[Bug c/53382] New: incorrect associativity in expressions

[miguel.barao at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382

             Bug #: 53382
           Summary: incorrect associativity in expressions
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: miguel.barao@gmail.com


I found the following behavior while writing a small RPN calculator.
The ideia is to perform a subtraction on the two top elements of a stack using

push(&mystack, -pop(&mystack) + pop(&mystack));

Since the + associativity is left-to-right, this should pop the stack, negate
this value, pop the second argument then add. But the order is reversed in gcc
leading to a wrong result.

The behavior was observed in gcc 4.4.5 (debian stable), 4.6.3 (ubuntu 12.04),
and the llvm-gcc versions shipping with MacOSX.
The clang frontend versions 1.1 (on debian stable), 3.0 (OSX and ubuntu 12.04)
work correctly.

Example:
Suppose a stack contains [ 1 2 ], the top of the stack being 2.
On gcc the above expression returns 1, the first pop executed is the second
one.
On clang the same expression returns -1, the first pop executed is the first
one as should be.