https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108548
Bug ID: 108548
Summary: gcc asm goto with outputs not implicitly volatile
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ndesaulniers at google dot com
CC: eli.friedman at gmail dot com, foom at fuhm dot net
Target Milestone: ---
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile mentions:
> GCC’s optimizers sometimes discard asm statements if they determine there is
> no need for the output variables. ... Using the volatile qualifier disables
> these optimizations. asm statements that have no output operands and asm goto
> statements, are implicitly volatile.
So what about asm goto statements that do have outputs, which GCC recently
gained support for?
Looks like I get different codegen targeting x86 at -O2 with:
```
int foo (void) {
void *x;
asm goto (
"movq %l1, %0\n\t"
"jmp *%0"
:"=r"(x):::bar);
return 0;
bar:
return 1;
}
```
based on whether the asm goto statement is marked volatile or not.
Should asm goto statements with outputs be implicitly volatile (implying a bug
currently in GCC) or not (implying the documentation could perhaps be updated)?
