LLVM generates unreachable call stack adjustments after noreturn functions when the frame is not reserved

[rnk]

Bugzilla Link	27117
Resolution	FIXED
Resolved on	May 20, 2016 10:52
Version	trunk
OS	Windows NT
Blocks	#26673
CC	@zmodem

Extended Description

Consider:

__declspec(noreturn) void crash(void );
void g(void);
void f(int c) {
void *p = _alloca(c);
if (c)
crash(p);
g(p);
}

Here is the call to 'crash':
.LBB0_1: # %if.then
callq "?crash@@YAXPEAX@Z"
addq $32, %rsp
ud2

The ADD RSP instruction is dead. On Windows, it is very common to not have a reserved call frame because of inalloca.

Maybe the right fix here is to teach MI more about noreturn functions so we can do more general DCE after them. It would also help us get rid of TrapOnUnreachable in favor of inserting int3 after all noreturn calls.

[rnk]

This is worth doing because it comes up on every assert-like construct.

[zmodem]

r270109

[rnk]

mentioned in issue llvm/llvm-bugzilla-archive#45064