LLVM  15.0.0git
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 /// \file
10 /// This file implements the C++20 <bit> header.
11 ///
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_ADT_BIT_H
15 #define LLVM_ADT_BIT_H
16 
17 #include "llvm/Support/Compiler.h"
18 #include <cstring>
19 #include <type_traits>
20 
21 namespace llvm {
22 
23 // This implementation of bit_cast is different from the C++17 one in two ways:
24 // - It isn't constexpr because that requires compiler support.
25 // - It requires trivially-constructible To, to avoid UB in the implementation.
26 template <
27  typename To, typename From,
28  typename = std::enable_if_t<sizeof(To) == sizeof(From)>
29 #if (__has_feature(is_trivially_constructible) && defined(_LIBCPP_VERSION)) || \
30  (defined(__GNUC__) && __GNUC__ >= 5)
31  ,
32  typename = std::enable_if_t<std::is_trivially_constructible<To>::value>
33 #elif __has_feature(is_trivially_constructible)
34  ,
35  typename = std::enable_if_t<__is_trivially_constructible(To)>
36 #else
37  // See comment below.
38 #endif
39 #if (__has_feature(is_trivially_copyable) && defined(_LIBCPP_VERSION)) || \
40  (defined(__GNUC__) && __GNUC__ >= 5)
41  ,
42  typename = std::enable_if_t<std::is_trivially_copyable<To>::value>,
43  typename = std::enable_if_t<std::is_trivially_copyable<From>::value>
44 #elif __has_feature(is_trivially_copyable)
45  ,
46  typename = std::enable_if_t<__is_trivially_copyable(To)>,
47  typename = std::enable_if_t<__is_trivially_copyable(From)>
48 #else
49 // This case is GCC 4.x. clang with libc++ or libstdc++ never get here. Unlike
50 // llvm/Support/type_traits.h's is_trivially_copyable we don't want to
51 // provide a good-enough answer here: developers in that configuration will hit
52 // compilation failures on the bots instead of locally. That's acceptable
53 // because it's very few developers, and only until we move past C++11.
54 #endif
55  >
56 inline To bit_cast(const From &from) noexcept {
57  To to;
58  std::memcpy(&to, &from, sizeof(To));
59  return to;
60 }
61 
62 } // namespace llvm
63 
64 #endif
llvm
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:17
to
Should compile to
Definition: README.txt:449
llvm::bit_cast
To bit_cast(const From &from) noexcept
Definition: bit.h:56
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