http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45903
Summary: unnecessary load of 32/64bit variable when only 8 bits
are needed
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
Created attachment 21967
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=21967
testcase for both cases
For the following testcase:
uint8_t f64(uint64_t a, uint64_t b)
{
return (a >> 8) + (b >> 8);
}
gcc (r164716) generates this code:
$ gcc -O3 -S -m32 tst10.s tst10.c -masm=intel
f64:
push ebx
mov ecx, DWORD PTR [esp+8]
mov ebx, DWORD PTR [esp+12]
mov eax, DWORD PTR [esp+16]
mov edx, DWORD PTR [esp+20]
shrd ecx, ebx, 8
pop ebx
shrd eax, edx, 8
add eax, ecx
ret
while it could use just something like:
f64:
mov al, DWORD PTR [esp+5]
add al, DWORD PTR [esp+9]
ret
The situation is better for 32bit case:
uint8_t f32(uint32_t a, uint32_t b)
{
return (a >> 8) + (b >> 8);
}
where gcc generates:
$ gcc -O3 -S -m32 tst10.s tst10.c -masm=intel
f32:
mov eax, DWORD PTR [esp+4]
mov edx, DWORD PTR [esp+8]
shr eax, 8
shr edx, 8
add eax, edx
ret
while it could generate the same code as for f64:
f32:
mov al, DWORD PTR [esp+5]
add al, DWORD PTR [esp+9]
ret
