[Parse] Use CapturedStmt for @finally on MSVC

The body of a `@finally` needs to be executed on both exceptional and
non-exceptional paths. On landingpad platforms, this is straightforward:
the `@finally` body is emitted as a normal (non-exceptional) cleanup,
and then a catch-all is emitted which branches to that cleanup (the
cleanup has code to conditionally re-throw based on a flag which is set
by the catch-all).

Unfortunately, we can't use the same approach for MSVC exceptions, where
the catch-all will be emitted as a catchpad. We can't just branch to the
cleanup from within the catchpad, since we can only exit it via a
catchret, at which point the exception is destroyed and we can't
rethrow. We could potentially emit the finally body inside the catchpad
and have the normal cleanup path somehow branch into it, but that would
require some new IR construct that could branch into a catchpad.

Instead, after discussing it with Reid Kleckner, we decided that
frontend outlining was the best approach, similar to how SEH `__finally`
works today. We decided to use CapturedStmt (which was also suggested by
Reid) rather than CaptureFinder (which is what `__finally` uses) since
the latter doesn't handle a lot of cases we care about, e.g. self
accesses, property accesses, block captures, etc. Extending
CaptureFinder to handle those additional cases proved unwieldy, whereas
CapturedStmt already took care of all of those.  In theory `__finally`
could also be moved over to CapturedStmt, which would remove some
existing limitations (e.g. the inability to capture this), although
CaptureFinder would still be needed for SEH filters.

The one case supported by `@finally` but not CapturedStmt (or
CaptureFinder for that matter) is arbitrary control flow out of the
`@finally`, e.g. having a return statement inside a `@finally`. We can
add that support as a follow-up, but in practice we've found it to be
used very rarely anyway.

Differential Revision: https://reviews.llvm.org/D47564

llvm-svn: 334224

5 files changed, 32 insertions, 2 deletions

diff --git a/clang/include/clang/AST/Stmt.h b/clang/include/clang/AST/Stmt.h
index 03c808d394e..3b678dcc754 100644
--- a/clang/include/clang/AST/Stmt.h
+++ b/clang/include/clang/AST/Stmt.h
@@ -2133,7 +2133,7 @@ private:
 
   /// The pointer part is the implicit the outlined function and the 
   /// int part is the captured region kind, 'CR_Default' etc.
-  llvm::PointerIntPair<CapturedDecl *, 1, CapturedRegionKind> CapDeclAndKind;
+  llvm::PointerIntPair<CapturedDecl *, 2, CapturedRegionKind> CapDeclAndKind;
 
   /// The record for captured variables, a RecordDecl or CXXRecordDecl.
   RecordDecl *TheRecordDecl = nullptr;
diff --git a/clang/include/clang/Basic/CapturedStmt.h b/clang/include/clang/Basic/CapturedStmt.h
index 6c8d6edab0b..324e1b1d3d0 100644
--- a/clang/include/clang/Basic/CapturedStmt.h
+++ b/clang/include/clang/Basic/CapturedStmt.h
@@ -16,6 +16,7 @@ namespace clang {
 /// The different kinds of captured statement.
 enum CapturedRegionKind {
   CR_Default,
+  CR_ObjCAtFinally,
   CR_OpenMP
 };
 
diff --git a/clang/include/clang/Sema/ScopeInfo.h b/clang/include/clang/Sema/ScopeInfo.h
index b0f6bac9946..fa1b1037a2f 100644
--- a/clang/include/clang/Sema/ScopeInfo.h
+++ b/clang/include/clang/Sema/ScopeInfo.h
@@ -748,6 +748,8 @@ public:
     switch (CapRegionKind) {
     case CR_Default:
       return "default captured statement";
+    case CR_ObjCAtFinally:
+      return "Objective-C @finally statement";
     case CR_OpenMP:
       return "OpenMP region";
     }
diff --git a/clang/lib/Parse/ParseObjc.cpp b/clang/lib/Parse/ParseObjc.cpp
index 6ca0ad855f5..ff33d5fb96b 100644
--- a/clang/lib/Parse/ParseObjc.cpp
+++ b/clang/lib/Parse/ParseObjc.cpp
@@ -2585,13 +2585,26 @@ StmtResult Parser::ParseObjCTryStmt(SourceLocation atLoc) {
       ParseScope FinallyScope(this,
                               Scope::DeclScope | Scope::CompoundStmtScope);
 
+      bool ShouldCapture =
+          getTargetInfo().getTriple().isWindowsMSVCEnvironment();
+      if (ShouldCapture)
+        Actions.ActOnCapturedRegionStart(Tok.getLocation(), getCurScope(),
+                                         CR_ObjCAtFinally, 1);
+
       StmtResult FinallyBody(true);
       if (Tok.is(tok::l_brace))
         FinallyBody = ParseCompoundStatementBody();
       else
         Diag(Tok, diag::err_expected) << tok::l_brace;
-      if (FinallyBody.isInvalid())
+
+      if (FinallyBody.isInvalid()) {
         FinallyBody = Actions.ActOnNullStmt(Tok.getLocation());
+        if (ShouldCapture)
+          Actions.ActOnCapturedRegionError();
+      } else if (ShouldCapture) {
+        FinallyBody = Actions.ActOnCapturedRegionEnd(FinallyBody.get());
+      }
+
       FinallyStmt = Actions.ActOnObjCAtFinallyStmt(AtCatchFinallyLoc,
                                                    FinallyBody.get());
       catch_or_finally_seen = true;
diff --git a/clang/test/SemaObjC/finally-msvc.m b/clang/test/SemaObjC/finally-msvc.m
new file mode 100644
index 00000000000..5db08a7f710
--- /dev/null
+++ b/clang/test/SemaObjC/finally-msvc.m
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -triple i686--windows-msvc -fexceptions -fobjc-exceptions -ast-dump %s 2>&1 | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64--windows-msvc -fexceptions -fobjc-exceptions -ast-dump %s 2>&1 | FileCheck %s
+
+void f() {
+  @try {
+  } @finally {
+  }
+}
+
+// CHECK:      ObjCAtFinallyStmt
+// CHECK-NEXT:   CapturedStmt
+// CHECK-NEXT:     CapturedDecl
+// CHECK-NEXT:       CompoundStmt
+// CHECK-NEXT:       ImplicitParamDecl