Re: [patch, arm] fix bug in long long divide-by-zero handling

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Fri, Jan 23, 2015 at 2:07 AM, Sandra Loosemore
<sandra@codesourcery.com> wrote:
> The ARM run-time ABI says that long long division by zero should return the
> result of calling __aeabi_ldiv0 with an argument that is either zero if the
> numerator is zero, the largest value of the type if the numerator is
> positive, or the smallest value of the type if the numerator is negative.
> We had an internal bug report from Mentor's Nucleus RTOS team that it is
> incorrectly passing the negative magic value for positive numerators between
> 0x80000000LL and 0xffffffffLL.
>
> There are three implementations of this code in libgcc -- ARM, Thumb-2, and
> ARMv6-m -- and they are all structured similarly and all have the same bug.
> The source of the trouble is that the divide-by-zero handling is first
> checking to see if the high word is zero and substituting the low word
> instead if so, before proceeding to use signed conditionals to handle the
> positive/negative cases.  So, it's doing the wrong thing if the high word is
> zero and the high-order bit is set in the low word. The solution I've
> implemented is to test the negative case on the high word first, and then
> bypass the rest of the code, which can simply test zero/nonzero on whichever
> word to distinguish the remaining two cases.
>
> I've regression-tested this on mainline head with arm-none-eabi, and also in
> our local 4.9-based tree with a much larger set of multilibs to get test
> coverage of all three implementations.
>
> This isn't a regression (AFAICT, this has never worked properly), so I don't
> know if it's appropriate to consider this for trunk at this time.  If not,
> is it OK for when trunk re-opens for GCC 6?

While we are at it I'd rather get this fixed in time for 5.0 and all
release branches rather than wait for 6.0.

Therefore ok.

regards
Ramana

>
> -Sandra
>
>
> 2015-01-22  Sandra Loosemore  <sandra@codesourcery.com>
>
>         libgcc/
>         * bpabi.S (test_div_by_zero): Make label names consistent
>         between thumb2 and arm mode cases.  Separate the signed
>         comparison on the high word of the numerator from the
>         unsigned comparison on the low word.
>         * bpabi-v6m.S (test_div_by_zero): Similarly separate signed
>         comparison.
>
>         gcc/testsuite/
>         * gcc.target/arm/divzero.c: New test case.
>