[libc] Fix exp2f and prevent misuse of likely/unlikely

[gchatelet]

Summary

Let's make sure that we only accept boolean expressions when using likely/unlikely.

[lntue]

libc/src/__support/macros/attributes.h

27

Shall you use LIBC_INLINE here?

[gchatelet]

• address comments

[gchatelet]

Done

libc/src/__support/macros/attributes.h

 // Most macros here are exposing GCC or Clang features, and are stubbed out for
 // other compilers.
 
 #ifndef LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H
 #define LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H
 
 #define LIBC_INLINE inline
 #define LIBC_INLINE_ASM __asm__ __volatile__
-#define LIBC_LIKELY(x) __builtin_expect(!!(x), 1)
-#define LIBC_UNLIKELY(x) __builtin_expect(x, 0)
+
+// We use a template to implement likely/unlikely to make sure that we don't
+// accidentally pass an integer.
+namespace __llvm_libc::details {
+template <typename T>
+constexpr LIBC_INLINE bool expects_bool_condition(T value, T expected) {
+  return __builtin_expect(value, expected);
+}
+} // namespace __llvm_libc::details
+#define LIBC_LIKELY(x) __llvm_libc::details::expects_bool_condition(x, true)
+#define LIBC_UNLIKELY(x) __llvm_libc::details::expects_bool_condition(x, false)
+
 #define LIBC_UNUSED __attribute__((unused))
 
 #endif // LLVM_LIBC_SUPPORT_MACROS_ATTRIBUTES_H

libc/src/math/generic/exp2f.cpp

if (x_u >= 0xc316'0000U) {
         return FPBits(FPBits::MIN_SUBNORMAL).get_val();
       if (x != 0.0f)
         errno = ERANGE;
       return 0.0f;
     }
   }
 
   // Check exceptional values.
-  if (LIBC_UNLIKELY(x_u & exval_mask) == exval_mask) {
+  if (LIBC_UNLIKELY((x_u & exval_mask) == exval_mask)) {
     if (LIBC_UNLIKELY(x_u == exval1)) { // x = 0x1.853a6ep-9f
       if (fputil::get_round() == FE_TONEAREST)
         return 0x1.00870ap+0f;
     } else if (LIBC_UNLIKELY(x_u == exval2)) { // x = -0x1.e7526ep-6f
       if (fputil::get_round() == FE_TONEAREST)
         return 0x1.f58d62p-1f;
     }
   }