[llvm-bugs] [Bug 27117] New: LLVM generates unreachable call stack adjustments after noreturn functions when the frame is not reserved

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27117

            Bug ID: 27117
           Summary: LLVM generates unreachable call stack adjustments
                    after noreturn functions when the frame is not
                    reserved
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: rnk at google.com
                CC: llvm-bugs at lists.llvm.org
            Blocks: 26299
    Classification: Unclassified

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 at 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.

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