On Tue, 19 Oct 2021 at 16:55, Richard Biener <rguent...@suse.de> wrote:
>
> On Tue, 19 Oct 2021, Prathamesh Kulkarni wrote:
>
> > On Tue, 19 Oct 2021 at 13:02, Richard Biener <richard.guent...@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 <rguent...@suse.de> wrote:
> > > > >
> > > > > On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:
> > > > >
> > > > > > On Mon, 18 Oct 2021 at 17:10, Richard Biener <rguent...@suse.de>
> > > > > > wrote:
> > > > > > >
> > > > > > > On Mon, 18 Oct 2021, Prathamesh Kulkarni wrote:
> > > > > > >
> > > > > > > > On Mon, 18 Oct 2021 at 16:18, Richard Biener
> > > > > > > > <rguent...@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.
Does the attached patch look OK after bootstrap+test ?
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 <rguent...@suse.de>
> > > > > > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409
> > > > > > > > > Nuernberg,
> > > > > > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> > > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > Richard Biener <rguent...@suse.de>
> > > > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409
> > > > > > > Nuernberg,
> > > > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> > > > > >
> > > > >
> > > > > --
> > > > > Richard Biener <rguent...@suse.de>
> > > > > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409
> > > > > Nuernberg,
> > > > > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> >
>
> --
> Richard Biener <rguent...@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
diff --git a/gcc/match.pd b/gcc/match.pd
index a9791ceb74a..827670fb59b 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -6147,6 +6147,24 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(floors tree_expr_nonnegative_p@0)
(truncs @0))))
+/* Simplify,
+ erfc(x) -> 1 - erf(x) if canonicalize_math_p().
+ 1 - erf(x) -> erfc(x) if !canonicalize_math_p(). */
+
+(if (flag_unsafe_math_optimizations)
+ (simplify
+ (ERFC @0)
+ (if (canonicalize_math_p ())
+ (minus { build_one_cst (TREE_TYPE (@0)); } (ERF @0))))
+ (simplify
+ (minus real_onep (ERF@1 @0))
+ (if (!canonicalize_math_p () && single_use (@1))
+ (ERFC @0))))
+ /* (x + 1) - erf(x) -> x + (1 - erf(x)). */
+ (simplify
+ (minus (plus @0 real_onep) (ERF @0))
+ (plus @0 (minus { build_one_cst (TREE_TYPE (@0)); } (ERF @0))))
+
(match double_value_p
@0
(if (TYPE_MAIN_VARIANT (TREE_TYPE (@0)) == double_type_node)))
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr83750-1.c
b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-1.c
new file mode 100644
index 00000000000..a1d456c0119
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-1.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fdump-tree-optimized -funsafe-math-optimizations
-fno-math-errno" } */
+
+double f(double x)
+{
+ double g (double, double);
+ double t1 = __builtin_erf (x);
+ double t2 = __builtin_erfc (x);
+ return g (t1, t2);
+}
+
+/* { dg-final { scan-tree-dump-not "__builtin_erfc" "optimized"} } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr83750-2.c
b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-2.c
new file mode 100644
index 00000000000..a55825bca47
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-2.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fdump-tree-optimized -funsafe-math-optimizations
-fno-math-errno" } */
+
+double f(double x)
+{
+ return __builtin_erfc (x);
+}
+
+/* { dg-final { scan-tree-dump-not "__builtin_erf \\(" "optimized"} } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr83750-3.c
b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-3.c
new file mode 100644
index 00000000000..3305cdf125c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr83750-3.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fdump-tree-optimized -funsafe-math-optimizations
-fno-math-errno" } */
+
+double f1(double x)
+{
+ return x + __builtin_erfc (x);
+}
+
+double f2(double x)
+{
+ return __builtin_erfc (x) + x;
+}
+
+/* { dg-final { scan-tree-dump-not "__builtin_erf \\(" "optimized"} } */
