On Fri, Jan 19, 2024 at 5:50 PM Jeff Law <jeffreya...@gmail.com> wrote:
>
>
>
> On 1/19/24 09:05, Georg-Johann Lay wrote:
> >
> >
> > Am 18.01.24 um 20:54 schrieb Roger Sayle:
> >>
> >> This patch tweaks RTL expansion of multi-word shifts and rotates to use
> >> PLUS rather than IOR for disjunctive operations. During expansion of
> >> these operations, the middle-end creates RTL like (X<<C1) | (Y>>C2)
> >> where the constants C1 and C2 guarantee that bits don't overlap.
> >> Hence the IOR can be performed by any any_or_plus operation, such as
> >> IOR, XOR or PLUS; for word-size operations where carry chains aren't
> >> an issue these should all be equally fast (single-cycle) instructions.
> >> The benefit of this change is that targets with shift-and-add insns,
> >> like x86's lea, can benefit from the LSHIFT-ADD form.
> >>
> >> An example of a backend that benefits is ARC, which is demonstrated
> >> by these two simple functions:
> >
> > But there are also back-ends where this is bad.
> >
> > The reason is that with ORI, the back-end needs only to operate no
> > these sub-words where the sub-mask is non-zero. But for PLUS this
> > is not the case because the back-end does not know that intermediate
> > carry will be zero. Hence, with PLUS, more instructions are needed.
> > An example is AVR, but maybe much more target with multi-word operations
> > are affected in a bad way.
> >
> > Take for example the case with 2 words and a value of 1.
> >
> > LO |= 1
> > HI |= 0
> >
> > can be optimized to
> >
> > LO |= 1
> >
> > but for addition this is not the case:
> >
> > LO += 1
> > HI +=c 0 ;; Does not know that always carry = 0.
> I think it's clear that the decision is target and possibly uarch
> specific within a target.
>
> Which means that expmed is probably the right place and that we're going
> to need to look for a good way for the target to control. I suspect
> rtx_cost isn't likely a good fit.
Perhaps related is PR108477 [1] and patch at [2], where x86 would
prefer PLUS instead of {X,I}OR, where we have disjoint bits in the
operands of {X,I}OR.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108477
[2] https://gcc.gnu.org/pipermail/gcc-patches/2024-January/642164.html
Uros.
