https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84757
Bug ID: 84757
Summary: Useless MOVs and PUSHes to store results of MUL
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: b7.10110111 at gmail dot com
Target Milestone: ---
Consider the following C code:
#ifdef __SIZEOF_INT128__
typedef __uint128_t Longer;
#else
typedef unsigned long long Longer;
#endif
typedef unsigned long Shorter;
Shorter mulSmarter(Shorter a, Shorter b, Shorter* upper)
{
const Longer ab=(Longer)a*b;
*upper=ab >> 8*sizeof(Shorter);
return ab;
}
On amd64 with -m64 option I get identical assembly on both gcc 7.x and 6.3. But
on x86 (or amd64 with -m32) assembly is different, and on gcc 7.x is less
efficient. See to compare:
# gcc 6.3
mulSmarter:
mov eax, DWORD PTR [esp+8]
mul DWORD PTR [esp+4]
mov ecx, edx
mov edx, DWORD PTR [esp+12]
mov DWORD PTR [edx], ecx
ret
# gcc 7.3
mulSmarter:
push esi
push ebx
mov eax, DWORD PTR [esp+16]
mul DWORD PTR [esp+12]
mov esi, edx
mov edx, DWORD PTR [esp+20]
mov ebx, eax
mov eax, ebx
mov DWORD PTR [edx], esi
pop ebx
pop esi
ret
The gcc 6.3 version is already not perfect, but it's much better than that of
7.3.
