[llvm] Riscv branch peephole opt (PR #90451)

[Craig Topper via llvm-commits]

topperc wrote:

> > Do you have an end-to-end C language example that will produce these instructions?
> 
> The following is the minimal code which can produce the `bne a0, a0, %bb1` instruction. It's cropped from spec2006/400.perlbench/Base64.c/XS_MIME__QuotedPrint_encode_qp function which can produce `bne` instruction too.
> 
> ```
> #define qp_isplain(c) ((c) == '\t' || (((c) >= ' ' && (c) <= '~') && (c) != '='))
> int test_bne(char *beg, char *end)
> {
>     char *p;
>     char *p_beg;
> 
>     p = beg;
>     while (1) {
>             p_beg = p;
> 
>             while (p < end && qp_isplain(*p)) {
>                 p++;
>             }
>             if (p == end || *p == '\n') {
>             while (p > p_beg && (*(p - 1) == '\t' || *(p - 1) == ' '))
>                p--;
>             }
>     }
> }
> ```
> 
> Build command: `clang -S -fno-PIC -g -march=rv64imafdc_zba_zbb_zbc_zbs -mabi=lp64d -Ofast -o xxx.S xxx.c`

That compiles to an empty function on compiler explorer https://godbolt.org/z/ss3fqYTo4

https://github.com/llvm/llvm-project/pull/90451