https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115506
Bug ID: 115506
Summary: Possible but missed "cmp" instruction merging (x86 &
ARM, optimization)
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: Explorer09 at gmail dot com
Target Milestone: ---
I'm not sure if the GCC developers are interested in this optimization or if
this issue has been reported before.
----
(Compiler Explorer link: https://godbolt.org/z/vqqf865cb)
```c
#include <stdint.h>
// Modified from <https://gitlab.com/-/snippets/3718423>
uint32_t utf8_to_code_point(const uint8_t **sequence) {
if (**sequence <= 0x7F) {
return *(*sequence)++;
}
unsigned int max_length;
uint32_t min_code_point;
if ((**sequence & 0xF0) == 0xE0) { /* NOTE 0 */
max_length = 3;
min_code_point = 0x0800;
} else if ((**sequence & 0xF0) < 0xE0) { /* NOTE 1 */
max_length = 2;
min_code_point = 0x80;
} else {
max_length = 4;
min_code_point = 0x10000;
}
uint32_t code_point = (uint32_t)**sequence - (0xFF & ~(0xFF >>
max_length));
while (1) {
(*sequence)++;
if (--max_length == 0) {
break;
}
unsigned int code_offset = (unsigned int)**sequence - 0x80;
if (code_offset > 0x3F) {
return (uint32_t)-1;
}
code_point = (code_point << 6) + code_offset;
}
if (code_point < min_code_point) {
return (uint32_t)-1;
}
if (code_point >= 0xD800 && code_point <= 0xDFFF) {
return (uint32_t)-1;
}
return code_point;
}
```
I wrote this code with the expectation that the compiler can optimize the "cmp"
instructions so that the same compare status can be used twice.
But instead I get this:
```x86asm
# ...
movl %eax, %ecx
andl $-16, %ecx
cmpb $-32, %cl
je .L8
cmpb $-33, %cl
ja .L9
# ...
```
The immediate constant for the second "cmp" instruction gets modified
off-by-one (I can guess the reason for that) and missed this particular chance
of merging the comparisons.
A workaround is to modify the line marked with the `/* NOTE 1 */` comment to
use `<=` instead of `<`. But I wish GCC can detect this optimization
opportunity better.
