support for GNU C hot+cold label attributes

[nickdesaulniers]

Bugzilla Link	47487
Version	unspecified
OS	All
Blocks	#4440
CC	@DougGregor,@isanbard,@jyknight,@zygoloid

Extended Description

https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#Label-Attributes
https://godbolt.org/z/Er3rP8

#include <stdio.h>
int foo(void) {
       asm goto ("some asm" : : : : NoError);

/* This branch (the fall-through from the asm) is less commonly used */
ErrorHandling: 
   __attribute__((cold, unused)); /* Semi-colon is required here */
   printf("error\n");
   return 0;

NoError:
   printf("no error\n");
   return 1;
}

clang:

warning: 'cold' attribute only applies to functions [-Wignored-attributes]
   __attribute__((cold, unused)); /* Semi-colon is required here */
                  ^

[nickdesaulniers]

This is going to become a problem for the Linux kernel sooner or later.
https://lore.kernel.org/lkml/20211108164711.mr2cqdcvedin2lvx@treble/

[isanbard]

Bleck!

That attribute should technically be useless, as the indirect branch from an asm goto block is already marked as "cold", at least in clang.

[nickdesaulniers]

It looks like clang already supports __attribute__((unused)) for -Wunused-label, so this is really just about __attribute__((hot)) and __attribute__((cold)). Re-titling the bug.

That attribute should technically be useless, as the indirect branch from an asm goto block is already marked as "cold", at least in clang.

Things get weird when outputs are involved, but without any I suspect it might be nice to mark which indirect branch is hot or hotter than the fallthrough BB.