[clang] [clang] Strict aliasing warning ala GCC [PR50066] (PR #74155)

[Vlad Serebrennikov via cfe-commits]

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@@ -2026,6 +2027,137 @@ static TryCastResult TryConstCast(Sema &Self, ExprResult &SrcExpr,
   return TC_Success;
 }
 
+// We're dereferencing E, either by turning into an RValue, or by dereferencing
+// it. Check whether it's a deref of a reinterpret cast that has aliasing
+// issues.
+void Sema::CheckStrictAliasingDeref(Expr const *E, bool IsLValue) {
+  if (Diags.getDiagnosticOptions().StrictAliasing < 3)
+    return;
+
+  assert(IsLValue || E->getType()->isAnyPointerType());
+  CastExpr const *CE = nullptr;
+  for (;;) {
+    CE = dyn_cast<CastExpr>(E->IgnoreParens());
+    if (!CE)
+      return;
+
+    if (IsLValue || CE->getCastKind() != CK_ArrayToPointerDecay)
+      break;
+
+    E = CE->getSubExpr();
+    IsLValue = true;
+  }
+
+  if (CE->getCastKind() != (IsLValue ? CK_LValueBitCast : CK_BitCast))
+    return;
+
+  if (CE->getSubExpr()->getType()->isVoidPointerType())
+    return;
+
+  QualType DestTy = CE->getType();
+  if (!IsLValue)
+    DestTy = DestTy->getPointeeType();
+
+  CheckStrictAliasing(CE->getSubExpr(), DestTy, IsLValue, CE->getSourceRange());
+}
+
+/// We're building a cast from E to pointer type DestType. If ISLValueCast is
+/// true, DestType is the pointer equivalent of the reference type we're casting
+/// to.
+void Sema::CheckStrictAliasingCast(Expr const *E, QualType DestType,
+                                   bool IsLValueCast, SourceRange Range) {
+  if (Diags.getDiagnosticOptions().StrictAliasing < 1 ||
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Endilll wrote:

Switch might be a better fit here.

https://github.com/llvm/llvm-project/pull/74155