[llvm] r227983 - Fix program crashes due to alignment exceptions generated for SSE memop instructions (PR22371).

[Nadav Rotem]

The change looks good to me, but I would wait a day and let Elena or Michael review it. 

> On Feb 3, 2015, at 3:19 PM, Hans Wennborg <hans at chromium.org> wrote:
> 
> On Tue, Feb 3, 2015 at 9:13 AM, Sanjay Patel <spatel at rotateright.com <mailto:spatel at rotateright.com>> wrote:
>> Author: spatel
>> Date: Tue Feb  3 11:13:04 2015
>> New Revision: 227983
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=227983&view=rev
>> Log:
>> Fix program crashes due to alignment exceptions generated for SSE memop instructions (PR22371).
>> 
>> r224330 introduced a bug by misinterpreting the "FeatureVectorUAMem" bit.
>> The commit log says that change did not affect anything, but that's not correct.
>> That change allowed SSE instructions to have unaligned mem operands folded into
>> math ops, and that's not allowed in the default specification for any SSE variant.
>> 
>> The bug is exposed when compiling for an AVX-capable CPU that had this feature
>> flag but without enabling AVX codegen. Another mistake in r224330 was not adding
>> the feature flag to all AVX CPUs; the AMD chips were excluded.
>> 
>> This is part of the fix for PR22371 ( http://llvm.org/bugs/show_bug.cgi?id=22371 ).
>> 
>> This feature bit is SSE-specific, so I've renamed it to "FeatureSSEUnalignedMem".
>> Changed the existing test case for the feature bit to reflect the new name and
>> renamed the test file itself to better reflect the feature.
>> Added runs to fold-vex.ll to check for the failing codegen.
>> 
>> Note that the feature bit is not set by default on any CPU because it may require a
>> configuration register setting to enable the enhanced unaligned behavior.
> 
> Sanjay requested in [1] that we should merge this to the 3.6 branch.
> I'm not qualified to determine if this is safe or not. Is there somone
> else with X86 expertise that would like to chime in? Nadav, you're
> listed as the code owner here.
> 
> Thanks,
> Hans
> 
> 1. http://llvm.org/bugs/show_bug.cgi?id=22374#c3 <http://llvm.org/bugs/show_bug.cgi?id=22374#c3>
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