On Wed, 2026-03-04 at 16:39:12 -0800, Chris Copeland wrote:
> >
> >>> +/* PR ipa/124218: ipa-reference must honor memory clobbers in inline
> >>> asm. */
> >>> +
> >>> +int flag;
> >>> +
> >>> +__attribute__ ((noinline))
> >>> +static void
> >>> +clobber_and_set (void)
> >>> +{
> >>> + __asm__ volatile ("ldr r0, =flag\n\t"
> >>> + "mov r1, #1\n\t"
> >>> + "str r1, [r0]"
> >>> + ::: "r0", "r1", "memory");
> >
> > At least withtout new Michal's heuristics, for making this safe with LTO
> > you need used attribute on flag and that disables ipa-reference on it.
>
> This was discussed some on the bug report, but in the real code this clobber
> is at the location where a context switch is triggered, which eventually
> leads to another task executing that modifies flag. I wrote the test this way
> because it's the simplest thing I could come up with to represent that
> situation. Would an unrelated and not-called function that does modify the
> flag obviate the need for ((used)) in this test, even without forthcoming
> changes?
>
The issue is with hardcoding symbols into the asm statement.
During LTO the "flag" may be renamed to "flag.lto_priv.1" but the asm
statement refers to the old name. Anything that would prevent renaming
would also disable ipa-reference.
You could do something like this:
int flag;
__attribute__ ((used))
void asm_set(void) {flag = 1;}
clobber_and_set (void) {
asm("call asm_set" ::: "memory");
}
