ADT: add <bit> header, implement C++20 bit_cast, use

[jfb]

Summary

I saw a few places that were punning through a union of FP and integer, and that made me sad. Luckily, C++20 adds bit_cast for exactly that purpose. Implement our own version in ADT (without constexpr, leaving us a bit sad), and use it in the few places my grep-fu found silly union punning.

[steven_wu]

LGTM!

[srhines]

Union punning is one of my pet peeves, so I am glad to see this. bit_cast seems generally useful too, so thanks for adding it.

[jfb]

Reverted by r341730. Looks like missing std::is_trivially_copyable, and a missing include in the ARM change (which is what I get for not building that target by default...).
@rsmith points out the workaround in llvm/Support/type_traits.h

[jfb]

• Fix is_trivially_copytable, and ARM build.

include/llvm/ADT/bit.h

+//===-- llvm/ADT/bit.h - C++20 <bit> ----------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the C++20 <bit> header.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_BIT_H
+#define LLVM_ADT_BIT_H
+
+#include "llvm/Support/type_traits.h"
+#include <cstring>
+
+namespace llvm {
+
+template <typename To, typename From,
+          typename = typename std::enable_if<sizeof(To) == sizeof(From)>::type,
+          typename = typename std::enable_if<isPodLike<To>::value>::type,
+          typename = typename std::enable_if<isPodLike<From>::value>::type>
+inline To bit_cast(const From &from) noexcept {
+  alignas(To) unsigned char storage[sizeof(To)];
+  std::memcpy(&storage, &from, sizeof(To));
+  return reinterpret_cast<To &>(storage);
+}
+
+} // namespace llvm
+
+#endif

lib/Support/APInt.cpp

 
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/bit.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include <climits>
 #include <cmath>
 #include <cstdlib>
if (A.ugt(B)) {
       B.lshrInPlace(B.countTrailingZeros() - Pow2);
     }
   }
 
   return A;
 }
 
 APInt llvm::APIntOps::RoundDoubleToAPInt(double Double, unsigned width) {
-  union {
-    double D;
-    uint64_t I;
-  } T;
-  T.D = Double;
+  uint64_t I = bit_cast<uint64_t>(Double);
 
   // Get the sign bit from the highest order bit
-  bool isNeg = T.I >> 63;
+  bool isNeg = I >> 63;
 
   // Get the 11-bit exponent and adjust for the 1023 bit bias
-  int64_t exp = ((T.I >> 52) & 0x7ff) - 1023;
+  int64_t exp = ((I >> 52) & 0x7ff) - 1023;
 
   // If the exponent is negative, the value is < 0 so just return 0.
   if (exp < 0)
     return APInt(width, 0u);
 
   // Extract the mantissa by clearing the top 12 bits (sign + exponent).
-  uint64_t mantissa = (T.I & (~0ULL >> 12)) | 1ULL << 52;
+  uint64_t mantissa = (I & (~0ULL >> 12)) | 1ULL << 52;
 
   // If the exponent doesn't shift all bits out of the mantissa
   if (exp < 52)
     return isNeg ? -APInt(width, mantissa >> (52 - exp)) :
                     APInt(width, mantissa >> (52 - exp));
 
   // If the client didn't provide enough bits for us to shift the mantissa into
   // then the result is undefined, just return 0
if (hiWord == 0) {
     assert(hiWord > 0 && "huh?");
     uint64_t hibits = Tmp.U.pVal[hiWord] << (52 - n % APINT_BITS_PER_WORD);
     uint64_t lobits = Tmp.U.pVal[hiWord-1] >> (11 + n % APINT_BITS_PER_WORD);
     mantissa = hibits | lobits;
   }
 
   // The leading bit of mantissa is implicit, so get rid of it.
   uint64_t sign = isNeg ? (1ULL << (APINT_BITS_PER_WORD - 1)) : 0;
-  union {
-    double D;
-    uint64_t I;
-  } T;
-  T.I = sign | (exp << 52) | mantissa;
-  return T.D;
+  uint64_t I = sign | (exp << 52) | mantissa;
+  return bit_cast<double>(I);
 }
 
 // Truncate to new width.
 APInt APInt::trunc(unsigned width) const {
   assert(width < BitWidth && "Invalid APInt Truncate request");
   assert(width && "Can't truncate to 0 bits");
 
   if (width <= APINT_BITS_PER_WORD)

lib/Target/AArch64/MCTargetDesc/AArch64AddressingModes.h

 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_AARCH64_MCTARGETDESC_AARCH64ADDRESSINGMODES_H
 #define LLVM_LIB_TARGET_AARCH64_MCTARGETDESC_AARCH64ADDRESSINGMODES_H
 
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/bit.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include <cassert>
 
 namespace llvm {
 
 /// AArch64_AM - AArch64 Addressing Mode Stuff
 namespace AArch64_AM {
static inline bool isValidDecodeLogicalImmediate(uint64_t val,
   return true;
 }
 
 //===----------------------------------------------------------------------===//
 // Floating-point Immediates
 //
 static inline float getFPImmFloat(unsigned Imm) {
   // We expect an 8-bit binary encoding of a floating-point number here.
-  union {
-    uint32_t I;
-    float F;
-  } FPUnion;
 
   uint8_t Sign = (Imm >> 7) & 0x1;
   uint8_t Exp = (Imm >> 4) & 0x7;
   uint8_t Mantissa = Imm & 0xf;
 
-  //   8-bit FP    iEEEE Float Encoding
+  //   8-bit FP    IEEE Float Encoding
   //   abcd efgh   aBbbbbbc defgh000 00000000 00000000
   //
   // where B = NOT(b);
 
-  FPUnion.I = 0;
-  FPUnion.I |= Sign << 31;
-  FPUnion.I |= ((Exp & 0x4) != 0 ? 0 : 1) << 30;
-  FPUnion.I |= ((Exp & 0x4) != 0 ? 0x1f : 0) << 25;
-  FPUnion.I |= (Exp & 0x3) << 23;
-  FPUnion.I |= Mantissa << 19;
-  return FPUnion.F;
+  uint32_t I = 0;
+  I |= Sign << 31;
+  I |= ((Exp & 0x4) != 0 ? 0 : 1) << 30;
+  I |= ((Exp & 0x4) != 0 ? 0x1f : 0) << 25;
+  I |= (Exp & 0x3) << 23;
+  I |= Mantissa << 19;
+  return bit_cast<float>(I);
 }
 
 /// getFP16Imm - Return an 8-bit floating-point version of the 16-bit
 /// floating-point value. If the value cannot be represented as an 8-bit
 /// floating-point value, then return -1.
 static inline int getFP16Imm(const APInt &Imm) {
   uint32_t Sign = Imm.lshr(15).getZExtValue() & 1;
   int32_t Exp = (Imm.lshr(10).getSExtValue() & 0x1f) - 15;  // -14 to 15

lib/Target/ARM/MCTargetDesc/ARMAddressingModes.h

 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_ARM_MCTARGETDESC_ARMADDRESSINGMODES_H
 #define LLVM_LIB_TARGET_ARM_MCTARGETDESC_ARMADDRESSINGMODES_H
 
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/bit.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include <cassert>
 
 namespace llvm {
 
 /// ARM_AM - ARM Addressing Mode Stuff
 namespace ARM_AM {
inline unsigned encodeNEONi32splat(unsigned Value) {
     return Value;
   }
 
   //===--------------------------------------------------------------------===//
   // Floating-point Immediates
   //
   inline float getFPImmFloat(unsigned Imm) {
     // We expect an 8-bit binary encoding of a floating-point number here.
-    union {
-      uint32_t I;
-      float F;
-    } FPUnion;
 
     uint8_t Sign = (Imm >> 7) & 0x1;
     uint8_t Exp = (Imm >> 4) & 0x7;
     uint8_t Mantissa = Imm & 0xf;
 
-    //   8-bit FP    iEEEE Float Encoding
+    //   8-bit FP    IEEE Float Encoding
     //   abcd efgh   aBbbbbbc defgh000 00000000 00000000
     //
     // where B = NOT(b);
-
-    FPUnion.I = 0;
-    FPUnion.I |= Sign << 31;
-    FPUnion.I |= ((Exp & 0x4) != 0 ? 0 : 1) << 30;
-    FPUnion.I |= ((Exp & 0x4) != 0 ? 0x1f : 0) << 25;
-    FPUnion.I |= (Exp & 0x3) << 23;
-    FPUnion.I |= Mantissa << 19;
-    return FPUnion.F;
+    uint32_t I = 0;
+    I |= Sign << 31;
+    I |= ((Exp & 0x4) != 0 ? 0 : 1) << 30;
+    I |= ((Exp & 0x4) != 0 ? 0x1f : 0) << 25;
+    I |= (Exp & 0x3) << 23;
+    I |= Mantissa << 19;
+    return bit_cast<float>(F);
   }
 
   /// getFP16Imm - Return an 8-bit floating-point version of the 16-bit
   /// floating-point value. If the value cannot be represented as an 8-bit
   /// floating-point value, then return -1.
   inline int getFP16Imm(const APInt &Imm) {
     uint32_t Sign = Imm.lshr(15).getZExtValue() & 1;
     int32_t Exp = (Imm.lshr(10).getSExtValue() & 0x1f) - 15;  // -14 to 15