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Bug#698950: marked as done (gcc-4.7: Miscompilation of simple loop depending on integer overflow for termination)

Your message dated Fri, 25 Jan 2013 09:31:44 -0800
with message-id <CADf0C44QrD0UWx6LxdQzexjEmndj4m54TAhWAeZXUdM6r=vO_Q@mail.gmail.com>
and subject line Isn't a bug to "miscompile" invalid code
has caused the Debian Bug report #698950,
regarding gcc-4.7: Miscompilation of simple loop depending on integer overflow for termination
to be marked as done.

This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.

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-- 
698950: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=698950
Debian Bug Tracking System
Contact owner@bugs.debian.org with problems
--- Begin Message --- 
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

--- End Message ---
--- Begin Message --- 
Sorry for the noise, it was just pointed out to me that the test
program depends on undefined behavior and is a known property of gcc
that can be worked around by passing -fwrapv.  So packages with the
failing mktime test should upgrade their autoconf version to get a
proper implementation of AC_FUNC_MKTIME.
-- 
Daniel Schepler

--- End Message ---
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