On Thu, 9 Mar 2023, Tamar Christina wrote: > Hi All, > > The testcase > > typedef unsigned int vec __attribute__((vector_size(32))); > vec > f3 (vec a, vec b, vec c) > { > vec d = a * b; > return d + ((c + d) >> 1); > } > > shows a case where we don't want to form an FMA due to the MUL not being > single > use. In this case to form an FMA we have to redo the MUL as well as we no > longer have it to share. > > As such making an FMA here would be a de-optimization. > > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues. > > Ok for master?
OK. Thanks, Richard. > Thanks, > Tamar > > gcc/ChangeLog: > > PR target/108583 > * tree-ssa-math-opts.cc (convert_mult_to_fma): Inhibit FMA in case not > single use. > > gcc/testsuite/ChangeLog: > > PR target/108583 > * gcc.dg/mla_1.c: New test. > > Co-Authored-By: Richard Sandiford <richard.sandif...@arm.com> > > --- inline copy of patch -- > diff --git a/gcc/testsuite/gcc.dg/mla_1.c b/gcc/testsuite/gcc.dg/mla_1.c > new file mode 100644 > index > 0000000000000000000000000000000000000000..a92ecf248116d89b1bc4207a907ea5ed95728a28 > --- /dev/null > +++ b/gcc/testsuite/gcc.dg/mla_1.c > @@ -0,0 +1,40 @@ > +/* { dg-do compile } */ > +/* { dg-require-effective-target vect_int } */ > +/* { dg-options "-O2 -msve-vector-bits=256 -march=armv8.2-a+sve > -fdump-tree-optimized" } */ > + > +unsigned int > +f1 (unsigned int a, unsigned int b, unsigned int c) { > + unsigned int d = a * b; > + return d + ((c + d) >> 1); > +} > + > +unsigned int > +g1 (unsigned int a, unsigned int b, unsigned int c) { > + return a * b + c; > +} > + > +__Uint32x4_t > +f2 (__Uint32x4_t a, __Uint32x4_t b, __Uint32x4_t c) { > + __Uint32x4_t d = a * b; > + return d + ((c + d) >> 1); > +} > + > +__Uint32x4_t > +g2 (__Uint32x4_t a, __Uint32x4_t b, __Uint32x4_t c) { > + return a * b + c; > +} > + > +typedef unsigned int vec __attribute__((vector_size(32))); vec > +f3 (vec a, vec b, vec c) > +{ > + vec d = a * b; > + return d + ((c + d) >> 1); > +} > + > +vec > +g3 (vec a, vec b, vec c) > +{ > + return a * b + c; > +} > + > +/* { dg-final { scan-tree-dump-times {\.FMA } 1 "optimized" { target > aarch64*-*-* } } } */ > diff --git a/gcc/tree-ssa-math-opts.cc b/gcc/tree-ssa-math-opts.cc > index > 5ab5b944a573ad24ce8427aff24fc5215bf05dac..26ed91d58fa4709a67c903ad446d267a3113c172 > 100644 > --- a/gcc/tree-ssa-math-opts.cc > +++ b/gcc/tree-ssa-math-opts.cc > @@ -3346,6 +3346,20 @@ convert_mult_to_fma (gimple *mul_stmt, tree op1, tree > op2, > param_avoid_fma_max_bits)); > bool defer = check_defer; > bool seen_negate_p = false; > + > + /* There is no numerical difference between fused and unfused integer FMAs, > + and the assumption below that FMA is as cheap as addition is unlikely > + to be true, especially if the multiplication occurs multiple times on > + the same chain. E.g., for something like: > + > + (((a * b) + c) >> 1) + (a * b) > + > + we do not want to duplicate the a * b into two additions, not least > + because the result is not a natural FMA chain. */ > + if (ANY_INTEGRAL_TYPE_P (type) > + && !has_single_use (mul_result)) > + return false; > + > /* Make sure that the multiplication statement becomes dead after > the transformation, thus that all uses are transformed to FMAs. > This means we assume that an FMA operation has the same cost > > > > > -- Richard Biener <rguent...@suse.de> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman; HRB 36809 (AG Nuernberg)