Subzero: Ignore variables with no actual uses.

Subzero: Ignore variables with no actual uses.

Liveness analysis uses a pair of bit vectors in each CFG node.  The early bits correspond to "global" variables that are referenced in more than one block, and the latter bits correspond to "local" variables that are referenced in only that particular single block.

Due to an oversight, variables that have no uses are conservatively classified as global, and consume space in every liveness bit vector.

This CL improves memory usage by reducing liveness bit vector size:

1. Identify variables with no actual uses and exclude them from the bit vectors.

2. Don't do liveness analysis on rematerializable variables, because they have no need to be involved in register allocation or dead code elimination.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
R=jpp@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=cc89c959c2f602361488e0fdc0bf62e5d197d15c

[Jim Stichnoth, 2016-03-31 18:19:16]

Description was changed from

==========
Subzero: Ignore variables with no actual uses.

BUG=
==========

to

==========
Subzero: Ignore variables with no actual uses.

Liveness analysis uses a pair of bit vectors in each CFG node.  The early bits
correspond to "global" variables that are referenced in more than one block, and
the latter bits correspond to "local" variables that are referenced in only that
particular single block.

Due to an oversight, variables that have no uses are conservatively classified
as global, and consume space in every liveness bit vector.

This CL improves memory usage by reducing liveness bit vector size:

1. Identify variables with no actual uses and exclude them from the bit vectors.

2. Don't do liveness analysis on rematerializable variables, because they have
no need to be involved in register allocation or dead code elimination.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
==========

[Jim Stichnoth, 2016-03-31 18:25:57]

stichnot@chromium.org changed reviewers:
+ eholk@chromium.org, jpp@chromium.org, kschimpf@google.com, sehr@chromium.org

[John, 2016-03-31 18:53:23]

lgtm

[Jim Stichnoth, 2016-03-31 18:55:27]

Description was changed from

==========
Subzero: Ignore variables with no actual uses.

Liveness analysis uses a pair of bit vectors in each CFG node.  The early bits
correspond to "global" variables that are referenced in more than one block, and
the latter bits correspond to "local" variables that are referenced in only that
particular single block.

Due to an oversight, variables that have no uses are conservatively classified
as global, and consume space in every liveness bit vector.

This CL improves memory usage by reducing liveness bit vector size:

1. Identify variables with no actual uses and exclude them from the bit vectors.

2. Don't do liveness analysis on rematerializable variables, because they have
no need to be involved in register allocation or dead code elimination.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
==========

to

==========
Subzero: Ignore variables with no actual uses.

Liveness analysis uses a pair of bit vectors in each CFG node.  The early bits
correspond to "global" variables that are referenced in more than one block, and
the latter bits correspond to "local" variables that are referenced in only that
particular single block.

Due to an oversight, variables that have no uses are conservatively classified
as global, and consume space in every liveness bit vector.

This CL improves memory usage by reducing liveness bit vector size:

1. Identify variables with no actual uses and exclude them from the bit vectors.

2. Don't do liveness analysis on rematerializable variables, because they have
no need to be involved in register allocation or dead code elimination.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4366
R=jpp@chromium.org

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

[Jim Stichnoth, 2016-03-31 18:55:28]

Committed patchset #2 (id:20001) manually as
cc89c959c2f602361488e0fdc0bf62e5d197d15c (presubmit successful).