Re: [match.pd] PR83750 - CSE erf/erfc pair

[Richard Biener <rguenther@suse.de>]

On Wed, 20 Oct 2021, Prathamesh Kulkarni wrote:

> On Tue, 19 Oct 2021 at 16:55, Richard Biener <rguenther@suse.de> wrote:
> >
> > On Tue, 19 Oct 2021, Prathamesh Kulkarni wrote:
> >
> > > On Tue, 19 Oct 2021 at 13:02, Richard Biener <richard.guenther@gmail.com> wrote:
> > > >
> > > > On Tue, Oct 19, 2021 at 9:03 AM Prathamesh Kulkarni via Gcc-patches
> > > > <gcc-patches@gcc.gnu.org> wrote:
> > > > >
> > > > > On Mon, 18 Oct 2021 at 17:23, Richard Biener <rguenther@suse.de> wrote:
> > > > > >
> > > > > > On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:
> > > > > >
> > > > > > > On Mon, 18 Oct 2021 at 17:10, Richard Biener <rguenther@suse.de> wrote:
> > > > > > > >
> > > > > > > > On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:
> > > > > > > >
> > > > > > > > > On Mon, 18 Oct 2021 at 16:18, Richard Biener <rguenther@suse.de> wrote:
> > > > > > > > > >
> > > > > > > > > > On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:
> > > > > > > > > >
> > > > > > > > > > > Hi Richard,
> > > > > > > > > > > As suggested in PR, I have attached WIP patch that adds two patterns
> > > > > > > > > > > to match.pd:
> > > > > > > > > > > erfc(x) --> 1 - erf(x) if canonicalize_math_p() and,
> > > > > > > > > > > 1 - erf(x) --> erfc(x) if !canonicalize_math_p().
> > > > > > > > > > >
> > > > > > > > > > > This works to remove call to erfc for the following test:
> > > > > > > > > > > double f(double x)
> > > > > > > > > > > {
> > > > > > > > > > >   double g(double, double);
> > > > > > > > > > >
> > > > > > > > > > >   double t1 = __builtin_erf (x);
> > > > > > > > > > >   double t2 = __builtin_erfc (x);
> > > > > > > > > > >   return g(t1, t2);
> > > > > > > > > > > }
> > > > > > > > > > >
> > > > > > > > > > > with .optimized dump shows:
> > > > > > > > > > >   t1_2 = __builtin_erf (x_1(D));
> > > > > > > > > > >   t2_3 = 1.0e+0 - t1_2;
> > > > > > > > > > >
> > > > > > > > > > > However, for the following test:
> > > > > > > > > > > double f(double x)
> > > > > > > > > > > {
> > > > > > > > > > >   double g(double, double);
> > > > > > > > > > >
> > > > > > > > > > >   double t1 = __builtin_erfc (x);
> > > > > > > > > > >   return t1;
> > > > > > > > > > > }
> > > > > > > > > > >
> > > > > > > > > > > It canonicalizes erfc(x) to 1 - erf(x), but does not transform 1 -
> > > > > > > > > > > erf(x) to erfc(x) again
> > > > > > > > > > > post canonicalization.
> > > > > > > > > > > -fdump-tree-folding shows that 1 - erf(x) --> erfc(x) gets applied,
> > > > > > > > > > > but then it tries to
> > > > > > > > > > > resimplify erfc(x), which fails post canonicalization. So we end up
> > > > > > > > > > > with erfc(x) transformed to
> > > > > > > > > > > 1 - erf(x) in .optimized dump, which I suppose isn't ideal.
> > > > > > > > > > > Could you suggest how to proceed ?
> > > > > > > > > >
> > > > > > > > > > I applied your patch manually and it does the intended
> > > > > > > > > > simplifications so I wonder what I am missing?
> > > > > > > > > Would it be OK to always fold erfc(x) -> 1 - erf(x) even when there's
> > > > > > > > > no erf(x) in the source ?
> > > > > > > >
> > > > > > > > I do think it's reasonable to expect erfc to be available when erf
> > > > > > > > is and vice versa but note both are C99 specified functions (either
> > > > > > > > requires -lm).
> > > > > > > OK, thanks. Would it be OK to commit the patch after bootstrap+test ?
> > > > > >
> > > > > > Yes, but I'm confused because you say the patch doesn't work for you?
> > > > > The patch works for me to CSE erf/erfc pair.
> > > > > However when there's only erfc in the source, it canonicalizes erfc(x)
> > > > > to 1 - erf(x) but later fails to uncanonicalize 1 - erf(x) back to
> > > > > erfc(x)
> > > > > with -O3 -funsafe-math-optimizations.
> > > > >
> > > > > For,
> > > > > t1 = __builtin_erfc(x),
> > > > >
> > > > > .optimized dump shows:
> > > > >   _2 = __builtin_erf (x_1(D));
> > > > >   t1_3 = 1.0e+0 - _2;
> > > > >
> > > > > and for,
> > > > > double t1 = x + __builtin_erfc(x);
> > > > >
> > > > > .optimized dump shows:
> > > > >   _3 = __builtin_erf (x_2(D));
> > > > >   _7 = x_2(D) + 1.0e+0;
> > > > >   t1_4 = _7 - _3;
> > > > >
> > > > > I assume in both cases, we want erfc in the code-gen instead ?
> > > > > I think the reason uncaonicalization fails is because the pattern 1 -
> > > > > erf(x) to erfc(x)
> > > > > gets applied, but then it fails in resimplifying erfc(x), and we end
> > > > > up with 1 - erf(x) in code-gen.
> > > > >
> > > > > From gimple-match.c, it hits the simplification:
> > > > >
> > > > >                                 gimple_seq *lseq = seq;
> > > > >                                 if (__builtin_expect (!dbg_cnt
> > > > > (match), 0)) goto next_after_fail1172;
> > > > >                                 if (__builtin_expect (dump_file &&
> > > > > (dump_flags & TDF_FOLDING), 0)) fprintf (dump_file, "Applying pattern
> > > > > %s:%d, %s:%d\n", "match.pd", 6162, __FILE__, __LINE__);
> > > > >                                 {
> > > > >                                   res_op->set_op (CFN_BUILT_IN_ERFC, type, 1);
> > > > >                                   res_op->ops[0] = captures[0];
> > > > >                                   res_op->resimplify (lseq, valueize);
> > > > >                                   return true;
> > > > >                                 }
> > > > >
> > > > > But res_op->resimplify returns false, and doesn't end up adding to lseq.
> > > >
> > > > There's nothing to add to lseq since there's also nothing to resimplify.
> > > > The only thing that could happen is that the replacement is not done
> > > > because replace_stmt_with_simplification via maybe_push_res_to_seq
> > > > doesn't pass the builtin_decl_implicit test:
> > > >
> > > >           /* Find the function we want to call.  */
> > > >           tree decl = builtin_decl_implicit (as_builtin_fn (fn));
> > > >           if (!decl)
> > > >             return NULL;
> > > >
> > > > btw, it did work for me since the call was present before and gimplification
> > > > should then mark the function eligible for implicit generation.
> > > >
> > > > > As you suggest, should we instead handle this in fre to transform
> > > > > erfc(x) to 1 - erf(x), only when
> > > > > there's a matching erf(x) in the source ?
> > > >
> > > > Note that's strictly less powerful and we'd have to handle erf(x) -> 1 +erfc (x)
> > > > to handle CSE in
> > > >
> > > >   tem = erfc (x);
> > > >   tem2 = erf (x);
> > > >
> > > > So no, I think the canonicalization is fine unless there's a compelling reason
> > > > for having both erfc and erf.
> > > >
> > > > Can you debug why the reverse transform doesn't work for you?
> > > It seems the issue was that erfc wasn't getting marked with const
> > > attribute, and failed the following test in
> > > maybe_push_res_to_seq:
> > >           /* We can't and should not emit calls to non-const functions.  */
> > >           if (!(flags_from_decl_or_type (decl) & ECF_CONST))
> > >             return NULL;
> > >
> > > Passing -fno-math-errno seems to work for the reverse transform:
> > >
> > > double f(double x)
> > > {
> > >   double g(double, double);
> > >
> > >   double t1 = __builtin_erfc (x);
> > >   return t1;
> > > }
> > >
> > > Compiling with -O3 -funsafe-math-optimizations -fno-math-errno:
> > >
> > > vrp2 dump shows:
> > > Folding statement: _2 = __builtin_erf (x_1(D));
> > > Not folded
> > > Folding statement: t1_3 = 1.0e+0 - _2;
> > > Applying pattern match.pd:6162, gimple-match.c:68450
> > > gimple_simplified to t1_3 = __builtin_erfc (x_1(D));
> > > Folded into: t1_3 = __builtin_erfc (x_1(D));
> > >
> > > and .optimized dump shows:
> > > double f (double x)
> > > {
> > >   double t1;
> > >
> > >   <bb 2> [local count: 1073741824]:
> > >   t1_3 = __builtin_erfc (x_1(D)); [tail call]
> > >   return t1_3;
> > > }
> > >
> > > Unfortunately, for the test-case involving erf/erfc pair, the reverse
> > > transform seems to undo the CSE:
> > >
> > > double f(double x)
> > > {
> > >   double g(double, double);
> > >
> > >   double t1 = __builtin_erf (x);
> > >   double t2 = __builtin_erfc (x);
> > >   return g(t1, t2);
> > > }
> > >
> > > gimplification turns erfc to 1 - erf:
> > > Applying pattern match.pd:6158, gimple-match.c:44479
> > > gimple_simplified to D.1988 = __builtin_erf (x);
> > > t2 = 1.0e+0 - D.1988;
> > >
> > >   t1 = __builtin_erf (x);
> > >   D.1988 = __builtin_erf (x);
> > >   t2 = 1.0e+0 - D.1988;
> > >   D.1987 = g (t1, t2);
> > >
> > > fre1 does the CSE:
> > >   t1_2 = __builtin_erf (x_1(D));
> > >   t2_4 = 1.0e+0 - t1_2;
> > >   _7 = g (t1_2, t2_4);
> > >
> > > and forwprop4 again converts 1 - erf(x) to erfc(x), "undoing" the CSE:
> > > Applying pattern match.pd:6162, gimple-match.c:68450
> > > gimple_simplified to t2_3 = __builtin_erfc (x_1(D));
> > >
> > >   t1_2 = __builtin_erf (x_1(D));
> > >   t2_3 = __builtin_erfc (x_1(D));
> > >   _6 = g (t1_2, t2_3);
> > >
> > > and .optimized dump shows:
> > >   t1_2 = __builtin_erf (x_1(D));
> > >   t2_3 = __builtin_erfc (x_1(D));
> > >   _6 = g (t1_2, t2_3); [tail call]
> >
> > You probably want an explicit && single_use () check on the
> > 1 - erf() -> erfc transform.
> single_use worked, thanks!
> As you pointed out, we reassociate x + erfc(x) to (x + 1) - erf(x) and
> don't uncanonicalize back.
> I added another pattern to reassociate (x + 1) - erf(x) to x + (1 -
> erf(x)), after which,
> it gets uncanonicalized back to x + erfc(x) in .optimized dump.

I think that's not the way to "fix" this case, instead if we'd care
we should change reassoc to either associate a -erf() operand and
a 1 next to each other or recognize it there.  But I think that
can be done as followup.

> Does the attached patch look OK after bootstrap+test ?

It's OK with removing the extra (x+1) - erf pattern and the
associated testcase.

Richard.

> Thanks,
> Prathamesh
> >
> > > Thanks,
> > > Prathamesh
> > > >
> > > > Richard.
> > > >
> > > > > Thanks,
> > > > > Prathamesh
> > > > > >
> > > > > > Btw, please add the testcase from the PR and also a testcase that shows
> > > > > > the canonicalization is undone.  Maybe you can also double-check that
> > > > > > we handle x + erfc (x) because I see we associate that as
> > > > > > (x + 1) - erf (x) which is then not recognized back to erfc.
> > > > > >
> > > > > > The less surprising (as to preserve the function called in the source)
> > > > > > variant for the PR would be to teach CSE to lookup erf(x) when
> > > > > > visiting erfc(x) and when found synthesize 1 - erf(x).
> > > > > >
> > > > > > That said, a mathematician should chime in on how important it is
> > > > > > to preserve erfc vs. erf (precision or even speed).
> > > > > >
> > > > > > Thanks,
> > > > > > Richard.
> > > > > >
> > > > > > > Thanks,
> > > > > > > Prathamesh
> > > > > > >
> > > > > > > >
> > > > > > > > Richard.
> > > > > > > >
> > > > > > > > > So for the following test:
> > > > > > > > > double f(double x)
> > > > > > > > > {
> > > > > > > > >   t1 = __builtin_erfc(x)
> > > > > > > > >   return t1;
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > .optimized dump shows:
> > > > > > > > > double f (double x)
> > > > > > > > > {
> > > > > > > > >   double t1;
> > > > > > > > >   double _2;
> > > > > > > > >
> > > > > > > > >   <bb 2> [local count: 1073741824]:
> > > > > > > > >   _2 = __builtin_erf (x_1(D));
> > > > > > > > >   t1_3 = 1.0e+0 - _2;
> > > > > > > > >   return t1_3;
> > > > > > > > > }
> > > > > > > > >
> > > > > > > > > while before patch, it has:
> > > > > > > > >   t1_4 = __builtin_erfc (x_2(D)); [tail call]
> > > > > > > > >   return t1_4;
> > > > > > > > >
> > > > > > > > > Thanks,
> > > > > > > > > Prathamesh
> > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > Richard.
> > > > > > > > > >
> > > > > > > > > > > Thanks,
> > > > > > > > > > > Prathamesh
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > --
> > > > > > > > > > Richard Biener <rguenther@suse.de>
> > > > > > > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> > > > > > > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> > > > > > > > >
> > > > > > > >
> > > > > > > > --
> > > > > > > > Richard Biener <rguenther@suse.de>
> > > > > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> > > > > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> > > > > > >
> > > > > >
> > > > > > --
> > > > > > Richard Biener <rguenther@suse.de>
> > > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> > > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> > >
> >
> > --
> > Richard Biener <rguenther@suse.de>
> > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)