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

[Georg-Johann Lay <avr@gjlay.de>]

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.

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
> --
>