https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118316
Bug ID: 118316
Summary: Missed optimization: while (len > 1) len -= 2;
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: blubban at gmail dot com
Target Milestone: ---
unsigned foo(char* buf, unsigned len)
{
while (len > 1)
len -= 2;
if (len)
*buf = 42;
return len;
}
unsigned bar(char* buf, unsigned len)
{
len %= 2;
if (len)
*buf = 42;
return len;
}
-O2
Result:
foo(char*, unsigned int):
mov eax, esi
and eax, 1
cmp esi, 1
cmovbe eax, esi
test eax, eax
je .L1
mov BYTE PTR [rdi], 42
.L1:
ret
bar(char*, unsigned int):
mov eax, esi
and eax, 1
je .L9
mov BYTE PTR [rdi], 42
.L9:
ret
Expected: Same for both
Reduced from an implementation of to_chars(int) using a two-chars-at-the-time
char[200] lookup table.
All three major compilers get confused by that code.
https://godbolt.org/z/1W1hroc67
