[coroutines] Promote cleanup.dest.slot allocas to registers to avoid storing it in the coroutine frame

Summary:
We don't want to store cleanup dest slot saved into the coroutine frame (as some of the cleanup code may
access them after coroutine frame destroyed).

This is an alternative to https://reviews.llvm.org/D37093

It is possible to do this for all functions, but, cursory check showed that in -O0, we get slightly longer function (by 1-3 instructions), thus, we are only limiting cleanup.dest.slot elimination to coroutines.

Reviewers: rjmccall, hfinkel, eric_niebler

Reviewed By: eric_niebler

Subscribers: EricWF, cfe-commits

Differential Revision: https://reviews.llvm.org/D39768

llvm-svn: 317981

1 files changed, 14 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 3b60b996ca19..37d174dd6635 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -33,9 +33,11 @@
 #include "clang/Frontend/CodeGenOptions.h"
 #include "clang/Sema/SemaDiagnostic.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/Transforms/Utils/PromoteMemToReg.h"
 using namespace clang;
 using namespace CodeGen;
 
@@ -419,6 +421,18 @@ void CodeGenFunction::FinishFunction(SourceLocation EndLoc) {
     I->first->replaceAllUsesWith(I->second);
     I->first->eraseFromParent();
   }
+
+  // Eliminate CleanupDestSlot alloca by replacing it with SSA values and
+  // PHIs if the current function is a coroutine. We don't do it for all
+  // functions as it may result in slight increase in numbers of instructions
+  // if compiled with no optimizations. We do it for coroutine as the lifetime
+  // of CleanupDestSlot alloca make correct coroutine frame building very
+  // difficult.
+  if (NormalCleanupDest && isCoroutine()) {
+    llvm::DominatorTree DT(*CurFn);
+    llvm::PromoteMemToReg(NormalCleanupDest, DT);
+    NormalCleanupDest = nullptr;
+  }
 }
 
 /// ShouldInstrumentFunction - Return true if the current function should be