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

pnacl/driver/pnacl-ld.py

 #!/usr/bin/python
 # Copyright (c) 2012 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # IMPORTANT NOTE: If you make local mods to this file, you must run:
 #   %  pnacl/build.sh driver
 # in order for them to take effect in the scons build.  This command
 # updates the copy in the toolchain/ tree.
 #
@@ skipping 340 matching lines @@
     if env.getbool('STATIC') and len(native_objects) == 0:
       passes = []
       if not env.getbool('ALLOW_CXX_EXCEPTIONS'):
         # '-lowerinvoke' prevents use of C++ exception handling, which
         # is not yet supported in the PNaCl ABI.  '-simplifycfg'
         # removes landingpad blocks made unreachable by
         # '-lowerinvoke'.
         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',
                  # Global cleanup needs to run after expand-tls because
                  # __tls_template_start etc are extern_weak before expansion
                  '-nacl-global-cleanup']
       chain.add(DoLLVMPasses(passes), 'expand_features.' + bitcode_type)
 
     if env.getone('OPT_LEVEL') != '' and env.getone('OPT_LEVEL') != '0':
       chain.add(DoLTO, 'opt.' + bitcode_type)
@@ skipping 162 matching lines @@
   -O<opt-level>               Optimize output file
   -M, --print-map             Print map file on standard output
   --whole-archive             Include all objects from following archives
   --no-whole-archive          Turn off --whole-archive
   -s, --strip-all             Strip all symbols
   -S, --strip-debug           Strip debugging symbols
   --undefined SYMBOL          Start with undefined reference to SYMBOL
 
   -help | -h                  Output this help.
 """