LLVM  14.0.0git
bit.h
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1 //===-- llvm/ADT/bit.h - C++20 <bit> ----------------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements the C++20 <bit> header.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #ifndef LLVM_ADT_BIT_H
14 #define LLVM_ADT_BIT_H
15 
16 #include "llvm/Support/Compiler.h"
17 #include <cstring>
18 #include <type_traits>
19 
20 namespace llvm {
21 
22 // This implementation of bit_cast is different from the C++17 one in two ways:
23 // - It isn't constexpr because that requires compiler support.
24 // - It requires trivially-constructible To, to avoid UB in the implementation.
25 template <
26  typename To, typename From,
27  typename = std::enable_if_t<sizeof(To) == sizeof(From)>
28 #if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
29  (defined(__GNUC__) && __GNUC__ >= 5)
30  ,
31  typename = std::enable_if_t<std::is_trivially_constructible<To>::value>
32 #elif __has_feature(is_trivially_constructible)
33  ,
34  typename = std::enable_if_t<__is_trivially_constructible(To)>
35 #else
36  // See comment below.
37 #endif
38 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
39  (defined(__GNUC__) && __GNUC__ >= 5)
40  ,
41  typename = std::enable_if_t<std::is_trivially_copyable<To>::value>,
42  typename = std::enable_if_t<std::is_trivially_copyable<From>::value>
43 #elif __has_feature(is_trivially_copyable)
44  ,
45  typename = std::enable_if_t<__is_trivially_copyable(To)>,
46  typename = std::enable_if_t<__is_trivially_copyable(From)>
47 #else
48 // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
49 // llvm/Support/type_traits.h's is_trivially_copyable we don't want to
50 // provide a good-enough answer here: developers in that configuration will hit
51 // compilation failures on the bots instead of locally. That's acceptable
52 // because it's very few developers, and only until we move past C++11.
53 #endif
54  >
55 inline To bit_cast(const From &from) noexcept {
56  To to;
57  std::memcpy(&to, &from, sizeof(To));
58  return to;
59 }
60 
61 } // namespace llvm
62 
63 #endif
llvm
This file implements support for optimizing divisions by a constant.
Definition: AllocatorList.h:23
to
Should compile to
Definition: README.txt:449
llvm::bit_cast
To bit_cast(const From &from) noexcept
Definition: bit.h:55
memcpy
<%struct.s * > cast struct s *S to sbyte *< sbyte * > sbyte uint cast struct s *agg result to sbyte *< sbyte * > sbyte uint cast struct s *memtmp to sbyte *< sbyte * > sbyte uint ret void llc ends up issuing two memcpy or custom lower memcpy(of small size) to be ldmia/stmia. I think option 2 is better but the current register allocator cannot allocate a chunk of registers at a time. A feasible temporary solution is to use specific physical registers at the lowering time for small(<
Compiler.h
From
BlockVerifier::State From
Definition: BlockVerifier.cpp:55