[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954

include/llvm/ADT/APInt.h

@@ -2212,6 +2212,15 @@ Optional<APInt> SolveQuadraticEquationWrap(APInt A, APInt B, APInt C,
 // See friend declaration above. This additional declaration is required in
 // order to compile LLVM with IBM xlC compiler.
 hash_code hash_value(const APInt &Arg);
-} // End of llvm namespace
+
+/// StoreIntToMemory - Fills the StoreBytes bytes of memory starting from Dst
+/// with the integer held in IntVal.
+void StoreIntToMemory(const APInt &IntVal, uint8_t *Dst, unsigned StoreBytes);
+
+/// LoadIntFromMemory - Loads the integer stored in the LoadBytes bytes starting
+/// from Src into IntVal, which is assumed to be wide enough and to hold zero.
+void LoadIntFromMemory(APInt &IntVal, uint8_t *Src, unsigned LoadBytes);
+
+} // namespace llvm
 
 #endif

lib/ExecutionEngine/ExecutionEngine.cpp

@@ -1019,32 +1019,6 @@ GenericValue ExecutionEngine::getConstantValue(const Constant *C) {
   return Result;
 }
 
-/// StoreIntToMemory - Fills the StoreBytes bytes of memory starting from Dst
-/// with the integer held in IntVal.
-static void StoreIntToMemory(const APInt &IntVal, uint8_t *Dst,
-                             unsigned StoreBytes) {
-  assert((IntVal.getBitWidth()+7)/8 >= StoreBytes && "Integer too small!");
-  const uint8_t *Src = (const uint8_t *)IntVal.getRawData();
-
-  if (sys::IsLittleEndianHost) {
-    // Little-endian host - the source is ordered from LSB to MSB.  Order the
-    // destination from LSB to MSB: Do a straight copy.
-    memcpy(Dst, Src, StoreBytes);
-  } else {
-    // Big-endian host - the source is an array of 64 bit words ordered from
-    // LSW to MSW.  Each word is ordered from MSB to LSB.  Order the destination
-    // from MSB to LSB: Reverse the word order, but not the bytes in a word.
-    while (StoreBytes > sizeof(uint64_t)) {
-      StoreBytes -= sizeof(uint64_t);
-      // May not be aligned so use memcpy.
-      memcpy(Dst + StoreBytes, Src, sizeof(uint64_t));
-      Src += sizeof(uint64_t);
-    }
-
-    memcpy(Dst, Src + sizeof(uint64_t) - StoreBytes, StoreBytes);
-  }
-}
-
 void ExecutionEngine::StoreValueToMemory(const GenericValue &Val,
                                          GenericValue *Ptr, Type *Ty) {
   const unsigned StoreBytes = getDataLayout().getTypeStoreSize(Ty);
@@ -1092,33 +1066,6 @@ void ExecutionEngine::StoreValueToMemory(const GenericValue &Val,
     std::reverse((uint8_t*)Ptr, StoreBytes + (uint8_t*)Ptr);
 }
 
-/// LoadIntFromMemory - Loads the integer stored in the LoadBytes bytes starting
-/// from Src into IntVal, which is assumed to be wide enough and to hold zero.
-static void LoadIntFromMemory(APInt &IntVal, uint8_t *Src, unsigned LoadBytes) {
-  assert((IntVal.getBitWidth()+7)/8 >= LoadBytes && "Integer too small!");
-  uint8_t *Dst = reinterpret_cast<uint8_t *>(
-                   const_cast<uint64_t *>(IntVal.getRawData()));
-
-  if (sys::IsLittleEndianHost)
-    // Little-endian host - the destination must be ordered from LSB to MSB.
-    // The source is ordered from LSB to MSB: Do a straight copy.
-    memcpy(Dst, Src, LoadBytes);
-  else {
-    // Big-endian - the destination is an array of 64 bit words ordered from
-    // LSW to MSW.  Each word must be ordered from MSB to LSB.  The source is
-    // ordered from MSB to LSB: Reverse the word order, but not the bytes in
-    // a word.
-    while (LoadBytes > sizeof(uint64_t)) {
-      LoadBytes -= sizeof(uint64_t);
-      // May not be aligned so use memcpy.
-      memcpy(Dst, Src + LoadBytes, sizeof(uint64_t));
-      Dst += sizeof(uint64_t);
-    }
-
-    memcpy(Dst + sizeof(uint64_t) - LoadBytes, Src, LoadBytes);
-  }
-}
-
 /// FIXME: document
 ///
 void ExecutionEngine::LoadValueFromMemory(GenericValue &Result,

lib/Support/APInt.cpp

@@ -2934,3 +2934,56 @@ llvm::APIntOps::SolveQuadraticEquationWrap(APInt A, APInt B, APInt C,
   LLVM_DEBUG(dbgs() << __func__ << ": solution (wrap): " << X << '\n');
   return X;
 }
+
+/// StoreIntToMemory - Fills the StoreBytes bytes of memory starting from Dst
+/// with the integer held in IntVal.
+void llvm::StoreIntToMemory(const APInt &IntVal, uint8_t *Dst,
+                            unsigned StoreBytes) {
+  assert((IntVal.getBitWidth()+7)/8 >= StoreBytes && "Integer too small!");
+  const uint8_t *Src = (const uint8_t *)IntVal.getRawData();
+
+  if (sys::IsLittleEndianHost) {
+    // Little-endian host - the source is ordered from LSB to MSB.  Order the
+    // destination from LSB to MSB: Do a straight copy.
+    memcpy(Dst, Src, StoreBytes);
+  } else {
+    // Big-endian host - the source is an array of 64 bit words ordered from
+    // LSW to MSW.  Each word is ordered from MSB to LSB.  Order the destination
+    // from MSB to LSB: Reverse the word order, but not the bytes in a word.
+    while (StoreBytes > sizeof(uint64_t)) {
+      StoreBytes -= sizeof(uint64_t);
+      // May not be aligned so use memcpy.
+      memcpy(Dst + StoreBytes, Src, sizeof(uint64_t));
+      Src += sizeof(uint64_t);
+    }
+
+    memcpy(Dst, Src + sizeof(uint64_t) - StoreBytes, StoreBytes);
+  }
+}
+
+/// LoadIntFromMemory - Loads the integer stored in the LoadBytes bytes starting
+/// from Src into IntVal, which is assumed to be wide enough and to hold zero.
+void llvm::LoadIntFromMemory(APInt &IntVal, uint8_t *Src, unsigned LoadBytes) {
+  assert((IntVal.getBitWidth()+7)/8 >= LoadBytes && "Integer too small!");
+  uint8_t *Dst = reinterpret_cast<uint8_t *>(
+                   const_cast<uint64_t *>(IntVal.getRawData()));
+
+  if (sys::IsLittleEndianHost)
+    // Little-endian host - the destination must be ordered from LSB to MSB.
+    // The source is ordered from LSB to MSB: Do a straight copy.
+    memcpy(Dst, Src, LoadBytes);
+  else {
+    // Big-endian - the destination is an array of 64 bit words ordered from
+    // LSW to MSW.  Each word must be ordered from MSB to LSB.  The source is
+    // ordered from MSB to LSB: Reverse the word order, but not the bytes in
+    // a word.
+    while (LoadBytes > sizeof(uint64_t)) {
+      LoadBytes -= sizeof(uint64_t);
+      // May not be aligned so use memcpy.
+      memcpy(Dst, Src + LoadBytes, sizeof(uint64_t));
+      Dst += sizeof(uint64_t);
+    }
+
+    memcpy(Dst + sizeof(uint64_t) - LoadBytes, Src, LoadBytes);
+  }
+}