Hi Richard,
> On 5 Feb 2025, at 09:57, Richard Sandiford <richard.sandif...@arm.com> wrote:
>
> gcc.target/aarch64/sve/acle/general/ldff1_8.c and
> gcc.target/aarch64/sve/ptest_1.c were failing because the
> aarch64 port was giving a zero (unknown) cost to instructions
> that compute two results in parallel. This was latent until
> r15-1575-gea8061f46a30, which fixed rtl-ssa to treat zero costs
> as unknown.
>
> A long-standing todo here is to make insn_cost derive costs from md
> information, rather than having to write a lot of matching code in
> aarch64_rtx_costs. But that's not something we can do for GCC 15.
>
> This patch instead treats the cost of a PARALLEL as being the maximum
> cost of its constituent sets. I don't like this very much, since it
> isn't really target-specific behaviour. If it were stage 1, I'd be
> trying to change pattern_cost instead.
>
> Tested on aarch64-linux-gnu. I'll push this tomorrow if there are no
> comments before then. In particular, please let me know if you'd be
> happy with experimenting with a pattern_cost change even at this stage.
> (The current behaviour is to bail out if more than one set in a parallel
> is live and neither of them is a comparison.)
IMO having this hunk in the backend and moving it into pattern_cost early in
GCC 16 is the optimal risk/benefit approach.
You could add a TODO or a PR marker to the comment to make it more explicit if
you’d like (I don’t insist).
>
> Richard
>
>
> gcc/
> * config/aarch64/aarch64.cc (aarch64_insn_cost): Give PARALLELs
> the same cost as the costliest SET.
> ---
> gcc/config/aarch64/aarch64.cc | 19 ++++++++++++++++++-
> 1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> index 16754fa9e7b..c1e40200806 100644
> --- a/gcc/config/aarch64/aarch64.cc
> +++ b/gcc/config/aarch64/aarch64.cc
> @@ -15889,7 +15889,24 @@ aarch64_insn_cost (rtx_insn *insn, bool speed)
> {
> if (rtx set = single_set (insn))
> return set_rtx_cost (set, speed);
> - return pattern_cost (PATTERN (insn), speed);
> +
> + /* If the instruction does multiple sets in parallel, use the cost
> + of the most expensive set. This copes with instructions that set
> + the flags to a useful value as a side effect. */
> + rtx pat = PATTERN (insn);
> + if (GET_CODE (pat) == PARALLEL)
> + {
> + int max_cost = 0;
> + for (int i = 0; i < XVECLEN (pat, 0); ++i)
> + {
> + rtx x = XVECEXP (pat, 0, i);
Not something for this patch, but would it make sense to add begin () and end
() to RTL vectors so that we can iterate over them with range-based for?
I haven’t thought this through, so it may well be not practical given the
specific structure of RTL fields.
The patch looks ok to me.
Thanks,
Kyrill
> + if (GET_CODE (x) == SET)
> + max_cost = std::max (max_cost, set_rtx_cost (x, speed));
> + }
> + return max_cost;
> + }
> +
> + return pattern_cost (pat, speed);
> }
>
> /* Implement TARGET_INIT_BUILTINS. */
> --
> 2.25.1
>
