On Thu, Jan 18, 2024 at 8:55 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> 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:
>
> 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?
For expand_shift_1 you add
+ where C is the bitsize of A. If N cannot be zero,
+ use PLUS instead of IOR.
but I don't see a check ensuring this other than mabe CONST_INT_P (op1)
suggesting that we enver end up with const0_rtx here. OTOH why is
N zero a problem and why is it not in the optabs.cc case where I don't
see any such check (at least not obvious)?
Since this doesn't seem to fix a regression it probably has to wait for
stage1 to re-open.
Thanks,
Richard.
>
> 2024-01-18 Roger Sayle <ro...@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
> --
>
