Support output constraints on "asm goto"

Summary:
Clang's "asm goto" feature didn't initially support outputs constraints. That
was the same behavior as gcc's implementation. The decision by gcc not to
support outputs was based on a restriction in their IR regarding terminators.
LLVM doesn't restrict terminators from returning values (e.g. 'invoke'), so
it made sense to support this feature.

Output values are valid only on the 'fallthrough' path. If an output value's used
on an indirect branch, then it's 'poisoned'.

In theory, outputs *could* be valid on the 'indirect' paths, but it's very
difficult to guarantee that the original semantics would be retained. E.g.
because indirect labels could be used as data, we wouldn't be able to split
critical edges in situations where two 'callbr' instructions have the same
indirect label, because the indirect branch's destination would no longer be
the same.

Reviewers: jyknight, nickdesaulniers, hfinkel

Reviewed By: jyknight, nickdesaulniers

Subscribers: MaskRay, rsmith, hiraditya, llvm-commits, cfe-commits, craig.topper, rnk

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D69876

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/lib/Analysis/UninitializedValues.cpp b/clang/lib/Analysis/UninitializedValues.cpp
index ff14e18..a4a900c 100644
--- a/clang/lib/Analysis/UninitializedValues.cpp
+++ b/clang/lib/Analysis/UninitializedValues.cpp
@@ -475,6 +475,7 @@ public:
   void VisitCallExpr(CallExpr *ce);
   void VisitDeclRefExpr(DeclRefExpr *dr);
   void VisitDeclStmt(DeclStmt *ds);
+  void VisitGCCAsmStmt(GCCAsmStmt *as);
   void VisitObjCForCollectionStmt(ObjCForCollectionStmt *FS);
   void VisitObjCMessageExpr(ObjCMessageExpr *ME);
   void VisitOMPExecutableDirective(OMPExecutableDirective *ED);
@@ -760,6 +761,16 @@ void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
   }
 }
 
+void TransferFunctions::VisitGCCAsmStmt(GCCAsmStmt *as) {
+  // An "asm goto" statement is a terminator that may initialize some variables.
+  if (!as->isAsmGoto())
+    return;
+
+  for (const auto &o : as->outputs())
+    if (const VarDecl *VD = findVar(o).getDecl())
+      vals[VD] = Initialized;
+}
+
 void TransferFunctions::VisitObjCMessageExpr(ObjCMessageExpr *ME) {
   // If the Objective-C message expression is an implicit no-return that
   // is not modeled in the CFG, set the tracked dataflow values to Unknown.
@@ -797,6 +808,10 @@ static bool runOnBlock(const CFGBlock *block, const CFG &cfg,
     if (Optional<CFGStmt> cs = I.getAs<CFGStmt>())
       tf.Visit(const_cast<Stmt *>(cs->getStmt()));
   }
+  CFGTerminator terminator = block->getTerminator();
+  if (GCCAsmStmt *as = dyn_cast_or_null<GCCAsmStmt>(terminator.getStmt()))
+    if (as->isAsmGoto())
+      tf.Visit(as);
   return vals.updateValueVectorWithScratch(block);
 }