[llvm-dev] Where's the optimiser gone? (part 3)

[Stefan Kanthak via llvm-dev]

Hi @ll,

when called with "-O3 -target i386", the compiler fails to
generate properly optimised code (as shown on the right side)
for two of the following (rather trivial) functions:
(see <https://godbolt.org/z/tiwJqX>)

Especially notice the difference between the signed and unsigned
variants: the latter are properly optimised!

regards
Stefan Kanthak

--- sample3.c ---

__inline
long long Int32x32To64(long x, long y)
{
    return (long long) x * y;
}

long Int32x32To64Div32(long a, long b, long c)
{
    return Int32x32To64(a, b) / c;
}

Int32x32To64Div32:  # @Int32x32To64Div32
    push    ebp                          |    push    ebp
    mov     ebp, esp                     |    mov     ebp, esp
    push    esi
    mov     eax, dword ptr [ebp + 12]
    mov     ecx, dword ptr [ebp + 16]    |    mov     eax, dword ptr [ebp + 16]
    imul    dword ptr [ebp + 8]          |    cdq
    mov     esi, ecx                     |    push    edx
    sar     esi, 31                      |    push    eax
    push    esi                          |    mov     eax, dword ptr [ebp + 12]
    push    ecx                          |    imul    dword ptr [ebp + 8]
    push    edx                          |    push    edx
    push    eax                          |    push    eax
    call    __divdi3                     |    call    __divdi3
    add     esp, 16                      |    add     esp, 16
    pop     esi
    pop     ebp                          |    pop     ebp
    ret                                  |    ret


long Int32x32To64Rem32(long a, long b, long c)
{
    return Int32x32To64(a, b) % c;
}

Int32x32To64Rem32:   # @Int32x32To64Rem32
    push    ebp                          |    push    ebp
    mov     ebp, esp                     |    mov     ebp, esp
    push    esi
    mov     eax, dword ptr [ebp + 12]
    mov     ecx, dword ptr [ebp + 16]    |    mov     eax, dword ptr [ebp + 16]
    imul    dword ptr [ebp + 8]          |    cdq
    mov     esi, ecx                     |    push    edx
    sar     esi, 31                      |    push    eax
    push    esi                          |    mov     eax, dword ptr [ebp + 12]
    push    ecx                          |    imul    dword ptr [ebp + 8]
    push    edx                          |    push    edx
    push    eax                          |    push    eax
    call    __moddi3                     |    call    __modddi3
    add     esp, 16                      |    add     esp, 16
    pop     esi
    pop     ebp                          |    pop     ebp
    ret                                  |    ret


__inline
unsigned long long __fastcall UInt32x32To64(unsigned long x, unsigned long y)
{
    return (unsigned long long) x * y;
}

unsigned long UInt32x32To64Div32(unsigned long a, unsigned long b, unsigned long c)
{
    return UInt32x32To64(a, b) / c;
}

UInt32x32To64Div32:  # @UInt32x32To64Div32
    push    ebp
    mov     ebp, esp
    mov     eax, dword ptr [ebp + 12]
    mul     dword ptr [ebp + 8]
    push    0
    push    dword ptr [ebp + 16]
    push    edx
    push    eax
    call    __udivdi3
    add     esp, 16
    pop     ebp
    ret

unsigned long UInt32x32To64Rem32(unsigned long a, unsigned long b, unsigned long c)
{
    return UInt32x32To64(a, b) % c;
}

UInt32x32To64Rem32:  # @UInt32x32To64Rem32
    push    ebp
    mov     ebp, esp
    mov     eax, dword ptr [ebp + 12]
    mul     dword ptr [ebp + 8]
    push    0
    push    dword ptr [ebp + 16]
    push    edx
    push    eax
    call    __umoddi3
    add     esp, 16
    pop     ebp
    ret