Use a more canonical way of computing a mask with N trailing 1s. NFC.

llvm-svn: 222137

Author: espindola

llvm/include/llvm/Bitcode/BitstreamReader.h

@@ -338,16 +338,14 @@ class BitstreamCursor {
   }
 
   word_t Read(unsigned NumBits) {
-    static const unsigned BitsInWord = sizeof(word_t) * 8;
-
-    assert(NumBits && NumBits <= BitsInWord &&
+    assert(NumBits && NumBits <= sizeof(word_t) * 8 &&
            "Cannot return zero or more than BitsInWord bits!");
 
     static const unsigned Mask = sizeof(word_t) > 4 ? 0x3f : 0x1f;
 
     // If the field is fully contained by CurWord, return it quickly.
     if (BitsInCurWord >= NumBits) {
-      word_t R = CurWord & (~word_t(0) >> (BitsInWord - NumBits));
+      word_t R = CurWord & ((word_t(1) << NumBits) - 1);
 
       // Use a mask to avoid undefined behavior.
       CurWord >>= (NumBits & Mask);
@@ -365,7 +363,7 @@ class BitstreamCursor {
     if (BitsLeft > BitsInCurWord)
       return 0;
 
-    word_t R2 = CurWord & (~word_t(0) >> (BitsInWord - BitsLeft));
+    word_t R2 = CurWord & ((word_t(1) << BitsLeft) - 1);
 
     // Use a mask to avoid undefined behavior.
     CurWord >>= (BitsLeft & Mask);