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?
2022-09-13 Roger Sayle <ro...@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.guent...@gmail.com>
> Sent: 08 August 2022 12:42
> To: Roger Sayle <ro...@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
> <ro...@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 <ro...@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
> > --
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index 74b8d8d..2644446 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -5312,6 +5312,30 @@ visit_nary_op (tree lhs, gassign *stmt)
}
}
break;
+ case LSHIFT_EXPR:
+ /* For X << C, use the value number of X * (1 << C). */
+ if (INTEGRAL_TYPE_P (type))
+ {
+ tree rhs2 = gimple_assign_rhs2 (stmt);
+ if (TREE_CODE (rhs2) == INTEGER_CST
+ && tree_fits_uhwi_p (rhs2)
+ && tree_to_uhwi (rhs2) < TYPE_PRECISION (type))
+ {
+ wide_int w = wi::set_bit_in_zero (tree_to_uhwi (rhs2),
+ TYPE_PRECISION (type));
+ gimple_match_op match_op (gimple_match_cond::UNCOND,
+ MULT_EXPR, type, rhs1,
+ wide_int_to_tree (type, w));
+ result = vn_nary_build_or_lookup (&match_op);
+ if (result)
+ {
+ bool changed = set_ssa_val_to (lhs, result);
+ vn_nary_op_insert_stmt (stmt, result);
+ return changed;
+ }
+ }
+ }
+ break;
default:
break;
}
diff --git a/gcc/testsuite/gcc.dg/pr71343-2.c b/gcc/testsuite/gcc.dg/pr71343-2.c
new file mode 100644
index 0000000..11800a9
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr71343-2.c
@@ -0,0 +1,34 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int test1(unsigned int a , unsigned int b)
+{
+ return (a << 2) + (b << 2) == a * 4 + b * 4;
+}
+
+unsigned int test2(unsigned int a , unsigned int b)
+{
+ return (a << 2) + (b << 2) == (a + b) << 2;
+}
+
+unsigned int test3(unsigned int a , unsigned int b)
+{
+ return a * 4 + b * 4 == (a + b) * 4;
+}
+
+unsigned int test4(unsigned int a , unsigned int b)
+{
+ return (a + b) << 2 == (a + b) * 4;
+}
+
+unsigned int test5(unsigned int a , unsigned int b)
+{
+ return (a << 2) + (b << 2) == (a + b) * 4;
+}
+
+unsigned int test6(unsigned int a , unsigned int b)
+{
+ return (a + b) << 2 == a * 4 + b * 4;
+}
+
+/* { dg-final { scan-tree-dump-times "return 1" 6 "optimized" } } */
