X86: Avoid using _chkstk when lowering WIN_ALLOCA instructions

This patch moves the expansion of WIN_ALLOCA pseudo-instructions
into a separate pass that walks the CFG and lowers the instructions
based on a conservative estimate of the offset between the stack
pointer and the lowest accessed stack address.

The goal is to reduce binary size and run-time costs by removing
calls to _chkstk. While it doesn't fix all the code quality problems
with inalloca calls, it's an incremental improvement for PR27076.

Differential Revision: http://reviews.llvm.org/D20263

llvm-svn: 269828

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86TargetMachine.cpp b/llvm/lib/Target/X86/X86TargetMachine.cpp
index e0f63de..1d7c126 100644
--- a/llvm/lib/Target/X86/X86TargetMachine.cpp
+++ b/llvm/lib/Target/X86/X86TargetMachine.cpp
@@ -266,6 +266,7 @@ void X86PassConfig::addPreRegAlloc() {
     addPass(createX86OptimizeLEAs());
 
   addPass(createX86CallFrameOptimization());
+  addPass(createX86WinAllocaExpander());
 }
 
 void X86PassConfig::addPostRegAlloc() {