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

[Nathan Sidwell via cfe-commits]

================
@@ -0,0 +1,192 @@
+// RUN: %clang_cc1 %s -O0 -Wstrict-aliasing -S -o %t -verify=quiet
+// RUN: %clang_cc1 %s -O2 -Wstrict-aliasing=0 -S -o %t -verify=quiet
+// RUN: %clang_cc1 %s -O2 -Wno-strict-aliasing -S -o %t -verify=quiet
+// RUN: %clang_cc1 %s -O2 -Wstrict-aliasing=1 -S -o %t -verify=level1,level12,level123
+// RUN: %clang_cc1 %s -O2 -Wstrict-aliasing=2 -S -o %t -verify=level2,level23,level12,level123
+// RUN: %clang_cc1 %s -O2 -Wstrict-aliasing=3 -S -o %t -verify=level23,level123
+// RUN: %clang_cc1 %s -O2 -Wstrict-aliasing -S -o %t -verify=level23,level123
+// RUN: %clang_cc1 %s -O2 -S -o %t -verify=level23,level123
+
+// quiet-no-diagnostics
+
+#if _LP64
+// These names make more sense on an ilp32 machine
+typedef long INT;
+typedef long long LONG;
+typedef unsigned long UINT;
+#else
+typedef int INT;
+typedef long LONG;
+typedef unsigned int UINT;
+#endif
+typedef short SHORT;
+
+INT ScalarINT;
+INT Ary[2];
+struct {int m;} Struct;
+
+_Complex int CPLx;
+
+void ByVal(long long);
+void ByPtr(void *);
+
+void VarPtr(INT *Ptr) {
+  // GCC: 1
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByPtr((LONG *)(Ptr));
+  // level1-note at -1{{not alias compatible}}
+
+  // GCC:
+  ByPtr((LONG *)((void *)(Ptr)));
+
+  // GCC: 1
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(Ptr));
+  // level1-note at -1{{not alias compatible}}
+}
+
+void Object() {
+  // GCC: 1, 2
+  // level2-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByPtr((LONG *)(&ScalarINT));
+  // level12-note at -1{{not alias compatible}}
+
+  // GCC:
+  ByPtr((LONG *)((void *)(&ScalarINT)));
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&ScalarINT));
+  // level123-note at -1{{not alias compatible}}
+}
+
+// Level 1, 2, 3 - 1, 2, 3
+void DetectedVariants() {
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&Ary[1]));
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&Struct.m));
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&(&Struct)->m));
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&__real__(CPLx)));
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*(LONG *)(&__imag__(CPLx)));
+  // level123-note at -1{{not alias compatible}}
+}
+
+void Ok() {
+  // GCC:
+  ByPtr((UINT *)(&ScalarINT));
+  // GCC:
+  ByPtr((UINT *)((void *)(&ScalarINT)));
+  // GCC:
+  ByVal(*(UINT *)(&ScalarINT));
+}
+
+// Level 1, 2, 3 - 1, 2, 3
+void Parens() {
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*((LONG *)((&(ScalarINT)))));
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ByVal(*((LONG *)((&(Ary[1])))));
+  // level123-note at -1{{not alias compatible}}
+}
+
+// Clang models may_alias as a decl attribute, not a type attribute.
+
+typedef int MA __attribute__((may_alias));
+
+void Frob(MA *a) {
+  ByPtr((short *)(a));
+  ByVal(*(short *)(a));
+}
+
+struct Inner { int m; };
+struct Outer1 { struct Inner i; };
+struct Outer2 { struct Outer1 o; };
+struct Inner i;
+struct Outer2 o;
+
+void ByValInner (struct Inner);
+void ByValOuter2 (struct Outer2);
+
+void Inherit() {
+  // Check we see through multiple levels
+  int in;
+
+  ByValOuter2(*(struct Outer2 *)&in);
+  ByValOuter2(*(struct Outer2 *)&i);
+  ByValInner(*(struct Inner *)&o.o);
+  ByValInner(*(struct Inner *)&o);
+  ByVal(*(int *)&o);
+}
+
+// PR 50066
+typedef unsigned char uchar;
+
+void Double(double);
+
+int main() {
+  double d = 2.34;
+  int i[2];
+  Double(d);
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  *(long long *)i =
+  // level123-note at -1{{not alias compatible}}
+
+      // GCC: 1, 2, 3
+      // level23-warning at +2{{type-punned pointer breaks}}
+      // level1-warning at +1{{type-punned pointer might break}}
+      *(long long *)&d;
+      // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2, 3
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ((int *)&d)[0] = i[0];
+  // level123-note at -1{{not alias compatible}}
+
+  // GCC: 1, 2
+  // level23-warning at +2{{type-punned pointer breaks}}
+  // level1-warning at +1{{type-punned pointer might break}}
+  ((int *)&d)[1] = i[1];
+  // level123-note at -1{{not alias compatible}}
+
+  Double(d);
+  ((uchar *)&d)[2] += 123;
+  Double(d);
+  return 0;
+}
+
----------------
urnathan wrote:

AFAICT that's the idiom for rvalue conversion -- what I've done with `ByVal` or `RValue` calls.  I've added an example though.

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