https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113203
Bug ID: 113203
Summary: __attribute__ ((always_inline)) fails with
C99/LTO/-Og.
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: stefan at bytereef dot org
Target Milestone: ---
This is similar to #107931. I'm opening a new issue because there are no
indirect function calls and the problem occurs with -std=c99 -flto -Og.
foo.c
========================================================
#include <stdbool.h>
#include "foo.h"
inline __attribute__ ((always_inline)) bool
f(int x)
{
return (x > 2);
}
========================================================
foo.h
========================================================
#include <stdbool.h>
bool f(int);
========================================================
main.c
========================================================
#include <stdio.h>
#include "foo.h"
int
main(int argc, char *argv[])
{
(void)argv;
if (f(argc)) {
puts("yes");
}
else {
puts("no");
}
return 0;
}
========================================================
$ gcc -Wall -Wextra -std=c99 -flto -Og -o main foo.c main.c
foo.c: In function ‘main’:
foo.c:5:1: error: inlining failed in call to ‘always_inline’ ‘f’: function not
considered for inlining
5 | f(int x)
| ^
main.c:9:8: note: called from here
9 | if (f(argc)) {
| ^
lto-wrapper: fatal error: /home/skrah/gcc/bin/gcc returned 1 exit status
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
This is extracted from the mpdecimal project that has used C99 and
always_inline for a decade without problems. The code was written before the
amendment to the always_inline documentation in 2014 and always_inline has
consistently produced a speedup of 1-2.5% even with -O3.
My questions:
1) Since this is C99, should always_inline work without errors when -std=c99 is
active? If not, should -std=c99 reject always_inline?
2) There is a clear demand for something like "really_inline" that ignores the
heuristics and just inlines whenever possible without errors or warnings. In
practice that is how MSVC __forceinline or clang always_inline behave. Could
that be added?
