Re: [PATCH] PR tree-optimization/71343: Value number X<<2 as X*4.

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Sep 13, 2022 at 7:55 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch is the second part of a fix for PR tree-optimization/71343,
> that implements Richard Biener's suggestion of using tree-ssa's value
> numbering instead of match.pd.  The change is that when assigning a
> value number for the expression X<<C, we actually look-up or insert
> the value number for the multiplication X*(1<<C).  This elegantly
> handles the fact that we (intentionally) don't canonicalize these as
> equivalent in GIMPLE, and the optimization/equivalence in PR 71343 now
> happens by (tree-ssa SCCVN) magic.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-32},
> with no new failures.  Ok for mainline?

Note that "insertion" is quite limited, in particular does not support
inserting a MULT_EXPR (see eliminate_dom_walker::eliminate_insert).

Your testcases have all expressions in one C statement, if you break
things down to enforce the various evaluation orders you seem to
test I think you'd see that CSEing

   tem1 = (a + b) << 2;
   tem2 = (a + b ) * 4;

does not actually work?  Amending eliminate_insert by for example
changing the BIT_AND_EXPR handling (with constant rhs2) to
covert all tcc_binary should make it work.

Can you double-check?

Otherwise this looks OK.

Thanks,
Richard.

>
> 2022-09-13  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR tree-optimization/71343
>         * tree-ssa-sccvn.cc (visit_nary_op) <case LSHIFT_EXPR>: Make
>         the value number of the expression X << C the same as the value
>         number for the multiplication X * (1<<C).
>
> gcc/testsuite/ChangeLog
>         PR tree-optimization/71343
>         * gcc.dg/pr71343-2.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>
> > -----Original Message-----
> > From: Richard Biener <richard.guenther@gmail.com>
> > Sent: 08 August 2022 12:42
> > To: Roger Sayle <roger@nextmovesoftware.com>
> > Cc: GCC Patches <gcc-patches@gcc.gnu.org>
> > Subject: Re: [PATCH] PR tree-optimization/71343: Optimize (X<<C)&(Y<<C) as
> > (X&Y)<<C.
> >
> > On Mon, Aug 8, 2022 at 10:07 AM Roger Sayle
> > <roger@nextmovesoftware.com> wrote:
> > >
> > > This patch resolves PR tree-optimization/71343, a missed-optimization
> > > enhancement request where GCC fails to see that (a<<2)+(b<<2) == a*4+b*4.
> > > This requires two related (sets of) optimizations to be added to match.pd.
> > >
> > > The first is that (X<<C) op (Y<<C) can be simplified to (X op Y) << C,
> > > for many binary operators, including AND, IOR, XOR, and (if overflow
> > > isn't an issue) PLUS and MINUS.  Likewise, the right shifts (both
> > > logical and arithmetic) and bit-wise logical operators can be
> > > simplified in a similar fashion.  These all reduce the number of
> > > GIMPLE binary operations from 3 to 2, by combining/eliminating a shift
> > operation.
> > >
> > > The second optimization reflects that the middle-end doesn't impose a
> > > canonical form on multiplications by powers of two, vs. left shifts,
> > > instead leaving these operations as specified by the programmer unless
> > > there's a good reason to change them.  Hence, GIMPLE code may contain
> > > the expressions "X * 8" and "X << 3" even though these represent the
> > > same value/computation.  The tweak to match.pd is that comparison
> > > operations whose operands are equivalent non-canonical expressions can
> > > be taught their equivalence.  Hence "(X * 8) == (X << 3)" will always
> > > evaluate to true, and "(X<<2) > 4*X" will always evaluate to false.
> > >
> > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > > and make -k check, both with and without --target_board=unix{-m32},
> > > with no new failures.  Ok for mainline?
> >
> > +/* Shifts by constants distribute over several binary operations,
> > +   hence (X << C) + (Y << C) can be simplified to (X + Y) << C.  */
> > +(for op (plus minus)
> > +  (simplify
> > +    (op (lshift:s @0 INTEGER_CST@1) (lshift:s @2 INTEGER_CST@1))
> > +    (if (INTEGRAL_TYPE_P (type)
> > +        && TYPE_OVERFLOW_WRAPS (type)
> > +        && !TYPE_SATURATING (type)
> > +        && tree_fits_shwi_p (@1)
> > +        && tree_to_shwi (@1) > 0
> > +        && tree_to_shwi (@1) < TYPE_PRECISION (type))
> >
> > I do wonder why we need to restrict this to shifts by constants?
> > Any out-of-bound shift was already there, no?
> >
> > +/* Some tree expressions are intentionally non-canonical.
> > +   We handle the comparison of the equivalent forms here.  */ (for cmp
> > +(eq le ge)
> > +  (simplify
> > +    (cmp:c (lshift @0 INTEGER_CST@1) (mult @0 integer_pow2p@2))
> > +    (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
> > +        && tree_fits_shwi_p (@1)
> > +        && tree_to_shwi (@1) > 0
> > +        && tree_to_shwi (@1) < TYPE_PRECISION  (TREE_TYPE (@0))
> > +        && wi::to_wide (@1) == wi::exact_log2 (wi::to_wide (@2)))
> > +      { constant_boolean_node (true, type); })))
> > +
> > +(for cmp (ne lt gt)
> > +  (simplify
> > +    (cmp:c (lshift @0 INTEGER_CST@1) (mult @0 integer_pow2p@2))
> > +    (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
> > +        && tree_fits_shwi_p (@1)
> > +        && tree_to_shwi (@1) > 0
> > +        && tree_to_shwi (@1) < TYPE_PRECISION  (TREE_TYPE (@0))
> > +        && wi::to_wide (@1) == wi::exact_log2 (wi::to_wide (@2)))
> > +      { constant_boolean_node (false, type); })))
> >
> > hmm.  I wonder if it makes more sense to handle this in value-numbering.
> > tree-ssa-sccvn.cc:visit_nary_op handles some cases that are not exactly
> > canonicalization issues but the shift vs mult could be handled there by just
> > performing the alternate lookup.  That would also enable CSE and by means of
> > that of course the comparisons you do above.
> >
> > Thanks,
> > Richard.
> >
> > >
> > > 2022-08-08  Roger Sayle  <roger@nextmovesoftware.com>
> > >
> > > gcc/ChangeLog
> > >         PR tree-optimization/71343
> > >         * match.pd (op (lshift @0 @1) (lshift @2 @1)): Optimize the
> > >         expression (X<<C) + (Y<<C) to (X+Y)<<C for multiple operators.
> > >         (op (rshift @0 @1) (rshift @2 @1)): Likwise, simplify (X>>C)^(Y>>C)
> > >         to (X^Y)>>C for binary logical operators, AND, IOR and XOR.
> > >         (cmp:c (lshift @0) (mult @1)): Optimize comparisons between
> > >         shifts by integer constants and multiplications by powers of 2.
> > >
> > > gcc/testsuite/ChangeLog
> > >         PR tree-optimization/71343
> > >         * gcc.dg/pr71343-1.c: New test case.
> > >         * gcc.dg/pr71343-2.c: Likewise.
> > >
> > >
> > > Thanks in advance,
> > > Roger
> > > --
>