On Tue, 5 Nov 2024, Soumya AR wrote:
>
>
> > On 29 Oct 2024, at 7:16 PM, Richard Biener <[email protected]> wrote:
> >
> > External email: Use caution opening links or attachments
> >
> >
> > On Mon, 28 Oct 2024, Soumya AR wrote:
> >
> >> This patch transforms the following POW calls to equivalent LDEXP calls, as
> >> discussed in PR57492:
> >>
> >> powi (2.0, i) -> ldexp (1.0, i)
> >>
> >> a * powi (2.0, i) -> ldexp (a, i)
> >>
> >> 2.0 * powi (2.0, i) -> ldexp (1.0, i + 1)
> >>
> >> pow (powof2, i) -> ldexp (1.0, i * log2 (powof2))
> >>
> >> powof2 * pow (2, i) -> ldexp (1.0, i + log2 (powof2))
> >
> > For the multiplication cases why not handle powof2 * ldexp (1., i)
> > to ldexp (1., i + log2 (powof2)) and a * ldexp (1., i) -> ldexp (a, i)
> > instead? exp2 * ldexp (1., i) is another candidate.
> >
> > So please split out the multiplication parts.
> >
> > + /* Simplify pow (powof2, i) to ldexp (1, i * log2 (powof2)). */
> >
> > the below pattern handles POWI, not POW.
> >
> > + (simplify
> > + (POWI REAL_CST@0 @1)
> > + (with { HOST_WIDE_INT tmp = 0;
> > + tree integer_arg1 = NULL_TREE; }
> > + (if (integer_valued_real_p (@0)
> > + && real_isinteger (&TREE_REAL_CST (@0), &tmp)
> > + && integer_pow2p (integer_arg1 = build_int_cst (integer_type_node,
> > tmp)))
> >
> > && tmp > 0
> > && pow2p_hwi (tmp)
> >
> > + (LDEXP { build_one_cst (type); }
> > + (mult @1 { build_int_cst (integer_type_node,
> > + tree_log2 (integer_arg1)); })))))
> >
> > build_int_cst (integer_type_node, exact_log2 (tmp))
> >
> > + /* Simplify powi (2.0, i) to ldexp (1, i). */
> > + (simplify
> > + (POWI REAL_CST@0 @1)
> > + (if (real_equal (TREE_REAL_CST_PTR (@0), &dconst2))
> > + (LDEXP { build_one_cst (type); } @1)))
> > +
> >
> > You'll have a duplicate pattern here, instead merge them. 2.0
> > is power-of-two so I wonder why the pattern is needed.
>
> Thanks for the feedback!
>
> I've merged the extra case that was specifically checking for 2.0.
>
> Like you suggested, I also added two ldexp specific transforms:
>
> • powof2 * ldexp (x, i) -> ldexp (x, i + log2 (powof2))
> • a * ldexp(1., i) -> ldexp (a, i)
>
> Regarding your suggestion to also fold exp2, a conversion like
> exp2 (x) -> ldexp (1., x) or exp2 (x) * ldexp (y, i) -> ldexp (y, i + x) is a
> bit tricky because we'd have to cast it to an integer before passing it to
> ldexp.
>
> real_to_integer only works for constants, which isn't helpful here as exp2
> (CST)
> becomes a power of 2 anyway and matches with the above patterns.
>
> We'll have to explicitly convert it for non constants and I'm not sure if that
> is worth it for this patch.
>
> Let me know what you think.
+ (simplify
+ (mult:c REAL_CST@0 (POWI REAL_CST@1 @2))
+ (with { HOST_WIDE_INT tmp = 0;
+ tree integer_arg1 = NULL_TREE; }
+ (if (integer_valued_real_p (@0)
+ && real_equal (TREE_REAL_CST_PTR (@1), &dconst2)
+ && real_isinteger (&TREE_REAL_CST (@0), &tmp)
+ && integer_pow2p (integer_arg1 = build_int_cst (integer_type_node,
tmp)))
as said you can use tmp > 0 && pow2p_hwi (tmp) instead of
integer_pow2p and build_int_cst. This applies to all patterns.
+ (if (integer_valued_real_p (@0)
+ && real_isinteger (&TREE_REAL_CST (@0), &tmp)
also is a redundant check, real_isinteger gives you the answer already.
+ (mult @1 {build_int_cst (integer_type_node,
+ tree_log2 (integer_arg1)); })))))
and use exact_log2 (tmp) instead of tree_log2 (integer_arg1).
+ /* Simplify a * powi (2.0, i) to ldexp (a, i). */
+ (simplify
+ (mult:c @0 (POWI REAL_CST@1 @2))
+ (if (real_equal (TREE_REAL_CST_PTR (@1), &dconst2))
+ (LDEXP @0 @2)))
...
+ /* Simplify powi (2.0, i) to ldexp (1, i). */
+ (simplify
+ (POWI REAL_CST@0 @1)
+ (if (real_equal (TREE_REAL_CST_PTR (@0), &dconst2))
+ (LDEXP { build_one_cst (type); } @1)))
you can expect the first to never hit as we should replace
a * powi (2.0, i) with a * ldexp (1, i) first. So you can
drop the first pattern.
Richard.
> Best,
> Soumya
>
>
>
> > Richard.
> >
> >>
> >> This is especially helpful for SVE architectures as LDEXP calls can be
> >> implemented using the FSCALE instruction, as seen in the following patch:
> >> https://gcc.gnu.org/pipermail/gcc-patches/2024-September/664160.html
> >>
> >> SPEC2017 was run with this patch, while there are no noticeable
> >> improvements,
> >> there are no non-noise regressions either.
> >>
> >> The patch was bootstrapped and regtested on aarch64-linux-gnu, no
> >> regression.
> >> OK for mainline?
> >>
> >> Signed-off-by: Soumya AR <[email protected]>
> >>
> >> gcc/ChangeLog:
> >> PR target/57492
> >> * match.pd: Added patterns to fold certain calls to pow to ldexp.
> >>
> >> gcc/testsuite/ChangeLog:
> >> PR target/57492
> >> * gcc.dg/tree-ssa/pow-to-ldexp.c: New test.
> >>
> >>
> >
> > --
> > Richard Biener <[email protected]>
> > SUSE Software Solutions Germany GmbH,
> > Frankenstrasse 146, 90461 Nuernberg, Germany;
> > GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
>
>
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
