[MIPS] Do not use pnacl-irt-link for MIPS target.

SConstruct

Index: SConstruct
diff --git a/SConstruct b/SConstruct
index f1ecbbed7d75053581579d28e07447e30b1564eb..6fb9470d3b9d7f3d7b1446fd77894e87ec90cbf1 100755
--- a/SConstruct
+++ b/SConstruct
@@ -3122,9 +3122,9 @@ if nacl_irt_env.Bit('bitcode'):
 # call to __nacl_read_tp, which the IRT code overrides to segregate
 # IRT-private TLS from user TLS.
 if nacl_irt_env.Bit('bitcode'):
-  nacl_irt_env.Append(LINKFLAGS=['--pnacl-allow-native',
-                                 '-Wt,-mtls-use-call',
-                                 '-Wl,--pnacl-irt-link'])
+  nacl_irt_env.Append(LINKFLAGS=['--pnacl-allow-native', '-Wt,-mtls-use-call'])
+  if nacl_irt_env.Bit('target_arm'):
+   nacl_irt_env.Append(LINKFLAGS=['-Wl,--pnacl-irt-link'])

[jvoung (off chromium), 2012/12/28 17:21:15]

The reason we have --pnacl-irt-link, is that the IRT is built with a different
ABI from PNaCl's "le32" ABI.  It's built with the ARM hard float gnueabi ABI on
ARM, for compatibility with GCC.  Then we have the "shim" built by clang to
bridge the le32 ABI with the gcc one, since clang knows about both ABIs and has
the source-level type information to do the ABI matching.

In order to achieve that, the bitcode needs to be biased toward that ABI, so the
bitcode libraries that the IRT uses, like libstdc++, has to be
architecture-biased libraries (not stock le32 bitcode libraries).

Perhaps file an issue and link it to a TODO for ironing out the MIPS ABI.  If we
want compatibility with some future MIPS gcc NaCl port, then I suspect we will
want to do something similar with mips-biased bitcode.  

(1) actually change the IRT build to use the chosen mips abi (like the code on
line 3112).

(2) build biased libraries for mips so that --pnacl-irt-link works.

(3) modify clang a bit to be able to generate the shim that bridges the two
ABIs.


nit: indentation (4 spaces instead of 3?)

[Mark Seaborn, 2012/12/28 18:06:39]

Fix indentation here, then LGTM.

petarj wrote:
Ah yes, of course, that makes sense.

Actually, this is the relevant part (line 3081):

# For x86 systems we build the IRT using the nnacl TC even when
# the pnacl TC is used otherwise
if nacl_irt_env.Bit('target_x86_64') or nacl_irt_env.Bit('target_x86_32'):
  nacl_irt_env.ClearBits('bitcode')

 elif nacl_irt_env.Bit('target_arm'):
   nacl_irt_env.Append(CCFLAGS=['-mtp=soft'])
 else: