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Re: [PATCH 2/2] PR libgcc/59714 complex division is surprising on aarch64
On Fri, Oct 13, 2017 at 10:25 PM, Richard Earnshaw
<Richard.Earnshaw@foss.arm.com> wrote:
> On 13/10/17 18:28, vladimir.mezentsev@oracle.com wrote:
>> On 10/12/2017 03:40 AM, Richard Earnshaw wrote:
>>> On 12/10/17 06:21, vladimir.mezentsev@oracle.com wrote:
>>>> From: Vladimir Mezentsev <vladimir.mezentsev@oracle.com>
>>>>
>>>> FMA (floating-point multiply-add) instructions are supported on aarch64.
>>>> These instructions can produce different result if two operations executed separately.
>>>> -ffp-contract=off doesn't allow the FMA instructions.
>>>>
>>>> Tested on aarch64-linux-gnu.
>>>> No regression. Two failed tests now passed.
>>>>
>>>> ChangeLog:
>>>> 2017-10-11 Vladimir Mezentsev <vladimir.mezentsev@oracle.com>
>>>>
>>>> PR libgcc/59714
>>>> * libgcc/config/aarch64/t-aarch64 (HOST_LIBGCC2_CFLAGS): Add -ffp-contract=off
>>>> ---
>>>> libgcc/config/aarch64/t-aarch64 | 1 +
>>>> 1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/libgcc/config/aarch64/t-aarch64 b/libgcc/config/aarch64/t-aarch64
>>>> index 3af933c..e33bef0 100644
>>>> --- a/libgcc/config/aarch64/t-aarch64
>>>> +++ b/libgcc/config/aarch64/t-aarch64
>>>> @@ -18,4 +18,5 @@
>>>> # along with GCC; see the file COPYING3. If not see
>>>> # <http://www.gnu.org/licenses/>.
>>>>
>>>> +HOST_LIBGCC2_CFLAGS += -ffp-contract=off
>>>> LIB2ADD += $(srcdir)/config/aarch64/sync-cache.c
>>>>
>>> Why would we want to do this on AArch64 only? If it's right for us,
>>> then surely it would be right for everyone and the option should be
>>> applied at the top level.
>>
>> It is a machine dependent option.
>> We don't need this option, for example, on sparc or intel machines.
>>
>
> No, it's a target-independent option (it's in the -f... option space).
> It might be a no-op on some machines, but either it should be on
> globally, or it should be off globally. If it's a no-op then it won't
> matter whether it's on or off.
Indeed and we do have this on AArch32 with vfpv4 so that's 2
architectures already affected. It maybe that the default builds on
x86 don't tickle fma but we may see the same behavior elsewhere
depending on (fp) architecture defaults.
I haven't tried this testcase yet on AArch32 but I suspect we're
likely to hit the same issue there too ...
regards
Ramana
>
> R.
>
>> -Vladimir
>>>
>>> Hint: I'm not convinced on the evidence here that it is right across the
>>> whole of libgcc. Before moving forward on this particular PR I think we
>>> need to understand what behaviour we do want out of the compiler.
>>>
>>> R.
>>
>