Bug#698950: gcc-4.7: Miscompilation of simple loop depending on integer overflow for termination

[Daniel Schepler <dschepler@gmail.com>]

Package: gcc-4.7
Version: 4.7.2-5
Severity: normal

With the following very simple test program:

extern int foo(int);
extern void bar();

void test_fn() {
  int i;
  for (i = 1; 0 < i; i *= 2)
    foo(i);
  bar();
}

and compiling with "gcc -S -O2 -Wall bugtest.c -o -" (on amd64) I get output:

        .file   "bugtest.c"
        .text
        .p2align 4,,15
        .globl  test_fn
        .type   test_fn, @function
test_fn:
.LFB0:
        .cfi_startproc
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        movl    $1, %ebx
        .p2align 4,,10
        .p2align 3
.L2:
        movl    %ebx, %edi
        addl    %ebx, %ebx
        call    foo
        jmp     .L2
        .cfi_endproc
.LFE0:
        .size   test_fn, .-test_fn
        .ident  "GCC: (Debian 4.7.2-5) 4.7.2"
        .section        .note.GNU-stack,"",@progbits

So it misoptimized this into an infinite loop, and then also optimized
out the code after the loop.  With -O1 it appears to produce a correct
loop executing the body 31 times.  This bug also happens with 4.7.2-19
from experimental.

(I found this issue while tracking down why a "checking for working
mktime" autoconf check was timing out.)
-- 
Daniel Schepler