[clang][sema] Ignore xor-used-as-pow if both sides are macros

[tbaeder]

Summary

I've read both https://reviews.llvm.org/D63423 and https://reviews.llvm.org/D66397, but I think both of them miss the case where both RHS and LHS of the XOR are macros, and thus the warning should not be emitted (I think).

I ran into this while compiling elfutils with clang and all the available changes to the code seem non-sensical and the warning in clang seems misplaced given that both operands are macros, even if the left side evaluates to 2 and the right side to 10, for example. They are just used as state.

I know this warning (in conjunction with macros especially) has been discussed a lot in the past (as far as I know regarding the Chrome code base), but this patch is very conservative regarding the change to the warning semantics. And the case it changes was not even covered by the tests. I've added a test for it now.

[xbolva00]

No problem, +1

[aaron.ballman]

LGTM with a small nit, thanks!

clang/lib/Sema/SemaExpr.cpp

12105

[tbaeder]

Thanks everyone!

clang/lib/Sema/SemaExpr.cpp

 
 static void diagnoseXorMisusedAsPow(Sema &S, const ExprResult &XorLHS,
                                     const ExprResult &XorRHS,
                                     const SourceLocation Loc) {
   // Do not diagnose macros.
   if (Loc.isMacroID())
     return;
 
+  // Do not diagnose if both LHS and RHS are macros.
+  if (XorLHS.get()->getExprLoc().isMacroID() &&
+      XorRHS.get()->getExprLoc().isMacroID())
+    return;
+
   bool Negative = false;
   bool ExplicitPlus = false;
   const auto *LHSInt = dyn_cast<IntegerLiteral>(XorLHS.get());
   const auto *RHSInt = dyn_cast<IntegerLiteral>(XorRHS.get());
 
   if (!LHSInt)
     return;
   if (!RHSInt) {

clang/test/SemaCXX/warn-xor-as-pow.cpp

void test(unsigned a, unsigned b) {
   res = res + (2 ^ ALPHA_OFFSET); // expected-warning {{result of '2 ^ ALPHA_OFFSET' is 1; did you mean '1 << ALPHA_OFFSET' (8)?}}
   // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:16-[[@LINE-1]]:32}:"1 << ALPHA_OFFSET"
   // expected-note@-2 {{replace expression with '0x2 ^ ALPHA_OFFSET' or use 'xor' instead of '^' to silence this warning}}
   res = 0x2 ^ 16;
   res = 2 xor 16;
 
   res = 2 ^ 0x4;
   res = 2 ^ 04;
+  res = TWO ^ TEN;
   res = 0x2 ^ 10;
   res = 0X2 ^ 10;
   res = 02 ^ 10;
   res = FOOBAR(2, 16);
   res = 0b10 ^ 16;
   res = 0B10 ^ 16;
   res = 2 ^ 0b100;
   res = XOR(2, 16);