[Bug tree-optimization/55869] New: different bit shift/or optimization.

[pluto at agmk dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55869

             Bug #: 55869
           Summary: different bit shift/or optimization.
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pluto@agmk.net


here're equvialent functions:

$ cat u.cpp
unsigned long long convert_1( bool flag )
{
        return (unsigned long long)(flag ? 1ull : 0ull) << 57;
}
unsigned long long convert_2( bool flag )
{
        return (unsigned long long)flag << 57;
}

current 4.7/4.8 gcc emits following different machine code:

_Z9convert_1b:

        testb   %dil, %dil      # 29    *cmpqi_ccno_1/1 [length = 3]
        movl    $0, %edx        # 32    *movdi_internal_rex64/1 [length = 5]
        movabsq $144115188075855872, %rax       # 28    *movdi_internal_rex64/3
[length = 10]
        cmove   %rdx, %rax      # 31    *movdicc_noc/1  [length = 4]
        ret     # 39    simple_return_internal  [length = 1]

_Z9convert_2b:
        movq    %rdi, %rax      # 19    *movdi_internal_rex64/2 [length = 3]
        salq    $57, %rax       # 8     *ashldi3_1/1    [length = 4]
        ret     # 27    simple_return_internal  [length = 1]


while clang/llvm emits equivalent machine code for both functions:

_Z9convert_1b:                          # @_Z9convert_1b
        movzbl  %dil, %eax
        shlq    $57, %rax
        ret

_Z9convert_2b:                          # @_Z9convert_2b
        movzbl  %dil, %eax
        shlq    $57, %rax
        ret