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

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.

This was originally committed as r341728 and reverted in r341730.

Reviewers: javed.absar, steven_wu, srhines

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D51693

llvm-svn: 341741

1 files changed, 9 insertions, 12 deletions

diff --git a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AddressingModes.h b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AddressingModes.h
index 62644ab2f45..34b732a1ff4 100644
--- a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AddressingModes.h
+++ b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64AddressingModes.h
@@ -16,6 +16,7 @@
 
 #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>
@@ -342,27 +343,23 @@ static inline bool isValidDecodeLogicalImmediate(uint64_t val,
 //
 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