https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86707
Bug ID: 86707
Summary: Missed optimization: optimizing set of if statements
Product: gcc
Version: tree-ssa
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zamazan4ik at tut dot by
Target Milestone: ---
gcc(trunk) with -O3 -std=c++17 for this code:
unsigned int foo(unsigned int x)
{
if(x % 2 == 0)
{
return x * 2;
}
if(x % 4 == 0)
{
return x * 4;
}
if(x % 8 == 0)
{
return x * 8;
}
if(x % 16 == 0)
{
return x * 16;
}
if(x % 32 == 0)
{
return x * 32;
}
return 100;
}
generates this:
foo(unsigned int):
test dil, 1
je .L9
test dil, 3
je .L10
test dil, 7
je .L11
mov eax, edi
test dil, 15
je .L12
sal eax, 5
and edi, 31
mov edi, 100
cmovne eax, edi
ret
.L10:
lea eax, [0+rdi*4]
ret
.L9:
lea eax, [rdi+rdi]
ret
.L12:
sal eax, 4
ret
.L11:
lea eax, [0+rdi*8]
ret
As you see, generated code is suboptimal: here we can leave only first 'if'
statement and otherwise return 100.
