[ADT] Simplify llvm::bit_cast (NFC)

[kazu]

Summary

This patch removes macro tricks to check GCC versions.

The commit message from 19262fc5966ab569f21f3d440f8b001bca666f17
states that "is_trivially_copyable is only in GCC 5.1 and later".
Note that we now require GCC 7.1 or higher.

Since both std::is_trivially_constructible and
std::is_trivially_copyable are C++11 features, and we now require
C++17, we probably don't need to worry about the availability of the
C++11 features.

[jloser]

llvm/include/llvm/ADT/bit.h

16–17

I believe that we can remove this include now if you'd like to, right?

[kazu]

Remove #include "llvm/Support/Compiler.h"

[kazu]

llvm/include/llvm/ADT/bit.h

16–17

Good point!

[RKSimon]

llvm/include/llvm/ADT/bit.h

34

Could we use the __builtin_bit_cast if its available?

[kazu]

llvm/include/llvm/ADT/bit.h

34

Could we use the __builtin_bit_cast if its available?

We should be able to. Would that lead to better code?

[RKSimon]

llvm/include/llvm/ADT/bit.h

34

That's unlikely although if it allows us to remove the <cstring> we get a reduction in build costs.

MathExtras.h has overtaken <string> as one of the largest include build costs and I've been curious how easily we could reduce this by using ADT/bit.h directly (possibly with more missing c++20 equivalent <bit> methods):

https://commondatastorage.googleapis.com/chromium-browser-clang/llvm-include-analysis.html

I'll continue looking at ways to reduce MathExtras.h use for now - thanks anyway.

llvm/include/llvm/ADT/bit.h

 //===-- llvm/ADT/bit.h - C++20 <bit> ----------------------------*- 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
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file implements the C++20 <bit> header.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_BIT_H
 #define LLVM_ADT_BIT_H
 
-#include "llvm/Support/Compiler.h"
 #include <cstring>
 #include <type_traits>
 
 namespace llvm {
 
 // This implementation of bit_cast is different from the C++17 one in two ways:
 //  - It isn't constexpr because that requires compiler support.
 //  - It requires trivially-constructible To, to avoid UB in the implementation.
 template <
     typename To, typename From,
-    typename = std::enable_if_t<sizeof(To) == sizeof(From)>
-#if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
-    (defined(__GNUC__) && __GNUC__ >= 5)
-    ,
-    typename = std::enable_if_t<std::is_trivially_constructible<To>::value>
-#elif __has_feature(is_trivially_constructible)
-    ,
-    typename = std::enable_if_t<__is_trivially_constructible(To)>
-#else
-  // See comment below.
-#endif
-#if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
-    (defined(__GNUC__) && __GNUC__ >= 5)
-    ,
+    typename = std::enable_if_t<sizeof(To) == sizeof(From)>,
+    typename = std::enable_if_t<std::is_trivially_constructible<To>::value>,
     typename = std::enable_if_t<std::is_trivially_copyable<To>::value>,
-    typename = std::enable_if_t<std::is_trivially_copyable<From>::value>
-#elif __has_feature(is_trivially_copyable)
-    ,
-    typename = std::enable_if_t<__is_trivially_copyable(To)>,
-    typename = std::enable_if_t<__is_trivially_copyable(From)>
-#else
-// This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
-// llvm/Support/type_traits.h's is_trivially_copyable we don't want to
-// provide a good-enough answer here: developers in that configuration will hit
-// compilation failures on the bots instead of locally. That's acceptable
-// because it's very few developers, and only until we move past C++11.
-#endif
-    >
+    typename = std::enable_if_t<std::is_trivially_copyable<From>::value>>
 inline To bit_cast(const From &from) noexcept {
   To to;
   std::memcpy(&to, &from, sizeof(To));
   return to;
 }
 
 } // namespace llvm
 
 #endif