Subzero: Make the szbuild.py script more automatic.

Subzero: Make the szbuild.py script more automatic.

One wants to omit the --init option to make things go faster, but then there's the risk that the pexe or llvm2ice or llc has changed and --init is actually necessary.

This change automatically compares the modification timestamps of the pexe and the llvm2ice and llc binaries against the modification timestamps of the object files to determine whether re-translation is needed.

The --init option forces re-translation regardless of timestamps.

BUG= none
R=jvoung@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=0fb613f88e44dc877a1b99cf324af6809a8391fd

[Jim Stichnoth, 2014-09-16 00:25:47]

stichnot@chromium.org changed reviewers:
+ jvoung@chromium.org

[jvoung (off chromium), 2014-09-16 00:43:58]

lgtm

https://codereview.chromium.org/551373008/diff/20001/pydir/szbuild.py
File pydir/szbuild.py (right):

https://codereview.chromium.org/551373008/diff/20001/pydir/szbuild.py#newcode109
pydir/szbuild.py:109: help='Force all re-translation of the pexe')
Would it make sense to call the flag --force, or something, now that its
behavior is a bit different?

[Jim Stichnoth, 2014-09-16 03:09:48]

https://codereview.chromium.org/551373008/diff/20001/pydir/szbuild.py
File pydir/szbuild.py (right):

https://codereview.chromium.org/551373008/diff/20001/pydir/szbuild.py#newcode109
pydir/szbuild.py:109: help='Force all re-translation of the pexe')
On 2014/09/16 00:43:58, jvoung wrote:
> Would it make sense to call the flag --force, or something, now that its
> behavior is a bit different?

You're right.  I changed it to --force.

[Jim Stichnoth, 2014-09-16 17:19:07]

Committed patchset #3 (id:40001) manually as 0fb613f (presubmit successful).