https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120963
Bug ID: 120963
Summary: Missed optimization of (a > b ? a - b : 0) pattern
Product: gcc
Version: 15.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: Explorer09 at gmail dot com
Target Milestone: ---
This could be multiple missed optimization issues in one same test code. I just
don't know how to reduce the issues to even simpler test cases.
```c
#include <stdint.h>
int32_t func1a(int32_t a, int32_t b) {
if (b - a < 0)
a = b;
return b - a;
}
int32_t func1b(int32_t a, int32_t b) {
if (b < a)
a = b;
return b - a;
}
int32_t func1c(int32_t a, int32_t b) {
if (b < a)
return 0;
return b - a;
}
```
x86-64 gcc 15.1 with `-Os -fno-wrapv` options produces the following assembly:
```x86asm
func1a:
subl %edi, %esi
movl $0, %eax
cmovns %esi, %eax
ret
func1b:
cmpl %edi, %esi
movl %esi, %eax
cmovle %esi, %edi
subl %edi, %eax
ret
func1c:
movl %esi, %eax
xorl %edx, %edx
subl %edi, %eax
cmpl %edi, %esi
cmovl %edx, %eax
ret
```
Note that the three functions are equivalent, and yet the generated assembly
codes are different, and none of them are optimal.
The optimal code, AFAIK, is this:
```x86asm
func1:
xorl %eax, %eax
subl %edi, %esi
cmovg %esi, %eax
ret
```
When compared with the assembly of three functions above:
* func1a missed the part that the `movl $0, %eax` can be simplified to `xorl
%eax, %eax` if the instruction is performed _before_ the `sub` instruction, the
condition code of which is checked later.
* As signed integer overflow is undefined behavior in C, the `cmovns` in func1a
could as well be transformed to `cmovg`. (This optimization should be disabled
for `-fwrapv`.)
* func1c missed that the `sub` and `cmp` could be merged to one operation. This
can also save the `mov` instruction at the beginning. (I have reported Bug
113680 as similar to this issue.)
