Re: [PATCH 6/7] riscv: Add support for strlen inline expansion

["Christoph Müllner" <christoph.muellner@vrull.eu>]

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On Mon, Nov 14, 2022 at 7:17 PM Jeff Law <jeffreyalaw@gmail.com> wrote:

>
> On 11/13/22 16:05, Christoph Muellner wrote:
> > From: Christoph Müllner <christoph.muellner@vrull.eu>
> >
> > This patch implements the expansion of the strlen builtin
> > using Zbb instructions (if available) for aligned strings
> > using the following sequence:
> >
> >        li      a3,-1
> >        addi    a4,a0,8
> > .L2:  ld      a5,0(a0)
> >        addi    a0,a0,8
> >        orc.b   a5,a5
> >        beq     a5,a3,6 <.L2>
> >        not     a5,a5
> >        ctz     a5,a5
> >        srli    a5,a5,0x3
> >        add     a0,a0,a5
> >        sub     a0,a0,a4
> >
> > This allows to inline calls to strlen(), with optimized code for
> > determining the length of a string.
> >
> > gcc/ChangeLog:
> >
> >       * config/riscv/riscv-protos.h (riscv_expand_strlen): New
> >         prototype.
> >       * config/riscv/riscv-string.cc (riscv_emit_unlikely_jump): New
> >         function.
> >       (GEN_EMIT_HELPER2): New helper macro.
> >       (GEN_EMIT_HELPER3): New helper macro.
> >       (do_load_from_addr): New helper function.
> >       (riscv_expand_strlen_zbb): New function.
> >       (riscv_expand_strlen): New function.
> >       * config/riscv/riscv.md (strlen<mode>): Invoke expansion
> >         functions for strlen.
> >
> >
> > +extern bool riscv_expand_strlen (rtx[]);
>
> Consider adding the number of elements in the RTX array here. Martin S's
> work from a little while ago will make use of it to try and catch
> over-reads and over-writes if the data is available.
>

Done.


>
>
> >
> >   /* Information about one CPU we know about.  */
> >   struct riscv_cpu_info {
> > diff --git a/gcc/config/riscv/riscv-string.cc
> b/gcc/config/riscv/riscv-string.cc
> > index 1137df475be..bf96522b608 100644
> > --- a/gcc/config/riscv/riscv-string.cc
> > +++ b/gcc/config/riscv/riscv-string.cc
> > @@ -38,6 +38,81 @@
> >   #include "predict.h"
> >   #include "optabs.h"
> >
> > +/* Emit unlikely jump instruction.  */
> > +
> > +static rtx_insn *
> > +riscv_emit_unlikely_jump (rtx insn)
> > +{
> > +  rtx_insn *jump = emit_jump_insn (insn);
> > +  add_reg_br_prob_note (jump, profile_probability::very_unlikely ());
> > +  return jump;
> > +}
>
> I was a bit surprised that we didn't have this as a generic routine.
> Consider adding this to emit-rtl.cc along with its companion
> emit_likely_jump.  Not a requirement to move forward, but it seems like
> the right thing to do.
>

I created both and called them emit_[un]likely_jump_insn() to match
emit_jump_insn().


>
>
>
>
> > +
> > +/* Emit proper instruction depending on type of dest.  */
>
> s/type/mode/
>

Done.


>
>
>
> > +
> > +/* Emit proper instruction depending on type of dest.  */
>
> s/type/mode/
>

Done.


>
>
> You probably want to undefine GEN_EMIT_HELPER once you're done when
> them.  That's become fairly standard practice for these kind of helper
> macros.
>

Done.


>
> OK with the nits fixed.  Your call on whether or not to move the
> implementation of emit_likely_jump and emit_unlikely_jump into emit-rtl.cc.
>

I've made all the requested and suggested changes and rested again.
Thanks!


>
>
> Jeff
>
>
>