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From: "swarren at nvidia dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/61131] New: [4.8 regression] ARM -Os: incorrect code generation
Date: Fri, 09 May 2014 19:50:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61131

            Bug ID: 61131
           Summary: [4.8 regression] ARM -Os: incorrect code generation
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: swarren at nvidia dot com

Created attachment 32771
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32771&action=edit
Source sample that exhibits the issue when compiled

When running gcc 4.8 for ARM with -Os, the following function:

int count_entries(struct s *p)
{
    int i = 0;

    while (p->fd[i] >= 0 && i < MAX_COUNT) {
        i++;
    }

    return i;
}

... gets compiled to essentially a no-op:

000083f8 <count_entries>:
    83f8:    e5900000     ldr    r0, [r0]
    83fc:    e1e00000     mvn    r0, r0
    8400:    e1a00fa0     lsr    r0, r0, #31
    8404:    e12fff1e     bx    lr

If I don't use -Os, then I get much larger, and working, code.

If I swap the order of the two conditions in the while expression, I get
working code:

    while (i < MAX_COUNT && p->fd[i] >= 0) {

If I use gcc-4.7 insteaad, I get working code.

A complete source sample is attached.

The bug is NOT present in
gcc-linaro-arm-linux-gnueabihf-4.7-2013.04-20130415_linux.tar.xz, which is
apparently GCC 4.7.2+svn197188.

The bug IS present in gcc-linaro-arm-linux-gnueabihf-4.8-2014.04_linux.tar.xz,
which is apparently GCC 4.8.3+svn208968.

I wasn't able to quickly track down any more recent gcc-4.8.x binaries, or
gcc-4.9.x binaries.

The bug IS present in some toolchain I received from a customer in directory
name yocto/gcc-4.8.1-glibc-2.18-hard/.