Bitcode: Add SimpleBitstreamCursor::getPointerToByte, etc.

Add API to SimpleBitstreamCursor to allow users to translate between
byte addresses and pointers.

  - jumpToPointer: move the bit position to a particular pointer.
  - getPointerToByte: get the pointer for a particular byte.
  - getPointerToBit: get the pointer for the byte of the current bit.
  - getCurrentByteNo: convenience function for assertions and tests.

Mainly adds unit tests (getPointerToBit/Byte already has a use), but
also preparation for eventually using jumpToPointer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264546 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitstreamReader.h

@@ -158,6 +158,7 @@ public:
 
   SimpleBitstreamCursor() = default;
 
+  explicit SimpleBitstreamCursor(BitstreamReader &R) : R(&R) {}
   explicit SimpleBitstreamCursor(BitstreamReader *R) : R(R) {}
 
   bool canSkipToPos(size_t pos) const {
@@ -180,6 +181,9 @@ public:
     return NextChar*CHAR_BIT - BitsInCurWord;
   }
 
+  // Return the byte # of the current bit.
+  uint64_t getCurrentByteNo() const { return GetCurrentBitNo() / 8; }
+
   BitstreamReader *getBitStreamReader() { return R; }
   const BitstreamReader *getBitStreamReader() const { return R; }
 
@@ -198,6 +202,37 @@ public:
       Read(WordBitNo);
   }
 
+  /// Reset the stream to the bit pointed at by the specified pointer.
+  ///
+  /// The pointer must be a dereferenceable pointer into the bytes in the
+  /// underlying memory object.
+  void jumpToPointer(const uint8_t *Pointer) {
+    auto *Pointer0 = getPointerToByte(0, 1);
+    assert((intptr_t)Pointer0 <= (intptr_t)Pointer &&
+           "Expected pointer into bitstream");
+
+    JumpToBit(8 * (Pointer - Pointer0));
+    assert((intptr_t)getPointerToByte(getCurrentByteNo(), 1) ==
+               (intptr_t)Pointer &&
+           "Expected to reach pointer");
+  }
+  void jumpToPointer(const char *Pointer) {
+    jumpToPointer((const uint8_t *)Pointer);
+  }
+
+  /// Get a pointer into the bitstream at the specified byte offset.
+  const uint8_t *getPointerToByte(uint64_t ByteNo, uint64_t NumBytes) {
+    return R->getBitcodeBytes().getPointer(ByteNo, NumBytes);
+  }
+
+  /// Get a pointer into the bitstream at the specified bit offset.
+  ///
+  /// The bit offset must be on a byte boundary.
+  const uint8_t *getPointerToBit(uint64_t BitNo, uint64_t NumBytes) {
+    assert(!(BitNo % 8) && "Expected bit on byte boundary");
+    return getPointerToByte(BitNo / 8, NumBytes);
+  }
+
   void fillCurWord() {
     if (Size != 0 && NextChar >= Size)
       report_fatal_error("Unexpected end of file");

lib/Bitcode/Reader/BitstreamReader.cpp

@@ -261,9 +261,7 @@ unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
     }
 
     // Otherwise, inform the streamer that we need these bytes in memory.
-    const char *Ptr =
-        (const char *)getBitStreamReader()->getBitcodeBytes().getPointer(
-            CurBitPos / 8, NumElts);
+    const char *Ptr = (const char *)getPointerToBit(CurBitPos, NumElts);
 
     // If we can return a reference to the data, do so to avoid copying it.
     if (Blob) {

unittests/Bitcode/BitstreamReaderTest.cpp

@@ -53,4 +53,47 @@ TEST(BitstreamReaderTest, AtEndOfStreamEmpty) {
   EXPECT_TRUE(Cursor.AtEndOfStream());
 }
 
+TEST(BitstreamReaderTest, getCurrentByteNo) {
+  uint8_t Bytes[] = {0x00, 0x01, 0x02, 0x03};
+  BitstreamReader Reader(std::begin(Bytes), std::end(Bytes));
+  SimpleBitstreamCursor Cursor(Reader);
+
+  for (unsigned I = 0, E = 33; I != E; ++I) {
+    EXPECT_EQ(I / 8, Cursor.getCurrentByteNo());
+    (void)Cursor.Read(1);
+  }
+  EXPECT_EQ(4u, Cursor.getCurrentByteNo());
+}
+
+TEST(BitstreamReaderTest, getPointerToByte) {
+  uint8_t Bytes[] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07};
+  BitstreamReader Reader(std::begin(Bytes), std::end(Bytes));
+  SimpleBitstreamCursor Cursor(Reader);
+
+  for (unsigned I = 0, E = 8; I != E; ++I) {
+    EXPECT_EQ(Bytes + I, Cursor.getPointerToByte(I, 1));
+  }
+}
+
+TEST(BitstreamReaderTest, getPointerToBit) {
+  uint8_t Bytes[] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07};
+  BitstreamReader Reader(std::begin(Bytes), std::end(Bytes));
+  SimpleBitstreamCursor Cursor(Reader);
+
+  for (unsigned I = 0, E = 8; I != E; ++I) {
+    EXPECT_EQ(Bytes + I, Cursor.getPointerToBit(I * 8, 1));
+  }
+}
+
+TEST(BitstreamReaderTest, jumpToPointer) {
+  uint8_t Bytes[] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07};
+  BitstreamReader Reader(std::begin(Bytes), std::end(Bytes));
+  SimpleBitstreamCursor Cursor(Reader);
+
+  for (unsigned I : {0, 6, 2, 7}) {
+    Cursor.jumpToPointer(Bytes + I);
+    EXPECT_EQ(I, Cursor.getCurrentByteNo());
+  }
+}
+
 } // end anonymous namespace