The subject line pretty much says it all; the count-trailing-zeros function
of -X produces 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]. Likewise, the undefined at zero remains
undefined.
I'd noticed this 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.
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?
2024-07-23 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* match.pd (ctz (-X) => ctz (X)): New simplification.
gcc/testsuite/ChangeLog
* gcc.dg/fold-ctz-1.c: New test case.
Thanks in advance,
Roger
--
diff --git a/gcc/match.pd b/gcc/match.pd
index 6818856..d6d61eb 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -9056,6 +9056,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
/* CTZ simplifications. */
(for ctz (CTZ)
+ /* ctz (-x) => ctz (x). */
+ (simplify
+ (ctz (nop_convert?@0 (negate @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
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"} } */
