[CHERI CSA] When inferring the alignment of the first field in a struct, use the alignment of the struct

itself to help infer higher alignment.

Author: resistor

clang/lib/StaticAnalyzer/Checkers/PointerAlignmentChecker.cpp

@@ -190,6 +190,20 @@ int getTrailingZerosCount(const MemRegion *R, ProgramStateRef State,
       return -1;
     unsigned NaturalAlign = ASTCtx.getTypeAlignInChars(PT).getQuantity();
 
+    if (const FieldRegion *FR = R->getAs<FieldRegion>()) {
+      // If this is the first field of a larger struct, we can use the alignment
+      // of the containing struct try to increase the assumed alignment.
+      const RegionOffset &Offset = FR->getAsOffset();
+      if (!Offset.hasSymbolicOffset() && Offset.getOffset() == 0) {
+        if (const auto *Base =
+                FR->getSuperRegion()->getAs<TypedValueRegion>()) {
+          auto BaseTy = Base->getValueType();
+          unsigned BaseAlign = ASTCtx.getTypeAlignInChars(BaseTy).getQuantity();
+          NaturalAlign = std::max(NaturalAlign, BaseAlign);
+        }
+      }
+    }
+
     if (const ElementRegion *ER = R->getAs<ElementRegion>()) {
       int ElTyTZ = APSInt::getUnsigned(NaturalAlign).countTrailingZeros();
 

clang/test/Analysis/pointer-alignment.c

@@ -202,3 +202,15 @@ void cast_and_assign(void) {
                      // no duplicate assign warn
   *i = 42;
 }
+
+// ----
+struct __attribute__((aligned(8))) AlignedParentStruct {
+    int a;
+    int b;
+};
+
+void write_to_first_member(struct AlignedParentStruct *chunk) {
+  // No warning because alignment of is inferred from the parent struct.
+  *(long long*)(&chunk->a) = 0;
+}
+