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

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

On Tue, 15 Nov 2022 at 01:46, Kito Cheng <kito.cheng@gmail.com> 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.

For the basic option (-m[no-]inline-str[n]cmp), I would punt to
-fno-builtin-str[n]cmp.
The limit-one sounds more like a --param?

> And add some option like this:
> -minline-str[n]cmp=[bitmanip|vector|auto] in future,

If we want to follow the lead of others, then x86 has a -mstringop-strategy=alg


> since I assume we'll have different versions of those things.
>
> >
> > 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.