Re: [PATCH] match: Do not select to branchless expression when target has movcc pattern [PR113095]

[Monk Chiang <monk.chiang@sifive.com>]

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Thanks for your advice!! I agree it should be fixed in the RISC-V backend
when expansion.


On Wed, Jan 17, 2024 at 10:37 PM Jeff Law <jeffreyalaw@gmail.com> wrote:

>
>
> On 1/17/24 05:14, Richard Biener wrote:
> > On Wed, 17 Jan 2024, Monk Chiang wrote:
> >
> >> This allows the backend to generate movcc instructions, if target
> >> machine has movcc pattern.
> >>
> >> branchless-cond.c needs to be updated since some target machines have
> >> conditional move instructions, and the experssion will not change to
> >> branchless expression.
> >
> > While I agree this pattern should possibly be applied during RTL
> > expansion or instruction selection on x86 which also has movcc
> > the multiplication is cheaper.  So I don't think this isn't the way to
> go.
> >
> > I'd rather revert the change than trying to "fix" it this way?
> WRT reverting -- the patch in question's sole purpose was to enable
> branchless sequences for that very same code.  Reverting would regress
> performance on a variety of micro-architectures.  IIUC, the issue is
> that the SiFive part in question has a fusion which allows it to do the
> branchy sequence cheaply.
>
> ISTM this really needs to be addressed during expansion and most likely
> with a RISC-V target twiddle for the micro-archs which have
> short-forward-branch optimizations.
>
> jeff
>