https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102888
Bug ID: 102888
Summary: missing case for combining / and % into one operation
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vanyacpp at gmail dot com
Target Milestone: ---
Normally GCC combines a/b and a%b into one operation when they are computed in
the same basic-block. The example below has two functions. For one GCC is able
to combine the operations and for other not (presumably because of complicated
control-flow). I believe the two functions are functionally equivalent.
unsigned long long reduce(unsigned long long a, unsigned long long b)
{
while ((a % b) == 0)
a /= b;
return a;
}
unsigned long long reduce_opt(unsigned long long a, unsigned long long b)
{
for (;;)
{
unsigned long long quot = a / b;
unsigned long long rem = a % b;
if (rem != 0)
break;
a = quot;
}
return a;
}
reduce.L3:
mov rax, r8
xor edx, edx
div rsi
xor edx, edx
mov r8, rax
div rsi
test rdx, rdx
je .L3
reduce_opt.L8:
xor edx, edx
mov r8, rax
div rsi
test rdx, rdx
je .L8
https://godbolt.org/z/9dqs8avE5
It would be great if GCC generated the same code for both of these functions.
