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, 3 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86.h b/llvm/lib/Target/X86/X86.h
index c12554c9b92..27b428e2151 100644
--- a/llvm/lib/Target/X86/X86.h
+++ b/llvm/lib/Target/X86/X86.h
@@ -59,6 +59,9 @@ FunctionPass *createX86FixupLEAs();
 /// recalculations.
 FunctionPass *createX86OptimizeLEAs();
 
+/// Return a pass that expands WinAlloca pseudo-instructions.
+FunctionPass *createX86WinAllocaExpander();
+
 /// Return a pass that optimizes the code-size of x86 call sequences. This is
 /// done by replacing esp-relative movs with pushes.
 FunctionPass *createX86CallFrameOptimization();