https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102032
Bug ID: 102032
Summary: missed optimization on 2 equivalent expressions when
-fwrapv is not used
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
Consider the following code from bug 30484:
int f (int a, int b, int c)
{
if (b < 0)
return a + b + c;
else
return a + c + b;
}
On x86_64 with -O3, GCC 11.2.0 gives
leal (%rdi,%rdx), %eax
addl %esi, %edi
addl %edx, %edi
addl %esi, %eax
testl %esi, %esi
cmovs %edi, %eax
But x86_64 processors (and most processors, AFAIK) do not care about overflows,
so that a + b + c and a + c + b are actually equivalent for the processor and
one should get only 2 instructions, corresponding to the 2 additions.
Surprisingly, -fwrapv has the effect to allow this optimization, with the
following generated code:
addl %edx, %edi
leal (%rdi,%rsi), %eax
