Fix _BitInt suffix width calculation

[aaron.ballman]

Summary

@mgehre-amd pointed out the following post-commit review feedback on the changes in 8cba72177dcd8de5d37177dbaf2347e5c1f0f1e8:

As an example, the paper says 3wb /* Yields an _BitInt(3); two value bits, one sign bit */.
So I would expect that 0xFwb gives _BitInt(5); four value bits, one sign bit, but with this implementation I get _BitInt(2).
This is because ResultVal as 4 bits, and getMinSignedBits() inteprets it as negative and thus says that 1 bit is enough to represent -1.

This corrects the behavior for calculating the bit-width and adds some test coverage.

[mgehre-amd]

Thanks a lot!

[aaron.ballman]

Thanks for the quick reviews, and thanks for catching the issue @mgehre-amd! I've committed in 9cf8f81ca45de198013f29442a7de6600b226d70.

clang/lib/Sema/SemaExpr.cpp

         }
       }
 
       // Bit-precise integer literals are automagically-sized based on the
       // width required by the literal.
       if (Literal.isBitInt) {
         // The signed version has one more bit for the sign value. There are no
         // zero-width bit-precise integers, even if the literal value is 0.
-        Width = Literal.isUnsigned ? std::max(ResultVal.getActiveBits(), 1u)
-                                   : std::max(ResultVal.getMinSignedBits(), 2u);
+        Width = std::max(ResultVal.getActiveBits(), 1u) +
+                (Literal.isUnsigned ? 0u : 1u);
 
         // Diagnose if the width of the constant is larger than BITINT_MAXWIDTH,
         // and reset the type to the largest supported width.
         unsigned int MaxBitIntWidth =
             Context.getTargetInfo().getMaxBitIntWidth();
         if (Width > MaxBitIntWidth) {
           Diag(Tok.getLocation(), diag::err_integer_literal_too_large)
               << Literal.isUnsigned;

clang/test/Lexer/bitint-constants.c

void ValidSuffixInvalidValue(void) {
   // the width of BITINT_MAXWIDTH. When this value changes in the future, the
   // test cases should pick a new value that can't be represented by a _BitInt,
   // but also add a test case that a 129-bit literal still behaves as-expected.
   _Static_assert(__BITINT_MAXWIDTH__ <= 128,
                "Need to pick a bigger constant for the test case below.");
   0xFFFF'FFFF'FFFF'FFFF'FFFF'FFFF'FFFF'FFFF'1wb; // expected-error {{integer literal is too large to be represented in any signed integer type}}
   0xFFFF'FFFF'FFFF'FFFF'FFFF'FFFF'FFFF'FFFF'1uwb; // expected-error {{integer literal is too large to be represented in any integer type}}
 }
+
+void TestTypes(void) {
+  // 2 value bits, one sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(3wb), _BitInt(3)));
+  // 2 value bits, one sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(-3wb), _BitInt(3)));
+  // 2 value bits, no sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(3uwb), unsigned _BitInt(2)));
+  // 4 value bits, one sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(0xFwb), _BitInt(5)));
+  // 4 value bits, one sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(-0xFwb), _BitInt(5)));
+  // 4 value bits, no sign bit
+  _Static_assert(__builtin_types_compatible_p(__typeof__(0xFuwb), unsigned _BitInt(4)));
+}