[Sema] Teach -Wcast-align to look at the aligned attribute of the

declared variables.

Teach Sema to check the aligned attribute attached to variable
declarations so that it doesn't issue spurious warnings.

rdar://problem/26517471

Differential revision: https://reviews.llvm.org/D21099

llvm-svn: 288267

2 files changed, 42 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 4fe30536fdf..9e16554c2f9 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -10264,6 +10264,19 @@ bool Sema::CheckParmsForFunctionDef(ArrayRef<ParmVarDecl *> Parameters,
   return HasInvalidParm;
 }
 
+/// A helper function to get the alignment of a Decl referred to by DeclRefExpr
+/// or MemberExpr.
+static CharUnits getDeclAlign(Expr *E, CharUnits TypeAlign,
+                              ASTContext &Context) {
+  if (const auto *DRE = dyn_cast<DeclRefExpr>(E))
+    return Context.getDeclAlign(DRE->getDecl());
+
+  if (const auto *ME = dyn_cast<MemberExpr>(E))
+    return Context.getDeclAlign(ME->getMemberDecl());
+
+  return TypeAlign;
+}
+
 /// CheckCastAlign - Implements -Wcast-align, which warns when a
 /// pointer cast increases the alignment requirements.
 void Sema::CheckCastAlign(Expr *Op, QualType T, SourceRange TRange) {
@@ -10298,6 +10311,15 @@ void Sema::CheckCastAlign(Expr *Op, QualType T, SourceRange TRange) {
   if (SrcPointee->isIncompleteType()) return;
 
   CharUnits SrcAlign = Context.getTypeAlignInChars(SrcPointee);
+
+  if (auto *CE = dyn_cast<CastExpr>(Op)) {
+    if (CE->getCastKind() == CK_ArrayToPointerDecay)
+      SrcAlign = getDeclAlign(CE->getSubExpr(), SrcAlign, Context);
+  } else if (auto *UO = dyn_cast<UnaryOperator>(Op)) {
+    if (UO->getOpcode() == UO_AddrOf)
+      SrcAlign = getDeclAlign(UO->getSubExpr(), SrcAlign, Context);
+  }
+
   if (SrcAlign >= DestAlign) return;
 
   Diag(TRange.getBegin(), diag::warn_cast_align)
diff --git a/clang/test/Sema/warn-cast-align.c b/clang/test/Sema/warn-cast-align.c
index 9d64699bb5b..e8f85bc14d8 100644
--- a/clang/test/Sema/warn-cast-align.c
+++ b/clang/test/Sema/warn-cast-align.c
@@ -39,3 +39,23 @@ void test2(char *P) {
 void test3(char *P) {
   struct B *b = (struct B*) P;
 }
+
+// Do not issue a warning. The aligned attribute changes the alignment of the
+// variables and fields.
+char __attribute__((aligned(4))) a[16];
+
+struct S0 {
+  char a[16];
+};
+
+struct S {
+  char __attribute__((aligned(4))) a[16];
+  struct S0 __attribute__((aligned(4))) s0;
+};
+
+void test4() {
+  struct S s;
+  int *i = (int *)s.a;
+  i = (int *)&s.s0;
+  i = (int *)a;
+}