[Bug c/67025] New: Missing aggressive loop optimization warning when -fPIC used

[Bug c/67025] New: Missing aggressive loop optimization warning when -fPIC used

[Nikola.Veljkovic at imgtec dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67025

            Bug ID: 67025
           Summary: Missing aggressive loop optimization warning when
                    -fPIC used
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Nikola.Veljkovic at imgtec dot com
  Target Milestone: ---

The issue here is not that the loop is over optimized, but the fact that the
warning is missing.
Without the warning debugging an issue like this takes much longer.

Source code:

main.c:
#include <stdio.h>

struct problem {
    int*    array1[4];
    int*    array2[4];
};

int* foo() { return 0; }

int main(int argc, int** argv) {
    int i;
    struct problem p;
    for(i = 0; i < 8; i++) {
        printf("i=%d\n", i); 
        p.array1[i] = foo();
        if(p.array1[i]) return -1; 
    }   
    return 0;
}

Compilation (with -fPIC):
$ gcc-5 -O2 -fPIC -o for-test main.c

Compilation (without -fPIC):
$ gcc-5 -O2 -o for-test main.c
main.c: In function ‘main’:
main.c:15:21: warning: iteration 4u invokes undefined behavior
[-Waggressive-loop-optimizations]
         p.array1[i] = foo();
                     ^
main.c:13:5: note: containing loop
     for(i = 0; i < 8; i++) {

Compilation is successful in both cases, but the warning is missing when
compiling with -fPIC.

Execution: The loop executes indefinitely in both cases.

Tested on last week's build of gcc-5.2 release built for mips, and on Ubuntu
14.04 (i686 package) gcc-5.1.
Should be reproducible on any architecture with compiler version 4.9 and above.

I can provide any additional info, if required.
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Subject: [Bug c++/66555] Fails to warn for if (j == 0 && i == i)
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf555

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Author: mpolacek
Date: Mon Jul 27 12:40:45 2015
New Revision: 226242

URL: https://gcc.gnu.org/viewcvs?rev"6242&root=gcc&view=rev
Log:
        PR c++/66555
        PR c/54979
        * c-common.c (find_array_ref_with_const_idx_r): New function.
        (warn_tautological_cmp): New function.
        * c-common.h (warn_tautological_cmp): Declare.
        * c.opt (Wtautological-compare): New option.

        * c-typeck.c (parser_build_binary_op): Call warn_tautological_cmp.

        * call.c (build_new_op_1): Call warn_tautological_cmp.
        * pt.c (tsubst_copy_and_build): Use sentinel to suppress tautological
        compare warnings.

        * doc/invoke.texi: Document -Wtautological-compare.

        * c-c++-common/Wtautological-compare-1.c: New test.

Added:
    trunk/gcc/testsuite/c-c++-common/Wtautological-compare-1.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-family/ChangeLog
    trunk/gcc/c-family/c-common.c
    trunk/gcc/c-family/c-common.h
    trunk/gcc/c-family/c.opt
    trunk/gcc/c/ChangeLog
    trunk/gcc/c/c-typeck.c
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/cp/pt.c
    trunk/gcc/doc/invoke.texi
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/g++.dg/cpp0x/decltype-54581.C
    trunk/gcc/testsuite/g++.dg/other/vector-compare.C