Fold ctz(-x) and ctz(abs(x)) as ctz(x) in match.pd.

The subject line pretty much says it all; the count-trailing-zeros function
of -X and abs(X) produce the same result as count-trailing-zeros of X.
This transformation eliminates a negation which may potentially overflow
with an equivalent expression that doesn't [much like the analogous
abs(-X) simplification in match.pd].

I'd noticed this -X equivalence, which isn't mentioned in Hacker's Delight,
investigating whether ranger's non_zero_bits can help determine whether
an integer variable may be converted to a floating point type exactly
(without raising FE_INEXACT), but it turns out this observation isn't
novel, as (disappointingly) LLVM already performs this same folding.

2024-07-27  Roger Sayle  <roger@nextmovesoftware.com>
	    Andrew Pinski  <quic_apinski@quicinc.com>

gcc/ChangeLog
	* match.pd (ctz (-X) => ctz (X)): New simplification.
	(ctz (abs (X)) => ctz (X)): Likewise.

gcc/testsuite/ChangeLog
	* gcc.dg/fold-ctz-1.c: New test case.
	* gcc.dg/fold-ctz-2.c: Likewise.

diff --git a/gcc/match.pd b/gcc/match.pd
index b2e7d61790dfc259765adad3478eb54946719bc2..1c8601229e3d08eba0e36429d739db5fd603f932 100644 (file)
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -9102,6 +9102,12 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 
 /* CTZ simplifications.  */
 (for ctz (CTZ)
+ /* ctz (-X) => ctz (X).  ctz (abs (X)) => ctz (X).  */
+ (for op (negate abs)
+  (simplify
+   (ctz (nop_convert?@0 (op @1)))
+    (with { tree t = TREE_TYPE (@0); }
+     (ctz (convert:t @1)))))
  (for op (ge gt le lt)
       cmp (eq eq ne ne)
   (simplify

diff --git a/gcc/testsuite/gcc.dg/fold-ctz-1.c b/gcc/testsuite/gcc.dg/fold-ctz-1.c
new file mode 100644 (file)
index 0000000..dcc444c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-ctz-1.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int foo(int x)
+{
+  return __builtin_ctz (-x);
+}
+
+/* { dg-final { scan-tree-dump-not "-x_" "optimized"} } */

diff --git a/gcc/testsuite/gcc.dg/fold-ctz-2.c b/gcc/testsuite/gcc.dg/fold-ctz-2.c
new file mode 100644 (file)
index 0000000..c685698
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-ctz-2.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int foo(int x)
+{
+  return __builtin_ctz (__builtin_abs (x));
+}
+
+/* { dg-final { scan-tree-dump-not "ABS_EXPR" "optimized"} } */