Re: [ARM] implement division using vrecpe/vrecps with -funsafe-math-optimizations

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 7 June 2016 at 14:07, Ramana Radhakrishnan <ramana.gcc@googlemail.com> wrote:
>>> Please find the updated patch attached.
>>> It passes testsuite for arm-none-linux-gnueabi, arm-none-linux-gnueabihf and
>>> arm-none-eabi.
>>> However the test-case added in the patch (neon-vect-div-1.c) fails to
>>> get vectorized at -O2
>>> for armeb-none-linux-gnueabihf.
>>> Charles suggested me to try with -O3, which worked.
>>> It appears the test-case fails to get vectorized with
>>> -fvect-cost-model=cheap (which is default enabled at -O2)
>>> and passes for -fno-vect-cost-model / -fvect-cost-model=dynamic
>>>
>>> I can't figure out why it fails -fvect-cost-model=cheap.
>>> From the vect dump (attached):
>>> neon-vect-div-1.c:12:3: note: Setting misalignment to -1.
>>> neon-vect-div-1.c:12:3: note: not vectorized: unsupported unaligned load.*_9
>> Hi,
>> I think I have some idea why the test-case fails attached with patch
>> fail to get vectorized on armeb with -O2.
>>
>> Issue with big endian vectorizer:
>> The patch does not cause regressions on big endian vectorizer but
>> fails to vectorize the test-cases attached with the patch, while they
>> get vectorized on
>> litttle-endian.
>> Fails with armeb with the following message in dump:
>> note: not vectorized: unsupported unaligned load.*_9
>>
>> The behavior of big and little endian vectorizer seems to be different
>> in arm_builtin_support_vector_misalignment() which overrides the hook
>> targetm.vectorize.support_vector_misalignment().
>>
>> targetm.vectorize.support_vector_misalignment is called by
>> vect_supportable_dr_alignment () which in turn is called
>> by verify_data_refs_alignment ().
>>
>> Execution upto following condition is common between arm and armeb
>> in vect_supportable_dr_alignment():
>>
>> if ((TYPE_USER_ALIGN (type) && !is_packed)
>>       || targetm.vectorize.support_vector_misalignment (mode, type,
>>                                             DR_MISALIGNMENT (dr), is_packed))
>>         /* Can't software pipeline the loads, but can at least do them.  */
>>         return dr_unaligned_supported;
>>
>> For little endian case:
>> arm_builtin_support_vector_misalignment() is called with
>> V2SF mode and misalignment == -1, and the following condition
>> becomes true:
>> /* If the misalignment is unknown, we should be able to handle the access
>>          so long as it is not to a member of a packed data structure.  */
>>   if (misalignment == -1)
>>     return true;
>>
>> Since the hook returned true we enter the condition above in
>> vect_supportable_dr_alignment() and return dr_unaligned_supported;
>>
>> For big-endian:
>> arm_builtin_support_vector_misalignment() is called with V2SF mode.
>> The following condition that gates the entire function body fails:
>>  if (TARGET_NEON && !BYTES_BIG_ENDIAN && unaligned_access)
>> and the default hook gets called with V2SF mode and the default hook
>> returns false because
>> movmisalign_optab does not exist for V2SF mode.
>>
>> So the condition above in vect_supportable_dr_alignment() fails
>> and we come here:
>>  /* Unsupported.  */
>> return dr_unaligned_unsupported;
>>
>> And hence we get the unaligned load not supported message in the dump
>> for armeb in verify_data_ref_alignment ():
>>
>> static bool
>> verify_data_ref_alignment (data_reference_p dr)
>> {
>>   enum dr_alignment_support supportable_dr_alignment
>>     = vect_supportable_dr_alignment (dr, false);
>>   if (!supportable_dr_alignment)
>>     {
>>       if (dump_enabled_p ())
>>         {
>>           if (DR_IS_READ (dr))
>>             dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
>>                              "not vectorized: unsupported unaligned load.");
>>
>> With -O3, the test-cases vectorize for armeb, because loop peeling for alignment
>> is turned on.
>> The above behavior is also reproducible with test-case which is
>> irrelevant to the patch.
>> for instance, we get the same unsupported unaligned load for following
>> test-case (replaced / with +)
>>
>> void
>> foo (int len, float * __restrict p, float *__restrict x)
>> {
>>   len = len & ~31;
>>   for (int i = 0; i < len; i++)
>>     p[i] = p[i] + x[i];
>> }
>> Is the patch OK to commit after bootstrap+test ?
>
>
> Thanks for the analysis - all the test needs is an additional marker
> to skip it on armeb (is there a helper for misaligned loads from the
> vectorizer ? ) - Ah probably vect_hw_misalign is sufficient for your
> usecase and you want to appropriately fix it for little endian arm
> with neon support enabled.
Hi,
I added dg-require-effective-target vect_hw_misalign to the tests in the patch,
and modified vect_hw_misalign to return true for little-endian arm configs
with neon support enabled. The patch makes the tests unsupported for
armeb.
Does it look correct ?

Unfortunately the change to vect_hw_misalign breaks gcc.dg/vect/vect-align-1.c,
which were passing before:
XPASS: gcc.dg/vect/vect-align-1.c scan-tree-dump-times vect "Alignment
of access forced using versioning" 1
FAIL: gcc.dg/vect/vect-align-1.c scan-tree-dump-times vect
"Vectorizing an unaligned access" 1

I am not sure how to fix this and would be grateful for suggestions.

Thanks,
Prathamesh
>
> From the patch.
>
>>>+   && flag_unsafe_math_optimizations && flag_reciprocal_math"
>
> Why do we need flag_unsafe_math_optimizations && flag_reciprocal_math
> ? flag_unsafe_math_optimizations should be sufficient since it enables
> flag_reciprocal_math - the reason for flag_unsafe_math_optimizations
> is to prevent loss of precision and the fact that on neon denormalized
> numbers are flushed to zero.
>
> Ok with that change and a quick test with vect_hw_misalign added to
> your testcase.
>
> Sorry about the delay in reviewing.
>
> Ramana
>
>
>>
>> Thanks,
>> Prathamesh
>>>
>>> Thanks,
>>> Prathamesh
>>>>
>>>> Thanks,
>>>> Ramana
>>>>
>>>>>
>>>>> Thanks,
>>>>> Prathamesh
>>>>>>
>>>>>>
>>>>>> moving on to the patches.
>>>>>>
>>>>>>> diff --git a/gcc/config/arm/neon.md b/gcc/config/arm/neon.md
>>>>>>> index 654d9d5..28c2e2a 100644
>>>>>>> --- a/gcc/config/arm/neon.md
>>>>>>> +++ b/gcc/config/arm/neon.md
>>>>>>> @@ -548,6 +548,32 @@
>>>>>>>                      (const_string "neon_mul_<V_elem_ch><q>")))]
>>>>>>>  )
>>>>>>>
>>>>>>
>>>>>> Please add a comment here.
>>>>>>
>>>>>>> +(define_expand "div<mode>3"
>>>>>>> +  [(set (match_operand:VCVTF 0 "s_register_operand" "=w")
>>>>>>> +        (div:VCVTF (match_operand:VCVTF 1 "s_register_operand" "w")
>>>>>>> +               (match_operand:VCVTF 2 "s_register_operand" "w")))]
>>>>>>
>>>>>> I want to double check that this doesn't collide with Alan's patches for FP16 especially if he reuses the VCVTF iterator for all the vcvt f16 cases.
>>>>>>
>>>>>>> +  "TARGET_NEON && flag_unsafe_math_optimizations && flag_reciprocal_math"
>>>>>>> +  {
>>>>>>> +    rtx rec = gen_reg_rtx (<MODE>mode);
>>>>>>> +    rtx vrecps_temp = gen_reg_rtx (<MODE>mode);
>>>>>>> +
>>>>>>> +    /* Reciprocal estimate */
>>>>>>> +    emit_insn (gen_neon_vrecpe<mode> (rec, operands[2]));
>>>>>>> +
>>>>>>> +    /* Perform 2 iterations of Newton-Raphson method for better accuracy */
>>>>>>> +    for (int i = 0; i < 2; i++)
>>>>>>> +      {
>>>>>>> +     emit_insn (gen_neon_vrecps<mode> (vrecps_temp, rec, operands[2]));
>>>>>>> +     emit_insn (gen_mul<mode>3 (rec, rec, vrecps_temp));
>>>>>>> +      }
>>>>>>> +
>>>>>>> +    /* We now have reciprocal in rec, perform operands[0] = operands[1] * rec */
>>>>>>> +    emit_insn (gen_mul<mode>3 (operands[0], operands[1], rec));
>>>>>>> +    DONE;
>>>>>>> +  }
>>>>>>> +)
>>>>>>> +
>>>>>>> +
>>>>>>>  (define_insn "mul<mode>3add<mode>_neon"
>>>>>>>    [(set (match_operand:VDQW 0 "s_register_operand" "=w")
>>>>>>>          (plus:VDQW (mult:VDQW (match_operand:VDQW 2 "s_register_operand" "w")
>>>>>>> diff --git a/gcc/testsuite/gcc.target/arm/vect-div-1.c b/gcc/testsuite/gcc.target/arm/vect-div-1.c
>>>>>>> new file mode 100644
>>>>>>> index 0000000..e562ef3
>>>>>>> --- /dev/null
>>>>>>> +++ b/gcc/testsuite/gcc.target/arm/vect-div-1.c
>>>>>>> @@ -0,0 +1,14 @@
>>>>>>> +/* { dg-do compile } */
>>>>>>> +/* { dg-require-effective-target arm_v8_neon_ok } */
>>>>>>> +/* { dg-options "-O2 -funsafe-math-optimizations -ftree-vectorize -fdump-tree-vect-all" } */
>>>>>>> +/* { dg-add-options arm_v8_neon } */
>>>>>>
>>>>>> No this is wrong.
>>>>>>
>>>>>> What is armv8 specific about this test ? This is just like another test that is for Neon. vrecpe / vrecps are not instructions that were introduced in the v8 version of the architecture. They've existed in the base Neon instruction set. The code generation above in the patterns will be enabled when TARGET_NEON is true which can happen when -mfpu=neon -mfloat-abi={softfp/hard} is true.
>>>>>>
>>>>>>> +
>>>>>>> +void
>>>>>>> +foo (int len, float * __restrict p, float *__restrict x)
>>>>>>> +{
>>>>>>> +  len = len & ~31;
>>>>>>> +  for (int i = 0; i < len; i++)
>>>>>>> +    p[i] = p[i] / x[i];
>>>>>>> +}
>>>>>>> +
>>>>>>> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect" } } */
>>>>>>> diff --git a/gcc/testsuite/gcc.target/arm/vect-div-2.c b/gcc/testsuite/gcc.target/arm/vect-div-2.c
>>>>>>> new file mode 100644
>>>>>>> index 0000000..8e15d0a
>>>>>>> --- /dev/null
>>>>>>> +++ b/gcc/testsuite/gcc.target/arm/vect-div-2.c
>>>>>>> @@ -0,0 +1,14 @@
>>>>>>> +/* { dg-do compile } */
>>>>>>> +/* { dg-require-effective-target arm_v8_neon_ok } */
>>>>>>
>>>>>> And likewise.
>>>>>>
>>>>>>> +/* { dg-options "-O2 -funsafe-math-optimizations -fno-reciprocal-math -ftree-vectorize -fdump-tree-vect-all" } */
>>>>>>> +/* { dg-add-options arm_v8_neon } */
>>>>>>> +
>>>>>>> +void
>>>>>>> +foo (int len, float * __restrict p, float *__restrict x)
>>>>>>> +{
>>>>>>> +  len = len & ~31;
>>>>>>> +  for (int i = 0; i < len; i++)
>>>>>>> +    p[i] = p[i] / x[i];
>>>>>>> +}
>>>>>>> +
>>>>>>> +/* { dg-final { scan-tree-dump-times "vectorized 0 loops" 1 "vect" } } */
>>>>>>
>>>>>>
>>>>>> regards
>>>>>> Ramana

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