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

[Palmer Dabbelt <palmer@dabbelt.com>]

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.

>> 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.