On Mon, Aug 7, 2017 at 1:36 PM, Michael Collison
<michael.colli...@arm.com> wrote:
> This patch improves code generation for shifts with subtract instructions
> where the first operand to the subtract is equal to the bit-size of the
> operation.
>
>
> long f1(long x, int i)
> {
> return x >> (64 - i);
> }
>
> int f2(int x, int i)
> {
> return x << (32 - i);
> }
>
>
> With trunk at -O2 we generate:
>
> f1:
> mov w2, 64
> sub w1, w2, w1
> asr x0, x0, x1
> ret
>
> f2:
> mov w2, 32
> sub w1, w2, w1
> lsl w0, w0, w1
> ret
>
> with the patch we generate:
>
> f1:
> neg w2, w1
> asr x0, x0, x2
> ret
> .size f1, .-f1
> .align 2
> .p2align 3,,7
> .global f2
> .type f2, %function
> f2:
> neg w2, w1
> lsl w0, w0, w2
> ret
>
> Okay for trunk?
Shouldn't this be handled in simplify-rtx instead of an aarch64
specific pattern?
That is simplify:
(SHIFT A (32 - B)) -> (SHIFT A (AND (NEG B) 31))
etc.
or maybe not. I don't mind either way after thinking about it more.
Thanks,
Andrew
>
> 2017-08-07 Michael Collison <michael.colli...@arm.com>
>
> * config/aarch64/aarch64.md (*aarch64_reg_<optab>_minus<mode>3):
> New pattern.
>
> 2016-08-07 Michael Collison <michael.colli...@arm.com>
>
> * gcc.target/aarch64/var_shift_mask_2.c: New test.
