[llvm-bugs] [Bug 26023] New: LLVM -0z push/pop optimization clobbers the red zone

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26023

            Bug ID: 26023
           Summary: LLVM -0z push/pop optimization clobbers the red zone
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.majnemer at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

consider:
$ cat t.c
#include <stdio.h>

int f() {
  volatile char x[120] __attribute__((aligned(1)));
  asm volatile("" : : "g"(x) : "memory");
  x[119] = -2;
  asm volatile("" : : "r"(-1));
  return x[119];
}

int main() {
  int x = f();
  printf("%d\n", x);
}

compile with:
$ ~/llvm/Debug+Asserts/bin/clang -Oz  t.c -o t

run it:
$ ./t
-1

the correct result is -2 but we push/pop'd -1 on to the stack, clobbering the
end of 'x' which exists on the red zone.

Reverting r255656, the commit which introduced the optimization, fixes this.

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