[RFC PATCH 16/19] riscv: Add accelerated strcmp routines

[Jeff Law]

On 2/7/23 07:15, Christoph Müllner wrote:

>      > diff --git a/sysdeps/riscv/multiarch/strcmp_zbb_unaligned.S
>      > b/sysdeps/riscv/multiarch/strcmp_zbb_unaligned.S
[ ... ]


>      > +
>      > +ENTRY_ALIGN (STRCMP, 6)
>      > +       /* off...delta from src1 to src2.  */
>      > +       sub     off, src2, src1
>      > +       li      m1, -1
>      > +       andi    tmp, off, SZREG-1
>      > +       andi    align1, src1, SZREG-1
>      > +       bnez    tmp, L(misaligned8)
>      > +       bnez    align1, L(mutual_align)
>      > +
>      > +       .p2align 4
>      > +L(loop_aligned):
>      > +       REG_L   data1, 0(src1)
>      > +       add     tmp, src1, off
>      > +       addi    src1, src1, SZREG
>      > +       REG_L   data2, 0(tmp)

So any thoughts on reducing the alignment?  Based on the data I've seen 
we very rarely ever take the branch to L(loop_aligned).    So aligning 
this particular label is of dubious value to begin with.  As it stands 
we have to emit 3 full sized nops to achieve the requested alignment and 
they can burn most of an issue cycle.

While it's highly dependent on pipeline state, there's a reasonable 
chance of shaving a cycle by reducing the alignment to p2align 3.

I haven't done much with analyzing the rest of the code as it just 
hasn't been hot in any of the cases I've looked at.

I'd be comfortable with this going in as-is or with the alignment 
adjustment.  Obviously wiring it up via ifunc is dependent upon settling 
the kernel->glibc interface.

Jeff