Re: [PATCH, ARM] Thumb-2 12-bit immediates in ADD and SUB instructions

[Chung-Lin Tang <cltang@codesourcery.com>]

On 2011/5/20 11:45 AM, Dmitry Plotnikov wrote:
> This patch adds support for 12-bit immediate values for Thumb-2 in ADD and
> SUB instructions.  We added two new alternatives for *arm_addsi3 which
> make use of two new constraints for 12-bit values.  Also we modified
> costs of PLUS rtx expression.
> This patch reduces size of libevas by 1788 bytes (from 464916 to
> 463128), and sqlite by 54 bytes (from 266156 to 266052).
> Regtested with Cortex-A8 QEMU.
> 
> Ok for trunk?
> 

Andrew Stubbs seem to have another patch related to ADDW/SUBW support,
which I think is not yet committed to trunk. I have not yet studied how
this patch and Andrew's relate.

That aside, I think the style of adding new alternatives for this
purpose is a little unneeded. I suggest:

1) Abstract out const_ok_for_arm() into const_ok_for_arm_outer() with an
OUTER rtx code argument, and a const_ok_for_arm() with OUTER passed 0.

2) Within const_ok_for_arm_outer(), test for OUTER==PLUS and
TARGET_THUMB2 as needed.

3) Migrate from const_ok_for_arm() to const_ok_for_arm_outer() as
needed: in pattern conditions, etc.

I'll also note here that ADD/SUB are not the only instructions with
12-bit immediate under Thumb-2; so does AND, ORR, etc.

Chung-Lin