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Re: Don't use integer "FMA" for shifts


Richard Biener <richard.guenther@gmail.com> writes:
> On Tue, Jul 30, 2019 at 12:04 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> tree-ssa-math-opts supports FMA optabs for integers as well as
>> floating-point types, even though there's no distinction between
>> fused and unfused there.  It turns out to be pretty handy for the
>> IFN_COND_* handling though, so I don't want to remove it, however
>> weird it might seem.
>>
>> Instead this patch makes sure that we don't apply it to integer
>> multiplications that are actually shifts (but that are represented
>> in gimple as multiplications because that's the canonical form).
>>
>> This is a preemptive strike.  The test doesn't fail for SVE as-is,
>> but would after a later patch.
>>
>> Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu.
>> OK to install?
>>
>> Richard
>>
>>
>> 2019-07-30  Richard Sandiford  <richard.sandiford@arm.com>
>>
>> gcc/
>>         * tree-ssa-math-opts.c (convert_mult_to_fma): Reject integer
>>         multiplications by a power of 2 or a negative power of 2.
>>
>> gcc/testsuite/
>>         * gcc.dg/vect/vect-cond-arith-8.c: New test.
>>
>> Index: gcc/tree-ssa-math-opts.c
>> ===================================================================
>> --- gcc/tree-ssa-math-opts.c    2019-07-30 10:51:51.827405171 +0100
>> +++ gcc/tree-ssa-math-opts.c    2019-07-30 10:52:27.327139149 +0100
>> @@ -3074,10 +3074,20 @@ convert_mult_to_fma (gimple *mul_stmt, t
>>        && flag_fp_contract_mode == FP_CONTRACT_OFF)
>>      return false;
>>
>> -  /* We don't want to do bitfield reduction ops.  */
>> -  if (INTEGRAL_TYPE_P (type)
>> -      && (!type_has_mode_precision_p (type) || TYPE_OVERFLOW_TRAPS (type)))
>> -    return false;
>> +  if (ANY_INTEGRAL_TYPE_P (type))
>> +    {
>> +      /* We don't want to do bitfield reduction ops.  */
>> +      tree itype = INTEGRAL_TYPE_P (type) ? type : TREE_TYPE (type);
>
> you can use element_type () for this.  But I think it's easier to
> leave the existing
> test in-place since vector or complex types cannot have non-mode precision
> components.

Ah, yeah.  Guess I was over-generalising here :-)

>> +      if (!type_has_mode_precision_p (itype) || TYPE_OVERFLOW_TRAPS (itype))
>> +       return false;
>> +
>> +      /* Don't use FMA for multiplications that are actually shifts.  */
>
> So I question this - if the FMA can do the shift "for free" and it eventually is
> same cost/latency as an add why do we want to not allow this (for all targets)?
> Esp. for vector types I would guess a add plus a shift may be more costly
> (not all ISAs implement shifts anyhow).

The shift doesn't really come for free.  By using FMA we're converting
the shift by a constant into a general multiplication.

> Can this be fixed up in the target instead?  (by a splitter or appropriate
> expander?)

OK, I'll try to do it that way instead.

Thanks,
Richard

>
>> +      tree val = VECTOR_TYPE_P (type) ? uniform_vector_p (op2) : op2;
>> +      if (val
>> +         && TREE_CODE (val) == INTEGER_CST
>> +         && wi::popcount (wi::abs (wi::to_wide (val))) == 1)
>> +       return false;
>> +    }
>>
>>    /* If the target doesn't support it, don't generate it.  We assume that
>>       if fma isn't available then fms, fnma or fnms are not either.  */
>> Index: gcc/testsuite/gcc.dg/vect/vect-cond-arith-8.c
>> ===================================================================
>> --- /dev/null   2019-07-30 08:53:31.317691683 +0100
>> +++ gcc/testsuite/gcc.dg/vect/vect-cond-arith-8.c       2019-07-30 10:52:27.327139149 +0100
>> @@ -0,0 +1,57 @@
>> +/* { dg-require-effective-target scalar_all_fma } */
>> +/* { dg-additional-options "-fdump-tree-optimized -ffp-contract=fast" } */
>> +
>> +#include "tree-vect.h"
>> +
>> +#define N (VECTOR_BITS * 11 / 64 + 3)
>> +
>> +#define DEF(INV)                                       \
>> +  void __attribute__ ((noipa))                         \
>> +  f_##INV (int *restrict a, int *restrict b,           \
>> +          int *restrict c)                             \
>> +  {                                                    \
>> +    for (int i = 0; i < N; ++i)                                \
>> +      {                                                        \
>> +       int mb = (INV & 1 ? -b[i] : b[i]);              \
>> +       int mc = (INV & 2 ? -c[i] : c[i]);              \
>> +       a[i] = b[i] < 10 ? mb * 8 + mc : 10;            \
>> +      }                                                        \
>> +  }
>> +
>> +#define TEST(INV)                                      \
>> +  {                                                    \
>> +    f_##INV (a, b, c);                                 \
>> +    for (int i = 0; i < N; ++i)                                \
>> +      {                                                        \
>> +       int mb = (INV & 1 ? -b[i] : b[i]);              \
>> +       int mc = (INV & 2 ? -c[i] : c[i]);              \
>> +       int truev = mb * 8 + mc;                        \
>> +       if (a[i] != (i % 17 < 10 ? truev : 10))         \
>> +         __builtin_abort ();                           \
>> +       asm volatile ("" ::: "memory");                 \
>> +      }                                                        \
>> +  }
>> +
>> +#define FOR_EACH_INV(T) \
>> +  T (0) T (1) T (2) T (3)
>> +
>> +FOR_EACH_INV (DEF)
>> +
>> +int
>> +main (void)
>> +{
>> +  int a[N], b[N], c[N];
>> +  for (int i = 0; i < N; ++i)
>> +    {
>> +      b[i] = i % 17;
>> +      c[i] = i % 13 + 14;
>> +      asm volatile ("" ::: "memory");
>> +    }
>> +  FOR_EACH_INV (TEST)
>> +  return 0;
>> +}
>> +
>> +/* { dg-final { scan-tree-dump-not { = \.COND_FMA } "optimized" } } */
>> +/* { dg-final { scan-tree-dump-not { = \.COND_FMS } "optimized" } } */
>> +/* { dg-final { scan-tree-dump-not { = \.COND_FNMA } "optimized" } } */
>> +/* { dg-final { scan-tree-dump-not { = \.COND_FNMS } "optimized" } } */


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