Hi Richard,
Right, so this is an alternative approach using costs - Combine won't try to
duplicate instructions if it increases costs, so increasing the ctz cost to 2
instructions (which is the correct cost for ctz anyway) ensures we still get
efficient code for this example:
[AArch64] Set ctz rtx_cost (PR93565)
Although GCC should understand the limited range of clz/ctz/cls results,
Combine sometimes behaves oddly and duplicates ctz to remove an unnecessary
sign extension. Avoid this by setting the cost for ctz to be higher than
that of a simple ALU instruction. Deepsjeng performance improves by ~0.6%.
Bootstrap OK.
ChangeLog:
2020-02-12 Wilco Dijkstra <wdijk...@arm.com>
PR rtl-optimization/93565
* config/aarch64/aarch64.c (aarch64_rtx_costs): Add CTZ costs.
* gcc.target/aarch64/pr93565.c: New test.
--
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index
e40750380cce202473da3cf572ebdbc28a4ecc06..7426629d6c973c06640f75d3de53a2815ff40f1b
100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -11459,6 +11459,13 @@ aarch64_rtx_costs (rtx x, machine_mode mode, int outer
ATTRIBUTE_UNUSED,
return false;
+ case CTZ:
+ *cost = COSTS_N_INSNS (2);
+
+ if (speed)
+ *cost += extra_cost->alu.clz + extra_cost->alu.rev;
+ return false;
+
case COMPARE:
op0 = XEXP (x, 0);
op1 = XEXP (x, 1);
diff --git a/gcc/testsuite/gcc.target/aarch64/pr93565.c
b/gcc/testsuite/gcc.target/aarch64/pr93565.c
new file mode 100644
index
0000000000000000000000000000000000000000..7200f80d1bb161f6a058cc6591f61b6b75cf1749
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr93565.c
@@ -0,0 +1,34 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+static const unsigned long long magic = 0x03f08c5392f756cdULL;
+
+static const char table[64] = {
+ 0, 1, 12, 2, 13, 22, 17, 3,
+ 14, 33, 23, 36, 18, 58, 28, 4,
+ 62, 15, 34, 26, 24, 48, 50, 37,
+ 19, 55, 59, 52, 29, 44, 39, 5,
+ 63, 11, 21, 16, 32, 35, 57, 27,
+ 61, 25, 47, 49, 54, 51, 43, 38,
+ 10, 20, 31, 56, 60, 46, 53, 42,
+ 9, 30, 45, 41, 8, 40, 7, 6,
+};
+
+static inline int ctz1 (unsigned long b)
+{
+ unsigned long lsb = b & -b;
+ return table[(lsb * magic) >> 58];
+}
+
+void f (unsigned long x, int *p)
+{
+ if (x != 0)
+ {
+ int a = ctz1 (x);
+ *p = a | p[a];
+ }
+}
+
+/* { dg-final { scan-assembler-times "rbit\t" 1 } } */
+/* { dg-final { scan-assembler-times "clz\t" 1 } } */
+
