[Clang] Support constexpr builtin fmin

[Izaron]

Summary

Support constexpr version of __builtin_fmin and its variations.

[Izaron]

This is a mirror of my previous patch about builtin max. I promised to make the patch for builtin min =) Here: https://reviews.llvm.org/D134369#3806131

It conforms to libc fmin implementation: https://github.com/llvm/llvm-project/blob/1ae405144d62299d87096de4dc2596a04a4fee84/libc/src/__support/FPUtil/BasicOperations.h#L26-L42

[Izaron]

Aligned slashes in defines

clang/docs/LanguageExtensions.rst

 * ``__builtin_ctz``
 * ``__builtin_ctzl``
 * ``__builtin_ctzll``
 * ``__builtin_ctzs``
 * ``__builtin_ffs``
 * ``__builtin_ffsl``
 * ``__builtin_ffsll``
 * ``__builtin_fmax``
+* ``__builtin_fmin``
 * ``__builtin_fpclassify``
 * ``__builtin_inf``
 * ``__builtin_isinf``
 * ``__builtin_isinf_sign``
 * ``__builtin_isfinite``
 * ``__builtin_isnan``
 * ``__builtin_isnormal``
 * ``__builtin_nan``

clang/lib/AST/ExprConstant.cpp

if (!EvaluateFloat(E->getArg(0), Result, Info) ||
       return false;
     // When comparing zeroes, return +0.0 if one of the zeroes is positive.
     if (Result.isZero() && RHS.isZero() && Result.isNegative())
       Result = RHS;
     else if (Result.isNaN() || RHS > Result)
       Result = RHS;
     return true;
   }
+
+  case Builtin::BI__builtin_fmin:
+  case Builtin::BI__builtin_fminf:
+  case Builtin::BI__builtin_fminl:
+  case Builtin::BI__builtin_fminf16:
+  case Builtin::BI__builtin_fminf128: {
+    // TODO: Handle sNaN.
+    APFloat RHS(0.);
+    if (!EvaluateFloat(E->getArg(0), Result, Info) ||
+        !EvaluateFloat(E->getArg(1), RHS, Info))
+      return false;
+    // When comparing zeroes, return -0.0 if one of the zeroes is negative.
+    if (Result.isZero() && RHS.isZero() && RHS.isNegative())
+      Result = RHS;
+    else if (Result.isNaN() || RHS < Result)
+      Result = RHS;
+    return true;
+  }
   }
 }
 
 bool FloatExprEvaluator::VisitUnaryReal(const UnaryOperator *E) {
   if (E->getSubExpr()->getType()->isAnyComplexType()) {
     ComplexValue CV;
     if (!EvaluateComplex(E->getSubExpr(), CV, Info))
       return false;

clang/test/Sema/constant-builtins-fmin.cpp

+// RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+constexpr double NaN = __builtin_nan("");
+constexpr double Inf = __builtin_inf();
+constexpr double NegInf = -__builtin_inf();
+
+#define FMIN_TEST_SIMPLE(T, FUNC)                           \
+    static_assert(T(1.2345) == FUNC(T(1.2345), T(6.7890))); \
+    static_assert(T(1.2345) == FUNC(T(6.7890), T(1.2345)));
+
+#define FMIN_TEST_NAN(T, FUNC)                          \
+    static_assert(Inf == FUNC(NaN, Inf));               \
+    static_assert(NegInf == FUNC(NegInf, NaN));         \
+    static_assert(0.0 == FUNC(NaN, 0.0));               \
+    static_assert(-0.0 == FUNC(-0.0, NaN));             \
+    static_assert(T(-1.2345) == FUNC(NaN, T(-1.2345))); \
+    static_assert(T(1.2345) == FUNC(T(1.2345), NaN));   \
+    static_assert(__builtin_isnan(FUNC(NaN, NaN)));
+
+#define FMIN_TEST_INF(T, FUNC)                        \
+    static_assert(NegInf == FUNC(NegInf, Inf));       \
+    static_assert(0.0 == FUNC(Inf, 0.0));             \
+    static_assert(-0.0 == FUNC(-0.0, Inf));           \
+    static_assert(T(1.2345) == FUNC(Inf, T(1.2345))); \
+    static_assert(T(-1.2345) == FUNC(T(-1.2345), Inf));
+
+#define FMIN_TEST_NEG_INF(T, FUNC)                     \
+    static_assert(NegInf == FUNC(Inf, NegInf));        \
+    static_assert(NegInf == FUNC(NegInf, 0.0));        \
+    static_assert(NegInf == FUNC(-0.0, NegInf));       \
+    static_assert(NegInf == FUNC(NegInf, T(-1.2345))); \
+    static_assert(NegInf == FUNC(T(1.2345), NegInf));
+
+#define FMIN_TEST_BOTH_ZERO(T, FUNC)                                 \
+    static_assert(__builtin_copysign(1.0, FUNC(0.0, 0.0)) == 1.0);   \
+    static_assert(__builtin_copysign(1.0, FUNC(-0.0, 0.0)) == -1.0); \
+    static_assert(__builtin_copysign(1.0, FUNC(0.0, -0.0)) == -1.0); \
+    static_assert(__builtin_copysign(1.0, FUNC(-0.0, -0.0)) == -1.0);
+
+#define LIST_FMIN_TESTS(T, FUNC) \
+    FMIN_TEST_SIMPLE(T, FUNC)    \
+    FMIN_TEST_NAN(T, FUNC)       \
+    FMIN_TEST_INF(T, FUNC)       \
+    FMIN_TEST_NEG_INF(T, FUNC)   \
+    FMIN_TEST_BOTH_ZERO(T, FUNC)
+
+LIST_FMIN_TESTS(double, __builtin_fmin)
+LIST_FMIN_TESTS(float, __builtin_fminf)
+LIST_FMIN_TESTS((long double), __builtin_fminl)
+LIST_FMIN_TESTS(__fp16, __builtin_fminf16)
+#ifdef __FLOAT128__
+LIST_FMIN_TESTS(__float128, __builtin_fminf128)
+#endif