Re: [middle-end PATCH] Prefer PLUS over IOR in RTL expansion of multi-word shifts/rotates.

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Jan 19, 2024 at 5:06 PM Georg-Johann Lay <avr@gjlay.de> 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 wonder if the PLUS can be done on the lowpart only to make this
detail obvious?

> Johann
>
>
> >
> > unsigned long long foo(unsigned long long x) { return x<<2; }
> >
> > which with -O2 is currently compiled to:
> >
> > foo:    lsr     r2,r0,30
> >          asl_s   r1,r1,2
> >          asl_s   r0,r0,2
> >          j_s.d   [blink]
> >          or_s    r1,r1,r2
> >
> > with this patch becomes:
> >
> > foo:    lsr     r2,r0,30
> >          add2    r1,r2,r1
> >          j_s.d   [blink]
> >          asl_s   r0,r0,2
> >
> > unsigned long long bar(unsigned long long x) { return (x<<2)|(x>>62); }
> >
> > which with -O2 is currently compiled to 6 insns + return:
> >
> > bar:    lsr     r12,r0,30
> >          asl_s   r3,r1,2
> >          asl_s   r0,r0,2
> >          lsr_s   r1,r1,30
> >          or_s    r0,r0,r1
> >          j_s.d   [blink]
> >          or      r1,r12,r3
> >
> > with this patch becomes 4 insns + return:
> >
> > bar:    lsr     r3,r1,30
> >          lsr     r2,r0,30
> >          add2    r1,r2,r1
> >          j_s.d   [blink]
> >          add2    r0,r3,r0
> >
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > and make -k check, both with and without --target_board=unix{-m32}
> > with no new failures.  Ok for mainline?
> >
> >
> > 2024-01-18  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >          * expmed.cc (expand_shift_1): Use add_optab instead of ior_optab
> >          to generate PLUS instead or IOR when unioning disjoint bitfields.
> >          * optabs.cc (expand_subword_shift): Likewise.
> >          (expand_binop): Likewise for double-word rotate.
> >
> >
> > Thanks in advance,
> > Roger
> > --
> >