Use the return of readBytes to find out if we are at the end of the stream.

This allows the removal of isObjectEnd and opens the way for reading 64 bits
at a time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221804 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Bitcode/BitstreamReader.h

@@ -169,6 +169,9 @@ class BitstreamCursor {
   BitstreamReader *BitStream;
   size_t NextChar;
 
+  // The size of the bicode. 0 if we don't know it yet.
+  size_t Size;
+
   /// This is the current data we have pulled from the stream but have not
   /// returned to the client. This is specifically and intentionally defined to
   /// follow the word size of the host machine for efficiency. We use word_t in
@@ -208,17 +211,13 @@ public:
 
     BitStream = R;
     NextChar = 0;
-    CurWord = 0;
+    Size = 0;
     BitsInCurWord = 0;
     CurCodeSize = 2;
   }
 
   void freeState();
 
-  bool isEndPos(size_t pos) {
-    return BitStream->getBitcodeBytes().isObjectEnd(static_cast<uint64_t>(pos));
-  }
-
   bool canSkipToPos(size_t pos) const {
     // pos can be skipped to if it is a valid address or one byte past the end.
     return pos == 0 || BitStream->getBitcodeBytes().isValidAddress(
@@ -226,7 +225,12 @@ public:
   }
 
   bool AtEndOfStream() {
-    return BitsInCurWord == 0 && isEndPos(NextChar);
+    if (BitsInCurWord != 0)
+      return false;
+    if (Size == NextChar)
+      return true;
+    fillCurWord();
+    return BitsInCurWord == 0;
   }
 
   /// Return the number of bits used to encode an abbrev #.
@@ -305,7 +309,6 @@ public:
     // Move the cursor to the right word.
     NextChar = ByteNo;
     BitsInCurWord = 0;
-    CurWord = 0;
 
     // Skip over any bits that are already consumed.
     if (WordBitNo) {
@@ -316,6 +319,31 @@ public:
     }
   }
 
+  void fillCurWord() {
+    assert(Size == 0 || NextChar < (unsigned)Size);
+
+    // Read the next word from the stream.
+    uint8_t Array[sizeof(word_t)] = {0};
+
+    uint64_t BytesRead =
+        BitStream->getBitcodeBytes().readBytes(Array, sizeof(Array), NextChar);
+
+    // If we run out of data, stop at the end of the stream.
+    if (BytesRead == 0) {
+      Size = NextChar;
+      return;
+    }
+    assert(BytesRead == sizeof(Array));
+
+    // Handle big-endian byte-swapping if necessary.
+    support::detail::packed_endian_specific_integral<
+        word_t, support::little, support::unaligned> EndianValue;
+    memcpy(&EndianValue, Array, sizeof(Array));
+
+    CurWord = EndianValue;
+    NextChar += sizeof(word_t);
+    BitsInCurWord = sizeof(word_t) * 8;
+  }
 
   uint32_t Read(unsigned NumBits) {
     assert(NumBits && NumBits <= 32 &&
@@ -324,48 +352,32 @@ public:
     // If the field is fully contained by CurWord, return it quickly.
     if (BitsInCurWord >= NumBits) {
       uint32_t R = uint32_t(CurWord) & (~0U >> (32-NumBits));
-      CurWord >>= NumBits;
+
+      // Use a mask to avoid undefined behavior.
+      CurWord >>= (NumBits & 0x1f);
+
       BitsInCurWord -= NumBits;
       return R;
     }
 
+    uint32_t R = BitsInCurWord ? uint32_t(CurWord) : 0;
+    unsigned BitsLeft = NumBits - BitsInCurWord;
+
+    fillCurWord();
+
     // If we run out of data, stop at the end of the stream.
-    if (isEndPos(NextChar)) {
-      CurWord = 0;
-      BitsInCurWord = 0;
+    if (BitsLeft > BitsInCurWord)
       return 0;
-    }
 
-    uint32_t R = uint32_t(CurWord);
+    uint32_t R2 = uint32_t(CurWord) & (~0U >> (sizeof(word_t) * 8 - BitsLeft));
 
-    // Read the next word from the stream.
-    uint8_t Array[sizeof(word_t)] = {0};
+    // Use a mask to avoid undefined behavior.
+    CurWord >>= (BitsLeft & 0x1f);
 
-    BitStream->getBitcodeBytes().readBytes(Array, sizeof(Array), NextChar);
+    BitsInCurWord -= BitsLeft;
 
-    // Handle big-endian byte-swapping if necessary.
-    support::detail::packed_endian_specific_integral
-      <word_t, support::little, support::unaligned> EndianValue;
-    memcpy(&EndianValue, Array, sizeof(Array));
+    R |= uint32_t(R2 << (NumBits - BitsLeft));
 
-    CurWord = EndianValue;
-
-    NextChar += sizeof(word_t);
-
-    // Extract NumBits-BitsInCurWord from what we just read.
-    unsigned BitsLeft = NumBits-BitsInCurWord;
-
-    // Be careful here, BitsLeft is in the range [1..32]/[1..64] inclusive.
-    R |= uint32_t((CurWord & (word_t(~0ULL) >> (sizeof(word_t)*8-BitsLeft)))
-                    << BitsInCurWord);
-
-    // BitsLeft bits have just been used up from CurWord.  BitsLeft is in the
-    // range [1..32]/[1..64] so be careful how we shift.
-    if (BitsLeft != sizeof(word_t)*8)
-      CurWord >>= BitsLeft;
-    else
-      CurWord = 0;
-    BitsInCurWord = sizeof(word_t)*8-BitsLeft;
     return R;
   }
 
@@ -426,7 +438,6 @@ private:
     }
 
     BitsInCurWord = 0;
-    CurWord = 0;
   }
 public: