LibM: Delegate rounding to fully to the FRNDINT instruction

This is only enabled for clang right now, because it does not seem to
work on GCC.

Also fixes the types of two intermediate results

Co-authored-by: kleines Filmröllchen <filmroellchen@serenityos.org>

Userland/Libraries/LibM/math.cpp

@@ -2,12 +2,14 @@
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021, Mițca Dumitru <dumitru0mitca@gmail.com>
  * Copyright (c) 2022, the SerenityOS developers.
+ * Copyright (c) 2022, Leon Albrecht <leon.a@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/BuiltinWrappers.h>
 #include <AK/ExtraMathConstants.h>
+#include <AK/FPControl.h>
 #include <AK/Math.h>
 #include <AK/Platform.h>
 #include <AK/StdLibExtras.h>
@@ -459,7 +461,7 @@ float truncf(float x) NOEXCEPT
 
 long double rintl(long double value)
 {
-    double res;
+    long double res;
     asm(
         "frndint\n"
         : "=t"(res)
@@ -477,7 +479,7 @@ double rint(double value)
 }
 float rintf(float value)
 {
-    double res;
+    float res;
     asm(
         "frndint\n"
         : "=t"(res)
@@ -628,6 +630,8 @@ long double frexpl(long double x, int* exp) NOEXCEPT
     return scalbnl(x, -(*exp));
 }
 
+#if !(ARCH(I386) || ARCH(X86_64))
+
 double round(double value) NOEXCEPT
 {
     return internal_to_integer(value, RoundingMode::ToEven);
@@ -703,6 +707,117 @@ long double ceill(long double value) NOEXCEPT
     return internal_to_integer(value, RoundingMode::Up);
 }
 
+#else
+
+double round(double x) NOEXCEPT
+{
+    // Note: This is break-tie-away-from-zero, so not the hw's understanding of
+    //       "nearest", which would be towards even.
+    if (x == 0.)
+        return x;
+    if (x > 0.)
+        return floor(x + .5);
+    return ceil(x - .5);
+}
+
+float roundf(float x) NOEXCEPT
+{
+    if (x == 0.f)
+        return x;
+    if (x > 0.f)
+        return floorf(x + .5f);
+    return ceilf(x - .5f);
+}
+
+long double roundl(long double x) NOEXCEPT
+{
+    if (x == 0.L)
+        return x;
+    if (x > 0.L)
+        return floorl(x + .5L);
+    return ceill(x - .5L);
+}
+
+long lroundf(float value) NOEXCEPT
+{
+    return static_cast<long>(roundf(value));
+}
+
+long lround(double value) NOEXCEPT
+{
+    return static_cast<long>(round(value));
+}
+
+long lroundl(long double value) NOEXCEPT
+{
+    return static_cast<long>(roundl(value));
+}
+
+long long llroundf(float value) NOEXCEPT
+{
+    return static_cast<long long>(roundf(value));
+}
+
+long long llround(double value) NOEXCEPT
+{
+    return static_cast<long long>(round(value));
+}
+
+long long llroundd(long double value) NOEXCEPT
+{
+    return static_cast<long long>(roundl(value));
+}
+
+float floorf(float value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::DOWN };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+double floor(double value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::DOWN };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+long double floorl(long double value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::DOWN };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+float ceilf(float value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::UP };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+double ceil(double value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::UP };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+long double ceill(long double value) NOEXCEPT
+{
+    AK::X87RoundingModeScope scope { AK::RoundingMode::UP };
+    asm("frndint"
+        : "+t"(value));
+    return value;
+}
+
+#endif
+
 long double modfl(long double x, long double* intpart) NOEXCEPT
 {
     return internal_modf(x, intpart);