[AArch64] Fix cost of (plus ... (const_int -C))

[James Greenhalgh]

On Mon, Sep 23, 2019 at 10:45:29AM +0100, Richard Sandiford wrote:
> The PLUS handling in aarch64_rtx_costs only checked for nonnegative
> constants, meaning that simple immediate subtractions like:
> 
>   (set (reg R1) (plus (reg R2) (const_int -8)))
> 
> had a cost of two instructions.
> 
> Tested on aarch64-linux-gnu (with and without SVE).  OK to install?

OK.

Thanks,
James

> 
> Richard
> 
> 
> 2019-09-23  Richard Sandiford  <richard.sandiford@arm.com>
> 
> gcc/
> 	* config/aarch64/aarch64.c (aarch64_rtx_costs): Use
> 	aarch64_plus_immediate rather than aarch64_uimm12_shift
> 	to test for valid PLUS immediates.
>