eax, edx
  movzx edx, BYTE PTR [rdi+3]
  sal edx, 24
  or eax, edx
  ret

clang-8 -O:

read_u32_le: # @read_u32_le
  mov eax, dword ptr [rdi]
  ret

https://gcc.godbolt.org/z/8lGeCF

[Bug tree-optimization/32605] Missing byte swap optimizations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32605

Andrew Pinski  changed:

   What|Removed |Added

 Target|x86_64-*-*, i?86-*-*|
   Last reconfirmed|2012-02-02 00:00:00 |2018-4-22
  Component|rtl-optimization|tree-optimization

--- Comment #4 from Andrew Pinski  ---
Confirmed still.  Happens on aarch64-linux-gnu also.