[llvm-bugs] [Bug 26603] New: Wrong optimization with aliasing

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26603

            Bug ID: 26603
           Summary: Wrong optimization with aliasing
           Product: clang
           Version: trunk
          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 <stdlib.h>
#include <stdio.h>

int main()
{
  void *volatile p = malloc(10);
  int *pi = p;
  double *pd = p;

  *pi = 1;
  printf("*pi = %d\n", *pi);

  int a = *pi;
  *pd = 0;
  *pi = a;

  printf("p = %p\n", p);
  printf("*pi = %d\n", *pi);
}

compiled with `clang -std=c11 -O2` prints this:

*pi = 1
p = 0x929010
*pi = 0

The last value is wrong, it should be 1.

Tested with clang 3.0, 3.5 and 3.9.0 (trunk 260831) -- all affected.

AFAICT the code above doesn't violate any aliasing rules. All reads are with
the right type. And writes to allocated objects are free to change the
effective type according to C11, 6.5p6: "If a value is stored into an object
having no declared type through an lvalue having a type that is not a character
type, then the type of the lvalue becomes the effective type of the object for
that access and for subsequent accesses that do not modify the stored value."

Similar gcc bug -- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69776 .

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