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

[jamieschmeiser]

Summary

Change the warning produced for subtraction from (or with) a null pointer
to only be produced when the code path is live.

Respond to review comments: add cpp test.

[efriedma]

LGTM

[anarazel]

So far the tests in pointer-subtraction.c are mirrored in pointer-subtraction.cpp - worth continuing to do?

[jamieschmeiser]

Respond to review comments: add cpp test.

clang/lib/Sema/SemaExpr.cpp

static void diagnoseSubtractionOnNullPointer(Sema &S, SourceLocation Loc,
   // Null - null is valid in C++ [expr.add]p7
   if (BothNull && S.getLangOpts().CPlusPlus)
     return;
 
   // Is this s a macro from a system header?
   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.
 static void diagnoseArithmeticOnTwoFunctionPointers(Sema &S, SourceLocation Loc,
                                                     Expr *LHS, Expr *RHS) {
   assert(LHS->getType()->isAnyPointerType());
   assert(RHS->getType()->isAnyPointerType());
   S.Diag(Loc, S.getLangOpts().CPlusPlus

clang/test/Sema/pointer-subtraction.c

 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c11 -isystem %S/Inputs
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wnull-pointer-subtraction -std=c11 -isystem %S/Inputs
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c11 -isystem %S/Inputs -Wsystem-headers -DSYSTEM_WARNINGS
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wnull-pointer-subtraction -std=c11 -isystem %S/Inputs -Wsystem-headers -DSYSTEM_WARNINGS
 
 #include <pointer-subtraction.h>
 
 void a(void) {
   char *f = (char *)0;
   f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
   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
   SYSTEM_MACRO(f); // expected-warning {{performing pointer subtraction with a null pointer has undefined behavior}}
 #endif
 }

clang/test/Sema/pointer-subtraction.cpp

 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c++11 -isystem %S/Inputs
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wnull-pointer-subtraction -std=c++11 -isystem %S/Inputs
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c++11 -isystem %S/Inputs -Wsystem-headers -DSYSTEM_WARNINGS
 // RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wnull-pointer-subtraction -std=c++11 -isystem %S/Inputs -Wsystem-headers -DSYSTEM_WARNINGS
 
 #include <pointer-subtraction.h>
 
 void a() {
   char *f = (char *)0;
   f = (char *)((char *)0 - f);         // expected-warning {{performing pointer subtraction with a null pointer may have undefined behavior}}
   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
   SYSTEM_MACRO(f); // expected-warning {{performing pointer subtraction with a null pointer may have undefined behavior}}
 #endif
 }