[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

1 files changed, 53 insertions, 0 deletions

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 3b40d9bd734..43173311cd8 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/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);
+  }
+}