match.pd: Canonicalize (X + (X >> (prec - 1))) ^ (X >> (prec - 1)) to abs (X) [PR94783]

The following patch canonicalizes M = X >> (prec - 1); (X + M) ^ M
for signed integral types into ABS_EXPR (X).  For X == min it is already
UB because M is -1 and min + -1 is UB, so we can use ABS_EXPR rather than
say ABSU_EXPR + cast.

The backend might then emit the abs code back using the shift and addition
and xor if it is the best sequence for the target, but could do something
different that is better.

2020-05-08  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94783
	* match.pd ((X + (X >> (prec - 1))) ^ (X >> (prec - 1)) to abs (X)):
	New simplification.

	* gcc.dg/tree-ssa/pr94783.c: New test.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 5bad3ff..75b0ed1 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,9 @@
 2020-05-08  Jakub Jelinek  <jakub@redhat.com>
 
+       PR tree-optimization/94783
+       * match.pd ((X + (X >> (prec - 1))) ^ (X >> (prec - 1)) to abs (X)):
+       New simplification.
+
        PR tree-optimization/94956
        * match.pd (FFS): Optimize __builtin_ffs* of non-zero argument into
        __builtin_ctz* + 1 if direct IFN_CTZ is supported.

diff --git a/gcc/match.pd b/gcc/match.pd
index 892df1e..e8c53e3 100644 (file)
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -120,6 +120,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (with { tree utype = unsigned_type_for (TREE_TYPE (@0)); }
    (convert (absu:utype @0)))))
 
+#if GIMPLE
+/* Optimize (X + (X >> (prec - 1))) ^ (X >> (prec - 1)) into abs (X).  */
+(simplify
+ (bit_xor:c (plus:c @0 (rshift@2 @0 INTEGER_CST@1)) @2)
+ (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0))
+      && !TYPE_UNSIGNED (TREE_TYPE (@0))
+      && wi::to_widest (@1) == element_precision (TREE_TYPE (@0)) - 1)
+  (abs @0)))
+#endif
 
 /* Simplifications of operations with one constant operand and
    simplifications to constants or single values.  */

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index e8c54c7..174198f 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2020-05-08  Jakub Jelinek  <jakub@redhat.com>
 
+       PR tree-optimization/94783
+       * gcc.dg/tree-ssa/pr94783.c: New test.
+
        PR tree-optimization/94956
        * gcc.target/i386/pr94956.c: New test.
 

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr94783.c b/gcc/testsuite/gcc.dg/tree-ssa/pr94783.c
new file mode 100644 (file)
index 0000000..c52f657
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr94783.c
@@ -0,0 +1,12 @@
+/* PR tree-optimization/94783 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump "ABS_EXPR" "optimized" } } */
+/* { dg-final { scan-tree-dump-not " >> 31" "optimized" } } */
+
+int
+foo (int v)
+{
+  int mask = v >> (__SIZEOF_INT__ * __CHAR_BIT__ - 1);
+  return (v + mask) ^ mask;
+}