[clang][sema] Unary not boolean to int conversion

[AshleyRoll]

Summary

I have modifyed GetExprRange() to capture the promotion
of boolean values to ints when applying unary not to ensure
-Wsign-compare identifies unsigned/signed comparisons as
identified in:

https://github.com/llvm/llvm-project/issues/18878

[AshleyRoll]

Rebased to remove clangd test failure

[erichkeane]

clang/lib/Sema/SemaChecking.cpp

12328–12335

Richard mentions UO_PreInc, UO_PreDec, UO_Minus, and UO_Not. What about the other 3?

12330

12332

Can you trace what MaxWidth ends up being every time? Are we sure this will always just be int-width and not long long or something?

[aaron.ballman]

clang/lib/Sema/SemaChecking.cpp

12328–12335

UO_Plus also.

[rsmith]

Thank you for the patch! This is certainly an improvement but I think there are still some cases where we compute the wrong range for ~ with this patch applied.

clang/lib/Sema/SemaChecking.cpp

12328–12335

Taking those in turn:

• UO_PreInc and UO_PreDec can only produce values in the range of the lvalue operand; we'll never have a more precise range for an lvalue than the range from its type unless it's a bit-field, in which case we *do* want to take the bit-field's width into account. So I think the current code is actually correct for those two -- looking at the range of the operand gives the right result even for bit-fields, whereas looking at the type of the operand would give a less precise result in that case.
• UO_Minus is not giving the right answer. I think we should model -expr in exactly the same way we model 0 - expr -- see the code for BO_Sub above.
• UO_Plus seems OK with the current code: the range of values of +expr is the same as the range of values of expr, even if a promotion happens.

In any case, I don't think we should have a default here that returns the width of the operand. Instead, I think the default should return the range for the type of E.

12334

I don't think falling through and picking up the expression range of the operand is ever correct for ~. For example:

bool f(char c) {
  return ~c > 0x10000;
}

... produces a bogus tautological comparison warning. (This is not tautological: the ~ operator will map negative chars to ints in the range [2^31 - 2^7, 2^31) and non-negative chars to ints in the range [-2^31, -2^31+2^7), so this is equivalent to c < 0.)

Also, using MaxWidth here is conservatively correct but isn't precise; for example, we should still warn on:

bool f(bool b) {
  return ~b > 0x1'0000'0000LL;
}

... because ~b always fits in an int, but I think MaxWidth here will be 64 so we won't warn. (It'd be nice to warn even on ~b > 0 but I don't think we can do that without some major changes to how IntRange is represented.)

The true result range of ~n will be something like [-2^N, -2^N + 2^M) u [2^N - 2^M, 2^N) if the input is signed. IntRange can't represent a range with a hole in the middle like that, but it can represent [-2^N, 2^N), which seems like the least wrong answer that we can give -- that is notably also the entire range of the type of the ~ expression. We get a contiguous range [-2^N, -2^N + 2^M) if the input is known non-negative, but IntRange still can't represent that, and so that doesn't help us make our result any more precise than using the entire range of the type of E, unfortunately. So I think UO_Not should never look at its subexpression -- the best result we can give is IntRange::forValueOfType(C, GetExprType(E)), and that's what we should use.

[NekoCdr]

@AshleyRoll are you still working on this patch?

[AshleyRoll]

I've not been able to get the time for a while, I hope to be able to spend some more time on it, but I'd probably do that through a GitHub PR now. If someone else wants to take it on, I'd be fine with that.

[NekoCdr]

In D126960#4643979, @AshleyRoll wrote:

If someone else wants to take it on, I'd be fine with that.

You can resume working with this patch, if you want. It's just a reminder. )

clang/lib/Sema/SemaChecking.cpp

if (const auto *UO = dyn_cast<UnaryOperator>(E)) {
     // Boolean-valued operations are white-listed.
     case UO_LNot:
       return IntRange::forBoolType();
 
     // Operations with opaque sources are black-listed.
     case UO_Deref:
     case UO_AddrOf: // should be impossible
       return IntRange::forValueOfType(C, GetExprType(E));
 
+    case UO_Not:
+      // unary not promotes boolean to integer
+      if (UO->getSubExpr()->isKnownToHaveBooleanValue())
+        return IntRange(MaxWidth, false);
+
+      LLVM_FALLTHROUGH;
+
     default:
       return GetExprRange(C, UO->getSubExpr(), MaxWidth, InConstantContext,
                           Approximate);
     }
   }
 
   if (const auto *OVE = dyn_cast<OpaqueValueExpr>(E))
     return GetExprRange(C, OVE->getSourceExpr(), MaxWidth, InConstantContext,

clang/test/Sema/compare.c

 };
 
 void pr36008(enum PR36008EnumTest lhs) {
   __typeof__(lhs) x = lhs;
   __typeof__(kPR36008Value) y = (kPR36008Value);
   if (x == y) x = y; // no warning
   if (y == x) y = x; // no warning
 }
+
+int warn_on_different_sign_after_unary_operator(unsigned a, int b) {
+  return
+      // unary not promotes boolean to int
+      (a > ~(!b)) // expected-warning {{comparison of integers of different signs: 'unsigned int' and 'int'}}
+      &&
+      (a > -(b)) // expected-warning {{comparison of integers of different signs: 'unsigned int' and 'int'}}
+      &&
+      (a > ++b) // expected-warning {{comparison of integers of different signs: 'unsigned int' and 'int'}}
+      &&
+      (a > --b) // expected-warning {{comparison of integers of different signs: 'unsigned int' and 'int'}}
+      &&
+      // unary not promotes boolean to int
+      (b > ~(!a)) // no warning
+      &&
+      (b > -(a)) // expected-warning {{comparison of integers of different signs: 'int' and 'unsigned int'}}
+      &&
+      (b > ++a) // expected-warning {{comparison of integers of different signs: 'int' and 'unsigned int'}}
+      &&
+      (b > --a) // expected-warning {{comparison of integers of different signs: 'int' and 'unsigned int'}}
+      ;
+}