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Re: [PATCH][AArch64] Use cinc mnemonic for *csinc2<mode>_insn
- From: James Greenhalgh <james dot greenhalgh at arm dot com>
- To: Kyrill Tkachov <kyrylo dot tkachov at arm dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Marcus Shawcroft <Marcus dot Shawcroft at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>
- Date: Wed, 15 Jul 2015 15:00:53 +0100
- Subject: Re: [PATCH][AArch64] Use cinc mnemonic for *csinc2<mode>_insn
- Authentication-results: sourceware.org; auth=none
- References: <55A663C8 dot 4070201 at arm dot com>
On Wed, Jul 15, 2015 at 02:44:40PM +0100, Kyrill Tkachov wrote:
> Hi all,
> This pattern performs a csinc of the same register in both operands.
> This form can be written using the shorter alias CINC.
> The ARMv8-A ARM says:
>
> "CINC <Wd>, <Wn>, <cond>
> is equivalent to
> CSINC <Wd>, <Wn>, <Wn>, invert(<cond>)"
>
> So the patch switches the condition output modifier from 'M' to the inverse 'm'.
>
> I don't think we emit cinc anywhere else in aarch64.md so this will exercise
> the assembler a tiny bit more (no fallout detected in my testing) and make
> the output a bit more concise.
More importantly this puts us in line with the preferred disassembly, which
I think is a desirable position. If we can ever get here using the
zero register you should really put out cset, but having different
alternatives in this pattern to cover the different aliasing conditions
does seem overkill!
> Again, this is an alias, not a different instruction, so there are no
> performance/behaviour implications
>
> Bootstrapped and tested on aarch64.
>
> Ok for trunk?
Yup, OK. Watch out for the mangling of the tab in your proposed ChangeLog.
Thanks,
James
>
> Thanks,
> Kyrill
>
> 2015-07-15 Kyrylo Tkachov <kyrylo.tkachov@arm.com>
>
> * config/aarch64/aarch64.md (*csinc2<mode>_insn): Use cinc mnemonic.
> commit ec05547074dd575471ebafdd10975f438e3361f6
> Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
> Date: Wed Jul 8 11:04:47 2015 +0100
>
> [AArch64] Use cinc mnemonic for *csinc2<mode>_insn
>
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index 300537e..57a3360 100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -3036,7 +3036,7 @@ (define_insn "*csinc2<mode>_insn"
> (plus:GPI (match_operand 2 "aarch64_comparison_operation" "")
> (match_operand:GPI 1 "register_operand" "r")))]
> ""
> - "csinc\\t%<w>0, %<w>1, %<w>1, %M2"
> + "cinc\\t%<w>0, %<w>1, %m2"
> [(set_attr "type" "csel")]
> )
>