Re: [PATCH][AArch64] Use preferred aliases for CSNEG, CSINC, CSINV

[James Greenhalgh <james.greenhalgh@arm.com>]

On Tue, Sep 01, 2015 at 11:08:10AM +0100, Kyrill Tkachov wrote:
> Hi all,
> 
> The ARMv8-A reference manual says:
> "CNEG <Wd>, <Wn>, <cond>
> is equivalent to
> CSNEG <Wd>, <Wn>, <Wn>, invert(<cond>)
> and is the preferred disassembly when Rn == Rm && cond != '111x'."
> 
> That is, when the two input registers are the same we can use the shorter CNEG mnemonic
> with the inverse condition instead of the longer CSNEG instruction. Similarly for the
> CSINV and CSINC instructions, they have shorter CINV and CINC forms.
> This patch adjusts the output templates to emit the preferred shorter sequences when possible.
> 
> The new mnemonics are just aliases, they map down to the same instruction in the end, so there
> are no performance or behaviour implications. But it does make the assembly a bit more readable
> IMO, since:
> "cneg    w27, w9, le"
> can be simply read as "if the condition is less or equal negate w9" instead of the previous:
> "csneg    w27, w9, w9, gt" where you have to remember which of the input registers is negated.
> 
> 
> Bootstrapped and tested on aarch64-linux-gnu.
> Ok for trunk?
>
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index 77bc7cd..2e4b26c 100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -3090,7 +3090,12 @@ (define_insn "csinc3<mode>_insn"
>  		    (const_int 1))
>  	  (match_operand:GPI 3 "aarch64_reg_or_zero" "rZ")))]
>    ""
> -  "csinc\\t%<w>0, %<w>3, %<w>2, %M1"
> +  {
> +    if (rtx_equal_p (operands[2], operands[3]))
> +      return "cinc\\t%<w>0, %<w>2, %m1";
> +    else
> +      return "csinc\\t%<w>0, %<w>3, %<w>2, %M1";
> +  }
>    [(set_attr "type" "csel")]
>  )

I guess you do it this way rather than just adding a new alternative in
the pattern to avoid any chance of constraining the register allocator, but
would this not be more natural to read as an {r, r, r, 2} alternative, or
similar?

If you've given that some thought and decided it doesn't work for you,
then this is OK for trunk.

Thanks,
James