[LLVMdev] Proposal: Adding aligned instruction bundle support to MC

[Derek Schuff]

Hi Greg,
Our repos are public already, hosted on the Chromium infrastructure.
Our builds are based out of the Native Client repository. The nacl git
mirror is at http://git.chromium.org/native_client/src/native_client.git
(no gitweb, sorry). That repo has pointers to a bunch of other repos
which make pnacl (LLVM, clang, binutils, and a few others. see the
file pnacl/DEPS for more).
But if you just want to see the code and the diff against upstream,
our LLVM repo is in
https://gerrit.chromium.org/gerrit/gitweb?p=native_client%2Fpnacl-llvm.git;a=shortlog;h=refs%2Fheads%2Fmaster
All of our clang changes are upstream already.

-Derek

On Tue, Dec 4, 2012 at 5:23 PM, Greg Fitzgerald <garious at gmail.com> wrote:
> Hi Eli,
>
>> we are splitting the effort to manageable chunks that can
>> be committed, tested and reviewed separately as
>> independently as feasible
>
> I'd be interested in experimenting with this stuff sooner than later.  Could
> you dump all the patches in a publicly accessible repo?  How about Github?
>
> Thanks,
> Greg
>
>
>
> On Tue, Dec 4, 2012 at 3:53 PM, Eli Bendersky <eliben at google.com> wrote:
>>
>> Hello,
>>
>> We (the Portable Native Client team) would like to start upstreaming
>> our LLVM modifications which contain support for Software Fault
>> Isolation (SFI) as required for sandboxing programs to run under
>> Native Client. Since the "total patch size" is quite big, we are
>> splitting the effort to manageable chunks that can be committed,
>> tested and reviewed separately as independently as feasible.
>>
>> One of the first things we'd like to start with is support for aligned
>> instruction bundles in MC (assembler) level. This support exists in
>> gas since binutils version 2.23
>>
>> (http://sourceware.org/binutils/docs/as/Bundle-directives.html#Bundle-directives).
>> Succinctly, the initial proposal is to add the following directives:
>>
>> .bundle_align_mode <num>
>> .bundle_lock
>> .bundle_unlock
>>
>> With the following semantics:
>>
>> When aligned instruction bundle mode ("bundling" in short) is enabled
>> (.bundle_align_mode was encountered with an argument > 0, which is the
>> power of 2 to which the bundle size is equal), single
>> instructions and groups of instructions between .bundle_lock and
>> .bundle_unlock directives cannot cross a bundle boundary.
>>
>> For example, consider the following:
>>
>> .bundle_align_mode 4
>> mov1
>> mov2
>> mov3
>>
>> Assuming that each of the mov instructions is 7 bytes long and mov1 is
>> aligned to a 16-byte boundary, two bytes of NOP padding will be
>> inserted between mov2 and mov3 to make sure that mov3 does not cross a
>> 16-byte bundle boundary.
>>
>> A slightly modified example:
>>
>> .bundle_align_mode 4
>> mov1
>> .bundle_lock
>> mov2
>> mov3
>> .bundle_unlock
>>
>> Here, since the bundle-locked sequence "mov2 mov3" cannot cross a
>> bundle boundary, 9 bytes of NOP padding will be inserted between mov1
>> and mov2.
>>
>> For information on how this ability is used for software fault
>> isolation by Native Client, see the following resources:
>>
>> * http://src.chromium.org/viewvc/native_client/data/site/NaCl_SFI.pdf [PDF
>> link]
>> *
>> http://www.chromium.org/nativeclient/reference/arm-overview#TOC-The-Native-Client-Solution:-Bundles-
>> * Other papers listed at
>> http://www.chromium.org/nativeclient/reference/research-papers
>>
>> We want to start with this feature because it is self-contained in MC,
>> makes LLVM more compatible with gas, level and can be easily tested.
>> Future upstreaming efforts will be able to build upon this
>> functionality.
>>
>> Any comments, ideas and suggestions are welcome.
>>
>> Eli
>> _______________________________________________
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>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
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