Re: [PATCH 7/7] riscv: Add support for str(n)cmp inline expansion

[Kito Cheng <kito.cheng@sifive.com>]

On Tue, Nov 15, 2022 at 8:53 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Mon, 14 Nov 2022 16:46:37 PST (-0800), Kito Cheng wrote:
> > Hi Christoph:
> >
> >> This patch implements expansions for the cmpstrsi and the cmpstrnsi
> >> builtins using Zbb instructions (if available).
> >> This allows to inline calls to strcmp() and strncmp().
> >>
> >> The expansion basically emits a peeled comparison sequence (i.e. a peeled
> >> comparison loop) which compares XLEN bits per step if possible.
> >>
> >> The emitted sequence can be controlled, by setting the maximum number
> >> of compared bytes (-mstring-compare-inline-limit).
> >
> > I would like to have a unified option interface,
> > maybe -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit.
> > And add some option like this:
> > -minline-str[n]cmp=[bitmanip|vector|auto] in future,
> > since I assume we'll have different versions of those things.
>
> Can we just decide that from mtune?  We'll probably have uarch-specific
> string functions at some point, might as well start planning for it now.

I assume you mean the -minline-str[n]cmp=[bitmanip|vector|auto] part?
I think this part should have more discussion and could defer that until
we reach consensus.

But -m[no-]inline-str[n]cmp and -minline-str[n]cmp-limit part I favor having
those two options to disable and/or fine tune those parameters.

>
> >> gcc/ChangeLog:
> >>
> >>         * config/riscv/riscv-protos.h (riscv_expand_strn_compare): New
> >>           prototype.
> >>         * config/riscv/riscv-string.cc (GEN_EMIT_HELPER3): New helper
> >>           macros.
> >>         (GEN_EMIT_HELPER2): New helper macros.
> >>         (expand_strncmp_zbb_sequence): New function.
> >>         (riscv_emit_str_compare_zbb): New function.
> >>         (riscv_expand_strn_compare): New function.
> >>         * config/riscv/riscv.md (cmpstrnsi): Invoke expansion functions
> >>           for strn_compare.
> >>         (cmpstrsi): Invoke expansion functions for strn_compare.
> >>         * config/riscv/riscv.opt: Add new parameter
> >>           '-mstring-compare-inline-limit'.
> >
> > We need to document this option.