Subzero: Fold the load instruction into the next cast instruction.

Subzero: Fold the load instruction into the next cast instruction.

This is similar to the way a load instruction may be folded into the next arithmetic instruction.

Usually the effect is to improve a sequence like:
  mov ax, WORD PTR [mem]
  movsx eax, ax
into this:
  movsx eax, WORD PTR [mem]
without actually improving register allocation, though other kinds of casts may have different improvements.

Existing tests needed to be fixed when they "inadvertently" did a cast to i32 return type and triggered the optimization when it wasn't wanted.  These were fixed by inserting a "dummy" instruction between the load and the cast.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4095
R=jvoung@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=c77f817f923b2fcbf043da76e1a3066f7729aef2

[Jim Stichnoth, 2015-05-30 00:26:07]

stichnot@chromium.org changed reviewers:
+ jvoung@chromium.org, kschimpf@google.com, mtrofin@google.com

[Jim Stichnoth, 2015-05-30 00:26:34]

This appears to give a 1% gain in geomean across spec2k components.

[jvoung (off chromium), 2015-05-31 17:27:56]

lgtm

https://codereview.chromium.org/1152783006/diff/1/src/IceTargetLoweringX8632.cpp
File src/IceTargetLoweringX8632.cpp (right):

https://codereview.chromium.org/1152783006/diff/1/src/IceTargetLoweringX8632....
src/IceTargetLoweringX8632.cpp:3980: // Fuse this load with a subsequent
Arithmetic instruction in the
"Fuse ... Arithmetic or Cast..." but may need to reword a bit ?

[Jim Stichnoth, 2015-06-01 06:34:49]

Committed patchset #2 (id:20001) manually as
c77f817f923b2fcbf043da76e1a3066f7729aef2 (presubmit successful).

[Jim Stichnoth, 2015-06-01 06:35:12]

https://codereview.chromium.org/1152783006/diff/1/src/IceTargetLoweringX8632.cpp
File src/IceTargetLoweringX8632.cpp (right):

https://codereview.chromium.org/1152783006/diff/1/src/IceTargetLoweringX8632....
src/IceTargetLoweringX8632.cpp:3980: // Fuse this load with a subsequent
Arithmetic instruction in the
On 2015/05/31 17:27:56, jvoung wrote:
> "Fuse ... Arithmetic or Cast..." but may need to reword a bit ?

Added a separate description for Cast.