Wilco Dijkstra <wilco.dijks...@arm.com> writes:
> 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)
...agreed...
> 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.
OK, thanks. Could you remove the bit about combine behaving oddly when
you commit though? I think this was simply a target bug and combine
was being given duff information.
Richard
>
> --
>
> 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 } } */
> +
