Subzero: Fix an Om1 crash from memset lowering.

Subzero: Fix an Om1 crash from memset lowering.

With a certain combination of memset arguments, legalizeToReg() is called but its result is unused.  Om1 register allocation does not like this because it sees an infinite-weight variable with a definition but no uses.  The simplest fix is to add a fake use.

The problem shows up when building spec2k with -Om1.

BUG= none
R=ascull@google.com

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=f6f9825e8b695d44c6cbd36d16eeb6f20f1b986e

[Jim Stichnoth, 2015-08-05 23:10:39]

stichnot@chromium.org changed reviewers:
+ ascull@google.com, jpp@chromium.org, jvoung@chromium.org

[ascull, 2015-08-05 23:15:17]

lgtm

https://codereview.chromium.org/1272823004/diff/20001/src/IceTargetLoweringX8...
File src/IceTargetLoweringX86BaseImpl.h (right):

https://codereview.chromium.org/1272823004/diff/20001/src/IceTargetLoweringX8...
src/IceTargetLoweringX86BaseImpl.h:3810:
Context.insert(InstFakeUse::create(Func, Base));
I'm doing some refactoring as I'm writing memcpy so I'll see if I can get
rid of this fake use.

[Jim Stichnoth, 2015-08-05 23:22:30]

https://codereview.chromium.org/1272823004/diff/20001/src/IceTargetLoweringX8...
File src/IceTargetLoweringX86BaseImpl.h (right):

https://codereview.chromium.org/1272823004/diff/20001/src/IceTargetLoweringX8...
src/IceTargetLoweringX86BaseImpl.h:3810:
Context.insert(InstFakeUse::create(Func, Base));
On 2015/08/05 23:15:16, ascull wrote:
> I'm doing some refactoring as I'm writing memcpy so I'll see if I can get
> rid of this fake use.

That sounds great, thanks!

I originally tried to delay initializing Base until its actual use, but that
turned out to be messy and tricky, and caused other lit tests to fail if I
accidentally initialized it more than once.

[Jim Stichnoth, 2015-08-05 23:24:50]

Committed patchset #2 (id:20001) manually as
f6f9825e8b695d44c6cbd36d16eeb6f20f1b986e (presubmit successful).