> On Mar 3, 2015, at 7:19 PM, Wilco Dijkstra <wdijk...@arm.com> wrote:
>
> Expand abs into a compare and conditional negate. This is the most obvious
> expansion, enables
> merging of the comparison into ALU instructions and is faster on all
> implementations.
> Bootstrapped &
> regression tested.
>
> int f(int x) { return abs (x + 1); }
>
> Before:
> add w0, w0, 1
> sxtw x0, w0
> eor x1, x0, x0, asr 63
> sub x1, x1, x0, asr 63
> mov x0, x1
> ret
>
> After:
> adds w0, w0, 1
> csneg w0, w0, w0, pl
> ret
>
> ChangeLog:
>
> 2015-03-03 Wilco Dijkstra <wdijk...@arm.com>
>
> * gcc/config/aarch64/aarch64.md (absdi2): optimize abs expansion.
> (csneg3<mode>_insn): enable expansion of pattern.
> * gcc/testsuite/gcc.target/aarch64/abs_1.c (abs64): update test
> for new abs expansion. (abs64_in_dreg): likewise.
>
> ---
> gcc/config/aarch64/aarch64.md | 33 +++++++-------------------------
> gcc/testsuite/gcc.target/aarch64/abs_1.c | 5 ++---
> 2 files changed, 9 insertions(+), 29 deletions(-)
>
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index 1f4169e..46b7a63 100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -2172,35 +2172,16 @@
> [(set_attr "type" "alu_ext")]
> )
>
> -(define_insn_and_split "absdi2"
> - [(set (match_operand:DI 0 "register_operand" "=&r,w")
> - (abs:DI (match_operand:DI 1 "register_operand" "r,w")))]
> +(define_expand "abs<mode>2"
> + [(match_operand:GPI 0 "register_operand" "")
> + (match_operand:GPI 1 "register_operand" "")]
> ""
> - "@
> - #
> - abs\\t%d0, %d1"
You are removing the 2nd alternative that generates "abs" with your patch.
While I agree that using "csneg" is faster on all implementations, can you say
the same for "abs"? Especially given the fact that csneg requires 4 operands
instead of abs'es 2?
Wouldn't it be better to have (define_expand "abs<mode>2") that would expand
into either csneg3<mode> or second alternative of current absdi2?
Other than this, the patch looks OK.
Thank you,
--
Maxim Kuvyrkov
www.linaro.org
