Only issue warning for subtraction involving null pointers on live code paths

Summary:
Change the warning produced for subtraction from (or with) a null pointer
to only be produced when the code path is live.
https://github.com/llvm/llvm-project/issues/54570

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: anarazel (Andres Freund)
Differential Revision: https://reviews.llvm.org/D126816

clang/lib/Sema/SemaExpr.cpp

@@ -10845,8 +10845,10 @@ static void diagnoseSubtractionOnNullPointer(Sema &S, SourceLocation Loc,
   if (S.Diags.getSuppressSystemWarnings() && S.SourceMgr.isInSystemMacro(Loc))
     return;
 
-  S.Diag(Loc, diag::warn_pointer_sub_null_ptr)
-      << S.getLangOpts().CPlusPlus << Pointer->getSourceRange();
+  S.DiagRuntimeBehavior(Loc, Pointer,
+                        S.PDiag(diag::warn_pointer_sub_null_ptr)
+                            << S.getLangOpts().CPlusPlus
+                            << Pointer->getSourceRange());
 }
 
 /// Diagnose invalid arithmetic on two function pointers.

clang/test/Sema/pointer-subtraction.c

@@ -11,6 +11,16 @@ void a(void) {
   f = (char *)(f - (char *)0);         // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
   f = (char *)((char *)0 - (char *)0); // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}} expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
 
+  if (1)
+    f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
+  else
+    f = (char *)((char *)0 - f);
+
+  if (0)
+    f = (char *)((char *)0 - f);
+  else
+    f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
+
 #ifndef SYSTEM_WARNINGS
   SYSTEM_MACRO(f);
 #else

clang/test/Sema/pointer-subtraction.cpp

@@ -11,6 +11,16 @@ void a() {
   f = (char *)(f - (char *)0);         // expected-warning {{performing pointer subtraction with a null pointer may have undefined behavior}}
   f = (char *)((char *)0 - (char *)0); // valid in C++
 
+  if (1)
+    f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer may have undefined behavior}}
+  else
+    f = (char *)((char *)0 - f);
+
+  if (0)
+    f = (char *)((char *)0 - f);
+  else
+    f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer may have undefined behavior}}
+
 #ifndef SYSTEM_WARNINGS
   SYSTEM_MACRO(f);
 #else