[libc] Fix fputil::multiply_add and fputil::polyeval template signatures.

[lntue]

Summary

Fix fputil::multiply_add and fputil::polyeval template function
signatures.

[sivachandra]

If you can add some inline commentary about how it affects volatile args, that will be even more helpful.

[jhuber6]

libc/src/__support/FPUtil/PolyEval.h

41

Isn't this more standard?

libc/src/__support/FPUtil/PolyEval.h

 // evaluated it as:  a_0 + x * (a_1 + x * ( ... (a_(n-1) + x * a_n) ... ) ) ).
 // We will use FMA instructions if available.
 // Example: to evaluate x^3 + 2*x^2 + 3*x + 4, call
 //   polyeval( x, 4.0, 3.0, 2.0, 1.0 )
 
 namespace __llvm_libc {
 namespace fputil {
 
-template <typename T> LIBC_INLINE T polyeval(const T &, const T &a0) {
+template <typename T>
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) > sizeof(void *)), T>
+polyeval(const T &, const T &a0) {
   return a0;
 }
 
+template <typename T>
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) <= sizeof(void *)), T> polyeval(T,
+                                                                        T a0) {
+  return a0;
+}
+
+template <typename T, typename... Ts>
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) > sizeof(void *)), T>
+polyeval(const T &x, const T &a0, const Ts &...a) {
+  return multiply_add(x, polyeval(x, a...), a0);
+}
+
 template <typename T, typename... Ts>
-LIBC_INLINE T polyeval(const T &x, const T &a0, const Ts &...a) {
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) <= sizeof(void *)), T>
+polyeval(T x, T a0, Ts... a) {
   return multiply_add(x, polyeval(x, a...), a0);
 }
 
 } // namespace fputil
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_FPUTIL_POLYEVAL_H

libc/src/__support/FPUtil/multiply_add.h

 //===-- Common header for multiply-add implementations ----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIBC_SRC_SUPPORT_FPUTIL_MULTIPLY_ADD_H
 #define LLVM_LIBC_SRC_SUPPORT_FPUTIL_MULTIPLY_ADD_H
 
+#include "src/__support/CPP/type_traits.h"
 #include "src/__support/common.h"
 #include "src/__support/macros/properties/architectures.h"
 #include "src/__support/macros/properties/cpu_features.h" // LIBC_TARGET_CPU_HAS_FMA
 
 namespace __llvm_libc {
 namespace fputil {
 
 // Implement a simple wrapper for multiply-add operation:
 //   multiply_add(x, y, z) = x*y + z
 // which uses FMA instructions to speed up if available.
 
 template <typename T>
-LIBC_INLINE T multiply_add(const T &x, const T &y, const T &z) {
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) > sizeof(void *)), T>
+multiply_add(const T &x, const T &y, const T &z) {
+  return x * y + z;
+}
+
+template <typename T>
+LIBC_INLINE cpp::enable_if_t<(sizeof(T) <= sizeof(void *)), T>
+multiply_add(T x, T y, T z) {
   return x * y + z;
 }
 
 } // namespace fputil
 } // namespace __llvm_libc
 
 #if defined(LIBC_TARGET_CPU_HAS_FMA)