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Re: [PATCH 2/2] PR libgcc/59714 complex division is surprising on aarch64
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.
-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.