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

[ahatanak]

Summary

Sema::CheckCastAlign currently ignores the aligned attribute attached to the declared variables, which causes clang to issue spurious warnings. This patch fixes the bug.

[ahatanak]

Rebase and ping.

[arphaman]

lib/Sema/SemaChecking.cpp

10304 ↗

(On Diff #78300)

NIT, but I think you don't need the extra variable and the if (SE) below if you extract the code inside if (SE) { into a static function that returns SrcAlign. Then the if/else below can be rewritten like:

if (auto *CE = dyn_cast<CastExpr>(Op)) {
  if (CE->getCastKind() == CK_ArrayToPointerDecay)
    SrcAlign = newFunction(CE->getSubExpr(), Context);
} else if (auto *UO = dyn_cast<UnaryOperator>(Op)) {
  if (UO->getOpcode() == UO_AddrOf)
    SrcAlign = newFunction(UO->getSubExpr(), Context);
}

10315 ↗

(On Diff #78300)

You can use const auto here.

10317 ↗

(On Diff #78300)

Same here.

[ahatanak]

Address Alex's review comments.

Define a static function for setting SrcAlign.

[arphaman]

lib/Sema/SemaChecking.cpp

10270 ↗

(On Diff #78399)

I'm not sure if you can, since I don't really know CharUnits, but I think you should just return it instead of passing it as a parameter if it's a POD type (if you will end up doing that you should rename this function as well).

10270 ↗

(On Diff #78399)

Please add a brief doc comment as well.

[ahatanak]

Make the helper function return the Decl's alignment by value.

[arphaman]

LGTM, Thanks

cfe/trunk/lib/Sema/SemaChecking.cpp

if (const auto *Attr = Param->getAttr<PassObjectSizeAttr>())
       if (!Param->getType().isConstQualified())
         Diag(Param->getLocation(), diag::err_attribute_pointers_only)
             << Attr->getSpelling() << 1;
   }
 
   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) {
   // This is actually a lot of work to potentially be doing on every
   // cast; don't do it if we're ignoring -Wcast_align (as is the default).
   if (getDiagnostics().isIgnored(diag::warn_cast_align, TRange.getBegin()))
     return;
 
void Sema::CheckCastAlign(Expr *Op, QualType T, SourceRange TRange) {
 
   // Whitelist casts from cv void*.  We already implicitly
   // whitelisted casts to cv void*, since they have alignment 1.
   // Also whitelist casts involving incomplete types, which implicitly
   // includes 'void'.
   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)
     << Op->getType() << T
     << static_cast<unsigned>(SrcAlign.getQuantity())
     << static_cast<unsigned>(DestAlign.getQuantity())
     << TRange << Op->getSourceRange();
 }

cfe/trunk/test/Sema/warn-cast-align.c

 void test2(char *P) {
   struct A *a = (struct A*) P; // expected-warning {{cast from 'char *' to 'struct A *' increases required alignment from 1 to 16}}
 }
 
 // Incomplete type.
 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;
+}