Lower signatures exposing struct registers to byval struct pointers

The new phase, NormalizeStructRegSignatures, converts signatures having parameters, or returning struct registers; or using structs that transitively reference such function types.

The phase relies on a subsequent phase to clean up the redundant alloca/load/store instructions, however, I'm still investigating which such phase should be.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3857
R=dschuff@chromium.org, jfb@chromium.org, mseaborn@chromium.org

Review URL: https://codereview.chromium.org/992493002

Author: mtrofin

include/llvm/InitializePasses.h

@@ -313,6 +313,7 @@ void initializeGlobalCleanupPass(PassRegistry&);
 void initializeGlobalizeConstantVectorsPass(PassRegistry&);
 void initializeInsertDivideCheckPass(PassRegistry&);
 void initializeNaClCcRewritePass(PassRegistry&);
+void initializeSimplifyStructRegSignaturesPass(PassRegistry&);
 void initializePNaClABIVerifyFunctionsPass(PassRegistry&);
 void initializePNaClABIVerifyModulePass(PassRegistry&);
 void initializePNaClSjLjEHPass(PassRegistry&);

include/llvm/Transforms/NaCl.h

@@ -52,6 +52,7 @@ ModulePass *createExpandVarArgsPass();
 ModulePass *createFlattenGlobalsPass();
 ModulePass *createGlobalCleanupPass();
 ModulePass *createGlobalizeConstantVectorsPass();
+ModulePass *createSimplifyStructRegSignaturesPass();
 ModulePass *createPNaClSjLjEHPass();
 ModulePass *createReplacePtrsWithIntsPass();
 ModulePass *createResolveAliasesPass();

lib/Transforms/NaCl/CMakeLists.txt

@@ -34,6 +34,7 @@ add_llvm_library(LLVMNaClTransforms
   RewriteLLVMIntrinsics.cpp
   RewritePNaClLibraryCalls.cpp
   SimplifyAllocas.cpp
+  SimplifyStructRegSignatures.cpp
   StripAttributes.cpp
   StripMetadata.cpp
   )

lib/Transforms/NaCl/PNaClABISimplify.cpp

@@ -154,6 +154,9 @@ void llvm::PNaClABISimplifyAddPostOptPasses(PassManagerBase &PM) {
   // ConstantExprs have already been expanded out.
   PM.add(createReplacePtrsWithIntsPass());
 
+  // Convert struct reg function params to struct* byval
+  PM.add(createSimplifyStructRegSignaturesPass());
+
   // The atomic cmpxchg instruction returns a struct, and is rewritten to an
   // intrinsic as a post-opt pass, we therefore need to expand struct regs.
   PM.add(createExpandStructRegsPass());