[Analysis] Ignore casts and unary ops for uninitialized values

A series of unary operators and casts may obscure the variable we're
trying to analyze. Ignore them for the uninitialized value analysis.
Other checks determine if the unary operators result in a valid l-value.

Link: ClangBuiltLinux/linux#1521

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D114848

Author: isanbard

clang/lib/Analysis/UninitializedValues.cpp

@@ -591,8 +591,8 @@ class TransferFunctions : public StmtVisitor<TransferFunctions> {
 
         if (AtPredExit == MayUninitialized) {
           // If the predecessor's terminator is an "asm goto" that initializes
-          // the variable, then it won't be counted as "initialized" on the
-          // non-fallthrough paths.
+          // the variable, then don't count it as "initialized" on the indirect
+          // paths.
           CFGTerminator term = Pred->getTerminator();
           if (const auto *as = dyn_cast_or_null<GCCAsmStmt>(term.getStmt())) {
             const CFGBlock *fallthrough = *Pred->succ_begin();
@@ -810,13 +810,22 @@ void TransferFunctions::VisitGCCAsmStmt(GCCAsmStmt *as) {
   if (!as->isAsmGoto())
     return;
 
-  for (const Expr *o : as->outputs())
-    if (const VarDecl *VD = findVar(o).getDecl())
+  ASTContext &C = ac.getASTContext();
+  for (const Expr *O : as->outputs()) {
+    const Expr *Ex = stripCasts(C, O);
+
+    // Strip away any unary operators. Invalid l-values are reported by other
+    // semantic analysis passes.
+    while (const auto *UO = dyn_cast<UnaryOperator>(Ex))
+      Ex = stripCasts(C, UO->getSubExpr());
+
+    if (const VarDecl *VD = findVar(Ex).getDecl())
       if (vals[VD] != Initialized)
         // If the variable isn't initialized by the time we get here, then we
         // mark it as potentially uninitialized for those cases where it's used
         // on an indirect path, where it's not guaranteed to be defined.
         vals[VD] = MayUninitialized;
+  }
 }
 
 void TransferFunctions::VisitObjCMessageExpr(ObjCMessageExpr *ME) {

clang/test/Analysis/uninit-asm-goto.cpp

@@ -57,3 +57,15 @@ int test5(int x) {
 indirect:
   return -2;
 }
+
+// test6: Expect no diagnostics.
+int test6(unsigned int *x) {
+  unsigned int val;
+
+  // See through casts and unary operators.
+  asm goto("nop" : "=r" (*(unsigned int *)(&val)) ::: indirect);
+  *x = val;
+  return 0;
+indirect:
+  return -1;
+}