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

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222137 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rafael Espindola 2014-11-17 15:46:21 +00:00
parent 18e5ce0188
commit 6e61dd242b
1 changed files with 3 additions and 5 deletions
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8
include/llvm/Bitcode/BitstreamReader.h
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@ -338,16 +338,14 @@ public:
} }
word_t Read(unsigned NumBits) { word_t Read(unsigned NumBits) {
static const unsigned BitsInWord = sizeof(word_t) * 8; assert(NumBits && NumBits <= sizeof(word_t) * 8 &&
assert(NumBits && NumBits <= BitsInWord &&
"Cannot return zero or more than BitsInWord bits!"); "Cannot return zero or more than BitsInWord bits!");
static const unsigned Mask = sizeof(word_t) > 4 ? 0x3f : 0x1f; static const unsigned Mask = sizeof(word_t) > 4 ? 0x3f : 0x1f;
// If the field is fully contained by CurWord, return it quickly. // If the field is fully contained by CurWord, return it quickly.
if (BitsInCurWord >= NumBits) { 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. // Use a mask to avoid undefined behavior.
CurWord >>= (NumBits & Mask); CurWord >>= (NumBits & Mask);
@ -365,7 +363,7 @@ public:
if (BitsLeft > BitsInCurWord) if (BitsLeft > BitsInCurWord)
return 0; 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. // Use a mask to avoid undefined behavior.
CurWord >>= (BitsLeft & Mask); CurWord >>= (BitsLeft & Mask);