Addressed some potential style comments:

- Changed order in MPCommon.h and .cpp

Added workaround for older MPFR version
- Support for acospi

Fixed some nit in acospif16.cpp for the comments to look nice

Author: amemov

libc/src/math/generic/acospif16.cpp

@@ -1,4 +1,4 @@
-//===-- Half-precision acospi function ------------------------------===//
+//===-- Half-precision acospi function ------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.

libc/utils/MPFRWrapper/MPCommon.cpp

@@ -64,16 +64,26 @@ MPFRNumber MPFRNumber::acos() const {
   return result;
 }
 
-MPFRNumber MPFRNumber::acospi() const {
+MPFRNumber MPFRNumber::acosh() const {
   MPFRNumber result(*this);
-  mpfr_acospi(result.value, value, mpfr_rounding);
+  mpfr_acosh(result.value, value, mpfr_rounding);
   return result;
 }
 
-MPFRNumber MPFRNumber::acosh() const {
+MPFRNumber MPFRNumber::acospi() const {
   MPFRNumber result(*this);
-  mpfr_acosh(result.value, value, mpfr_rounding);
+
+#if MPFR_VERSION_MAJOR > 4 ||                                                  \
+    (MPFR_VERSION_MAJOR == 4 && MPFR_VERSION_MINOR >= 2)
+  mpfr_acospi(result.value, value, mpfr_rounding);
+  return result;
+#else
+  mpfr_acos(result.value, value, mpfr_rounding);
+  MPFRNumber value_pi(0.0, 1280);
+  mpfr_const_pi(value_pi.value, MPFR_RNDN);
+  mpfr_div(result.value, result.value, value_pi.value, mpfr_rounding);
   return result;
+#endif
 }
 
 MPFRNumber MPFRNumber::add(const MPFRNumber &b) const {

libc/utils/MPFRWrapper/MPCommon.h

@@ -180,8 +180,8 @@ class MPFRNumber {
   bool is_nan() const;
   MPFRNumber abs() const;
   MPFRNumber acos() const;
-  MPFRNumber acospi() const;
   MPFRNumber acosh() const;
+  MPFRNumber acospi() const;
   MPFRNumber add(const MPFRNumber &b) const;
   MPFRNumber asin() const;
   MPFRNumber asinh() const;