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Re: [PATCH 4/4] match.pd: Add x + ((-x) & m) -> (x + m) & ~m pattern


On Thu, 30 Apr 2015, Richard Biener wrote:

On Wed, Jan 21, 2015 at 11:49 AM, Rasmus Villemoes
<rv@rasmusvillemoes.dk> wrote:
Generalizing the x+(x&1) pattern, one can round up x to a multiple of
a 2^k by adding the negative of x modulo 2^k. But it is fewer
instructions, and presumably requires fewer registers, to do the more
common (x+m)&~m where m=2^k-1.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
---
 gcc/match.pd                      |  9 ++++++
 gcc/testsuite/gcc.dg/20150120-4.c | 59 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/20150120-4.c

diff --git gcc/match.pd gcc/match.pd
index 47865f1..93c2298 100644
--- gcc/match.pd
+++ gcc/match.pd
@@ -273,6 +273,15 @@ along with GCC; see the file COPYING3.  If not see
  (if (TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
   (bit_ior @0 (bit_not @1))))

+/* x + ((-x) & m) -> (x + m) & ~m when m == 2^k-1.  */
+(simplify
+ (plus:c @0 (bit_and@2 (negate @0) CONSTANT_CLASS_P@1))

I think you want to restrict this to INTEGER_CST@1

Is this only to make the following test easier (a good enough reason for me) or is there some fundamental reason why this transformation would be wrong for vectors?

+ (with { tree cst = fold_binary (PLUS_EXPR, TREE_TYPE (@1),
+                                @1, build_one_cst (TREE_TYPE (@1))); }

We shouldn't dispatch to fold_binary in patterns.  int_const_binop would
be the appropriate function to use - but what happens for @1 == INT_MAX
where @1 + 1 overflows?  Similar, is this also valid for negative @1
and thus signed mask types?  IMHO we should check whether @1
is equal to wi::mask (TYPE_PRECISION (TREE_TYPE (@1)) - wi::clz (@1),
false, TYPE_PRECISION (TREE_TYPE (@1)).

As with the other patch a ChangeLog entry is missing as well as stating
how you tested the patch.

Thanks,
Richard.

+  (if ((TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
+       && cst && integer_pow2p (cst))
+   (bit_and (plus @0 @1) (bit_not @1)))))

--
Marc Glisse


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