Re: [RFC PATCH v2] target/loongarch: Fix incorrect rounding in fnm{add,sub} under certain modes

[Richard Henderson <richard.henderson@linaro.org>]

On 5/7/25 08:25, WANG Rui wrote:
> Hi Richard,
> 
> On Wed, May 7, 2025 at 11:04 PM Richard Henderson
> <richard.henderson@linaro.org> wrote:
>>
>> On 5/7/25 02:14, WANG Rui wrote:
>>> This patch fixes incorrect results for `[xv]fnm{add,sub}.{s,d}`
>>> instructions when rounding toward {zero, positive, negative}.
>>>
>>> According to the LoongArch ISA specification, the result of an
>>> instruction like `FNMSUB.D` is computed as:
>>>
>>>     FR[fd] = -FP64_fusedMultiplyAdd(FR[fj], FR[fk], -FR[fa])
>>>
>>> Here, `FP64_fusedMultiplyAdd()` performs a fused multiply-add operation
>>> compliant with IEEE 754-2008. The negation is applied to the fully
>>> rounded result of the fused operation - not to any intermediate value.
>>> This behavior is specifiec to LoongArch and differs from other arches,
>>> which is why the existing `float_muladd_negate_result` flag does not
>>> model it correctly.
>>
>> Loongarch does not differ from other arches; we got it wrong for everyone.
>> There's no need for a new flag.
> 
> I'm not familiar with PowerPC and s390x. The official PowerPC docs[^1]
> doesn't clearly specify the order of negation and rounding operations.

Certainly it does.

   This instruction produces the same result as would be
   obtained by using the Floating Multiply-Subtract
   instruction and then negating the result, with the
   following exceptions.

Round is done the same as FMSUB, *and then negating*.
So the negation must happen second.


> I also don't have access to the hardware to run experiments. However,
> I did find the Linux kernel's emulation code[^2][^3] for the PowerPC
> `fnmsub` instruction, which seems to suggest that negation occurs
> before rounding - though it's possible that interpretation is
> incorrect.

That code gets this case wrong, just like we did.


> 
> [^1]: https://www.ibm.com/docs/en/aix/7.2?topic=set-fnmsub-fnms-floating-negative-multiply-subtract-instruction
> [^2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/math-emu/fnmsub.c?h=v6.15-rc5#n49
> [^3}: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/math-emu/fnmsub.c?h=v6.15-rc5#n55
> 
> Regards,
> -hev
> 
>>
>>
>> r~
>