[llvm-bugs] [Bug 27604] New: Wrong optimization when goto jumps backwards over declaration

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27604

            Bug ID: 27604
           Summary: Wrong optimization when goto jumps backwards over
                    declaration
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cherepan at mccme.ru
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The program:

#include <stdio.h>

int main()
{
  int *p = 0;

 start:
  if (p) {
    printf("%d\n", *p);
    return 0;
  }

  int x;
  x = 1;
  p = &x;
  goto start;
}

should print "1" but prints "0" when compiled with clang -O1.
Tested on clang version 3.9.0 (trunk 268228).
Tests with https://gcc.godbolt.org/ show that it started between 3.6 and 3.7.

The reason probably is insertion of llvm.lifetime.start where a declaration
occurs instead of where the corresponding block starts.
The relevant part of C11 is 6.2.4p5-6: "An object whose identifier is declared
with no linkage and without the storage-class specifier static has automatic
storage duration, as do some compound literals. [...] For such an object that
does not have a variable length array type, its lifetime extends from entry
into the block with which it is associated until execution of that block ends
in any way."

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