WholeProgramDevirt: introduce.

This pass implements whole program optimization of virtual calls in cases
where we know (via bitset information) that the list of callees is fixed. This
includes the following:

- Single implementation devirtualization: if a virtual call has a single
  possible callee, replace all calls with a direct call to that callee.

- Virtual constant propagation: if the virtual function's return type is an
  integer <=64 bits and all possible callees are readnone, for each class and
  each list of constant arguments: evaluate the function, store the return
  value alongside the virtual table, and rewrite each virtual call as a load
  from the virtual table.

- Uniform return value optimization: if the conditions for virtual constant
  propagation hold and each function returns the same constant value, replace
  each virtual call with that constant.

- Unique return value optimization for i1 return values: if the conditions
  for virtual constant propagation hold and a single vtable's function
  returns 0, or a single vtable's function returns 1, replace each virtual
  call with a comparison of the vptr against that vtable's address.

Differential Revision: http://reviews.llvm.org/D16795

llvm-svn: 260312

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 4798d4e88d8..18d321b3106 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -651,6 +651,16 @@ void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   PM.add(createJumpThreadingPass());
 }
 
+void PassManagerBuilder::addEarlyLTOOptimizationPasses(
+    legacy::PassManagerBase &PM) {
+  // Remove unused virtual tables to improve the quality of code generated by
+  // whole-program devirtualization and bitset lowering.
+  PM.add(createGlobalDCEPass());
+
+  // Apply whole-program devirtualization and virtual constant propagation.
+  PM.add(createWholeProgramDevirtPass());
+}
+
 void PassManagerBuilder::addLateLTOOptimizationPasses(
     legacy::PassManagerBase &PM) {
   // Delete basic blocks, which optimization passes may have killed.
@@ -675,6 +685,9 @@ void PassManagerBuilder::populateLTOPassManager(legacy::PassManagerBase &PM) {
   if (VerifyInput)
     PM.add(createVerifierPass());
 
+  if (OptLevel != 0)
+    addEarlyLTOOptimizationPasses(PM);
+
   if (OptLevel > 1)
     addLTOOptimizationPasses(PM);