On Thu, May 07, 2020 at 10:04:35AM +0200, Richard Biener wrote:
> On Thu, 7 May 2020, Jakub Jelinek wrote:
> > The ffs expanders on several targets (x86, ia64, aarch64 at least)
> > emit a conditional move or similar code to handle the case when the
> > argument is 0, which makes the code longer.
> > If we know from VRP that the argument will not be zero, we can (if the
> > target has also an ctz expander) just use ctz which is undefined at zero
> > and thus the expander doesn't need to deal with that.
> >
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> can you use direct_internal_fn_supported_p (IFN_CTZ, type,
> OPTIMIZE_FOR_SPEED)?
Only if it is guarded with #if GIMPLE (because otherwise the fn
isn't declared).
Though, restricting this to GIMPLE seems like a good idea anyway to me.
Ok for trunk if it passes bootstrap/regtest?
2020-05-07 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/94956
* match.pd (FFS): Optimize __builtin_ffs* of non-zero argument into
__builtin_ctz* + 1 if direct IFN_CTZ is supported.
* gcc.target/i386/pr94956.c: New test.
--- gcc/match.pd.jj 2020-05-06 15:03:51.618058839 +0200
+++ gcc/match.pd 2020-05-07 16:16:48.466970168 +0200
@@ -5986,6 +5986,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
&& direct_internal_fn_supported_p (IFN_POPCOUNT, type,
OPTIMIZE_FOR_BOTH))
(convert (IFN_POPCOUNT:type @0)))))
+
+/* __builtin_ffs needs to deal on many targets with the possible zero
+ argument. If we know the argument is always non-zero, __builtin_ctz + 1
+ should lead to better code. */
+(simplify
+ (FFS tree_expr_nonzero_p@0)
+ (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
+ && direct_internal_fn_supported_p (IFN_CTZ, TREE_TYPE (@0),
+ OPTIMIZE_FOR_SPEED))
+ (plus (CTZ:type @0) { build_one_cst (type); })))
#endif
/* Simplify:
--- gcc/testsuite/gcc.target/i386/pr94956.c.jj 2020-05-06 16:35:47.085876237
+0200
+++ gcc/testsuite/gcc.target/i386/pr94956.c 2020-05-06 16:39:52.927140038
+0200
@@ -0,0 +1,28 @@
+/* PR tree-optimization/94956 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { scan-assembler-not "\tcmovne\t" } } */
+/* { dg-final { scan-assembler-not "\tsete\t" } } */
+
+int
+foo (unsigned x)
+{
+ if (x == 0) __builtin_unreachable ();
+ return __builtin_ffs (x) - 1;
+}
+
+int
+bar (unsigned long x)
+{
+ if (x == 0) __builtin_unreachable ();
+ return __builtin_ffsl (x) - 1;
+}
+
+#ifdef __x86_64__
+int
+baz (unsigned long long x)
+{
+ if (x == 0) __builtin_unreachable ();
+ return __builtin_ffsll (x) - 1;
+}
+#endif
Jakub
