Hi Victor,
> -----Original Message-----
> From: Victor Do Nascimento <victor.donascime...@arm.com>
> Sent: Wednesday, October 2, 2024 5:26 PM
> To: gcc-patches@gcc.gnu.org
> Cc: Tamar Christina <tamar.christ...@arm.com>; richard.guent...@gmail.com;
> Victor Do Nascimento <victor.donascime...@arm.com>
> Subject: [PATCH] middle-end: reorder masking priority of math functions
>
> Given the categorization of math built-in functions as `ECF_CONST',
> when if-converting their uses, their calls are not masked and are thus
> called with an all-true predicate.
>
> This, however, is not appropriate where built-ins have library
> equivalents, wherein they may exhibit highly architecture-specific
> behaviors. For example, vectorized implementations may delegate the
> computation of values outside a certain acceptable numerical range to
> special (non-vectorized) routines which considerably slow down
> computation.
>
> As numerical simulation programs often do bounds check on input values
> prior to math calls, conditionally assigning default output values for
> out-of-bounds input and skipping the math call altogether, these
> fallback implementations should seldom be called in the execution of
> vectorized code. If, however, we don't apply any masking to these
> math functions, we end up effectively executing both if and else
> branches for these values, leading to considerable performance
> degradation on scientific workloads.
>
> We therefore invert the order of handling of math function calls in
> `if_convertible_stmt_p' to prioritize the handling of their
> library-provided implementations over the equivalent internal function.
I think this makes sense to me from a technical standpoint and from an SVE
one. Though I think the original order may have been there because of the
assumption that on some uarches unpredicated implementations are faster than
predicated ones.
So there may be some concerns about this order being slower for some.
I'll leave it up to Richi since e.g. I don't know the perf characteristics of
the
x86 variants here, but if there is a concern you could use the
conditional_operation_is_expensive target hook to decide on the preferred order.
But other than that the change itself looks good to be but you still need
approval.
Cheers,
Tamar
>
> Regression tested on aarch64-none-linux-gnu & x86_64-linux-gnu w/ no
> new regressions.
>
> gcc/ChangeLog:
>
> * tree-if-conv.cc (if_convertible_stmt_p): Check for explicit
> function declaration before IFN fallback.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/vect/vect-fncall-mask-math.c: New.
> ---
> .../gcc.dg/vect/vect-fncall-mask-math.c | 33 +++++++++++++++++++
> gcc/tree-if-conv.cc | 18 +++++-----
> 2 files changed, 42 insertions(+), 9 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/vect/vect-fncall-mask-math.c
>
> diff --git a/gcc/testsuite/gcc.dg/vect/vect-fncall-mask-math.c
> b/gcc/testsuite/gcc.dg/vect/vect-fncall-mask-math.c
> new file mode 100644
> index 00000000000..15e22da2807
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/vect-fncall-mask-math.c
> @@ -0,0 +1,33 @@
> +/* Test the correct application of masking to autovectorized math function
> calls.
> + Test is currently set to xfail pending the release of the relevant lmvec
> + support. */
> +/* { dg-do compile { target { aarch64*-*-* } } } */
> +/* { dg-additional-options "-march=armv8.2-a+sve -fdump-tree-ifcvt-raw
> -Ofast"
> { target { aarch64*-*-* } } } */
> +
> +#include <math.h>
> +
> +const int N = 20;
> +const float lim = 101.0;
> +const float cst = -1.0;
> +float tot = 0.0;
> +
> +float b[20];
> +float a[20] = { [0 ... 9] = 1.7014118e39, /* If branch. */
> + [10 ... 19] = 100.0 }; /* Else branch. */
> +
> +int main (void)
> +{
> + #pragma omp simd
> + for (int i = 0; i < N; i += 1)
> + {
> + if (a[i] > lim)
> + b[i] = cst;
> + else
> + b[i] = expf (a[i]);
> + tot += b[i];
> + }
> + return (0);
> +}
> +
> +/* { dg-final { scan-tree-dump-not { gimple_call <expf, _2, _1>} ifcvt {
> xfail {
> aarch64*-*-* } } } } */
> +/* { dg-final { scan-tree-dump { gimple_call <.MASK_CALL, _2, expf, _1,
> _30>} ifcvt
> { xfail { aarch64*-*-* } } } } */
> diff --git a/gcc/tree-if-conv.cc b/gcc/tree-if-conv.cc
> index 3b04d1e8d34..90c754a4814 100644
> --- a/gcc/tree-if-conv.cc
> +++ b/gcc/tree-if-conv.cc
> @@ -1133,15 +1133,6 @@ if_convertible_stmt_p (gimple *stmt,
> vec<data_reference_p> refs)
>
> case GIMPLE_CALL:
> {
> - /* There are some IFN_s that are used to replace builtins but have the
> - same semantics. Even if MASK_CALL cannot handle them vectorable_call
> - will insert the proper selection, so do not block conversion. */
> - int flags = gimple_call_flags (stmt);
> - if ((flags & ECF_CONST)
> - && !(flags & ECF_LOOPING_CONST_OR_PURE)
> - && gimple_call_combined_fn (stmt) != CFN_LAST)
> - return true;
> -
> tree fndecl = gimple_call_fndecl (stmt);
> if (fndecl)
> {
> @@ -1160,6 +1151,15 @@ if_convertible_stmt_p (gimple *stmt,
> vec<data_reference_p> refs)
> }
> }
>
> + /* There are some IFN_s that are used to replace builtins but have the
> + same semantics. Even if MASK_CALL cannot handle them vectorable_call
> + will insert the proper selection, so do not block conversion. */
> + int flags = gimple_call_flags (stmt);
> + if ((flags & ECF_CONST)
> + && !(flags & ECF_LOOPING_CONST_OR_PURE)
> + && gimple_call_combined_fn (stmt) != CFN_LAST)
> + return true;
> +
> return false;
> }
>
> --
> 2.34.1
