[PATCH] For the 16-bit unallocated NOP hints, make disassembly representation consistent with their 32-bit counterparts

[Artyom Skrobov]

1) Attaching the patch split in two, so that someone having the necessary
permissions could commit it

2) There are several issues about the range of hints to be supported in MC.

First of all, given that the tests for out-of-range hints don't exist in
Thumb2 nor in ARM (in fact, Joey's r191744 explicitly deleted such tests!),
it's arguable whether they should go in as part of the "fairly simple change
to bring Thumb1 in-line with Thumb2 and ARM support for hints". If
necessary, they can be added in a separate commit.

A more important point is that the range of supported hints seems too
loosely defined. Currently, ARM MC supports HINT #0..#255, out of which the
last 16 overlap with DBG #0..#15 (distinct MCInsts having identical
encodings, which isn't great). At the same time, Thumb2 MC supports only
HINT #0..#7 as distinct encodings; the range corresponding to HINT #8..#239
is treated as undefined when disassembling, and HINT #8..#255, although
accepted by the assembler, are wrapped-around to #0..#7 (distinct MCInsts
having identical encodings again). I guess this needs some fixing, e.g. by
defining the valid range to be #0..#239, so we can have a consistent
one-to-one correspondence between MCInsts and encodings. Then we can add
(back) some meaningful tests for this range.

As far as 16-bit hints are concerned, the range encodable in a narrow tHINT
is #0..#15, so if we want HINT #16 onwards to assemble in Thumb to the wide
t2HINT, rather than to produce an error, it would require even more
tinkering with the implementation.


-----Original Message-----
From: Richard Barton 
Sent: 18 October 2013 10:53
To: Artyom Skrobov; llvm-commits at cs.uiuc.edu
Subject: RE: [PATCH] For the 16-bit unallocated NOP hints, make disassembly
representation consistent with their 32-bit counterparts

Hi Artyom

This patch looks fine, but two points:

 * The refactoring in ARMInstPrinter.cpp should go in a separate commit.
 * There need to be error tests for assembly with out of range immediates
for the hints. I think these are missing from the wide hints and the arm
hints too, so it would be great to add them here.

LGTM with those two changed made.

Regards,
Rich

> -----Original Message-----
> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
> bounces at cs.uiuc.edu] On Behalf Of Artyom Skrobov
> Sent: 17 October 2013 14:16
> To: llvm-commits at cs.uiuc.edu
> Subject: [PATCH] For the 16-bit unallocated NOP hints, make disassembly
> representation consistent with their 32-bit counterparts
> 
> Hello,
> 
> Recently the ARM and Thumb2 backends have been changed to give unallocated
> memory hints a disassembly form "hint #<immediate>", but the 16-bit
> unallocated hints have not been similarly changed.
> 
> The 32-bit hints with defined disassembly (NOP through to SEVL) are
> implemented as InstAliases to a top level MCInst called HINT (for ARM) or
> t2HINT (for Thumb2) that holds the "hint" disassembly notation. Thus for
ARM
> and Thumb2 "hint #4" can be used to mean "SEV", for example. However, the
> 16-bit Thumb hints are implemented as their own MCInsts, and the top level
> MCInst called T1SystemEncoding does not have a generic hint definition.
> 
> We've implemented a fairly simple change to bring Thumb1 in-line with
Thumb2
> and ARM support for hints. This also adds support for wide vs. narrow hint
> notation: assembling "hint #7" gives the 16-bit encoding, with "hint.w #7"
> required to get the 32-bit encoding. Although there's no pressing need for
this,
> as the UAL and ARMARM don't cover disassembly for these instructions, but
it
> makes the hints consistent with the rest of the instruction set.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: nophints16.patch1
Type: application/octet-stream
Size: 8060 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20131018/2a8256a8/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: nophints16.patch2
Type: application/octet-stream
Size: 7130 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20131018/2a8256a8/attachment-0001.obj>