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[APInt] Make clearUnusedBits branch free.

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This makes the WordBits calculation calculate a value between 1 and 64 for the number of bits in the last word. Previously if the BitWidth was a multiple of 64 bits the WordBits value was 0 and we had to bail out early to avoid an undefined shift. Now with a value of 64 we no longer have an undefined shift issue.

This shows a 15-16k reduction in the size of the opt binary on my local x86-64 build.

llvm-svn: 301134
This commit is contained in:
Craig Topper 2017-04-23 17:16:26 +00:00
parent 976b7b2cf5
commit 72d61a457f
1 changed files with 2 additions and 7 deletions
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9
include/llvm/ADT/APInt.h
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@ -138,15 +138,10 @@ private:
/// zero'd out.
APInt &clearUnusedBits() {
// Compute how many bits are used in the final word
unsigned wordBits = BitWidth % APINT_BITS_PER_WORD;
if (wordBits == 0)
// If all bits are used, we want to leave the value alone. This also
// avoids the undefined behavior of >> when the shift is the same size as
// the word size (64).
return *this;
unsigned WordBits = ((BitWidth-1) % APINT_BITS_PER_WORD) + 1;
// Mask out the high bits.
uint64_t mask = WORD_MAX >> (APINT_BITS_PER_WORD - wordBits);
uint64_t mask = WORD_MAX >> (APINT_BITS_PER_WORD - WordBits);
if (isSingleWord())
VAL &= mask;
else