Subzero, MIPS32: Changes for improving sandbox crosstest results

pydir/crosstest.py

 #!/usr/bin/env python2
 
 import argparse
 import os
 import subprocess
 import sys
 import tempfile
 
 import targets
 from szbuild import LinkNonsfi
@@ skipping 140 matching lines @@
                   '--sandbox=' + str(args.sandbox),
                   '--nonsfi=' + str(args.nonsfi),
                   '--prefix=' + args.prefix,
                   '-allow-uninitialized-globals',
                   '-externalize',
                   '-filetype=' + args.filetype,
                   '-o=' + (obj_sz if args.filetype == 'obj' else asm_sz),
                   bitcode] + arch_sz_flags[args.target])
         if args.filetype != 'obj':
             shellcmd(['{bin}/llvm-mc'.format(bin=bindir),
-                      '-triple=' + triple,
+                      '-triple=' + ('mipsel-linux-gnu'

[Jim Stichnoth, 2016/11/21 20:08:39]

Is it possible to use a nacl-specific tuple for MIPS?  This is what the other
targets do, and that may also mean that the new .bundle_align_mode directive in
IceTargetLoweringMIPS32.cpp would not be needed.

[Stefan Maksimovic, 2016/11/22 11:38:13]

This change stems from the fact that when we run the crosstests in asm mode,
llvm-mc does its own sandboxing which results in duplicate instructions
generated, therefore exceeding the maximum bundle size.
We had two options, either to let subzero do the sandboxing and somehow
prevent llvm-mc from doing it, or vice versa.

We opted for the former, since letting llvm-mc do the sandboxing somehow
doesn't isolate t6, t7 and t8 registers which are used for storing masks
used in sandboxing, which in turn get used where they shouldn't be.
The way in which we accomplished that is to change the triple in this very
spot, i.e. from mipsel-nacl to mipsel-linux-gnu.
This in turn required that we generate the .bundle_align_mode directive.

We tried going the other way, but I am unaware whether there is a way to
tell llvm-mc to sequester said registers.

[Jim Stichnoth, 2016/11/22 18:25:30]

Ah, OK, that's very interesting.  Thanks for the explanation.

+                                    if args.target == 'mips32' and args.sandbox
+                                    else triple),
                       '-filetype=obj',
                       '-o=' + obj_sz,
                       asm_sz])
 
         # Each separately translated Subzero object file contains its own
         # definition of the __Sz_block_profile_info profiling symbol.  Avoid
         # linker errors (multiply defined symbol) by making all copies weak.
         # (This could also be done by Subzero if it supported weak symbol
         # definitions.)  This approach should be OK because cross tests are
         # currently the only situation where multiple translated files are
@@ skipping 98 matching lines @@
     objs.append(obj_llc)
     if args.nonsfi:
         LinkNonsfi(objs, os.path.join(args.dir, args.output), args.target)
     elif args.sandbox:
         LinkSandbox(objs, os.path.join(args.dir, args.output), args.target)
     else:
         LinkNative(objs, os.path.join(args.dir, args.output), args.target)
 
 if __name__ == '__main__':
     main()