PNaCl: Enable ExpandByVal pass for expanding out passing structs by value

pnacl/driver/pnacl-ld.py

Index: pnacl/driver/pnacl-ld.py
diff --git a/pnacl/driver/pnacl-ld.py b/pnacl/driver/pnacl-ld.py
index a70a16a520549a55fc25845551f2e7a6b8c05ce6..dc57ee1f9d86855caf6095ce51a43cf6a91296fa 100755
--- a/pnacl/driver/pnacl-ld.py
+++ b/pnacl/driver/pnacl-ld.py
@@ -358,6 +358,7 @@ def main(argv):
         passes += ['-lowerinvoke',
                    '-simplifycfg']
       passes += ['-expand-varargs',
+                 '-expand-byval',

[jvoung (off chromium), 2013/04/24 19:59:12]

When you did the byval -> reference pass did you add any noalias hints (was it
valid to do so)?

I ask because the ArgumentPromotionPass (which may run after this, at -O3) can
simplify byval arguments for internal functions.  I'm not sure it would do as
much with just a raw pointer.  It does try to simplify pointer arguments too,
given that the alias analysis says that it is valid to do so.

[Mark Seaborn, 2013/04/24 20:36:42]

No, I didn't.  (See https://codereview.chromium.org/13973018/)
It seems that way from LangRef, although I've got no experience with alias
analyses (aside from debugging that "tail" attribute problem this week).  I'd
want to be sure I'm not risking a miscompilation before adding a noalias attr.
:-)
So, should I move -expand-byval to the second set of ABI-simplification passes,
which run after -std-link-opts?

[jvoung (off chromium), 2013/04/24 21:43:09]

Well, I'm okay with leaving it here, and filing an issue to investigate noalias
or changing the order.  It might not actually matter much =/

                  '-nacl-expand-ctors',
                  '-resolve-aliases',
                  '-nacl-expand-tls',