[APInt] Make clearUnusedBits branch free.

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

diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
index c697402..4b966fa 100644 (file)
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -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