[APInt] Optimize APInt creation from uint64_t

[craig.topper]

Summary

This patch moves the clearUnusedBits calls into the two different initialization paths for APInt from a uint64_t. This allows the compiler to better optimize the clearing of the unused bits for the single word case. And it puts the clearing for the multi word case into the initSlowCase function to save code. In the common case of initializing with 0 this allows the clearing to be completely optimized out for the single word case.

On my local x86 build this is showing a ~45kb reduction in the size of the opt binary.

[hans]

lgtm

include/llvm/ADT/APInt.h

246

Maybe it's just me, but I always find } else to look weird. I'd suggest putting braces around the else-branch too.

include/llvm/ADT/APInt.h

public:
   /// the range of val are zero filled).
   ///
   /// \param numBits the bit width of the constructed APInt
   /// \param val the initial value of the APInt
   /// \param isSigned how to treat signedness of val
   APInt(unsigned numBits, uint64_t val, bool isSigned = false)
       : BitWidth(numBits), VAL(0) {
     assert(BitWidth && "bitwidth too small");
-    if (isSingleWord())
+    if (isSingleWord()) {
       VAL = val;
-    else
-      initSlowCase(val, isSigned);
       clearUnusedBits();
+    } else
+      initSlowCase(val, isSigned);
   }
 
   /// \brief Construct an APInt of numBits width, initialized as bigVal[].
   ///
   /// Note that bigVal.size() can be smaller or larger than the corresponding
   /// bit width but any extraneous bits will be dropped.
   ///
   /// \param numBits the bit width of the constructed APInt

lib/Support/APInt.cpp

 
 
 void APInt::initSlowCase(uint64_t val, bool isSigned) {
   pVal = getClearedMemory(getNumWords());
   pVal[0] = val;
   if (isSigned && int64_t(val) < 0)
     for (unsigned i = 1; i < getNumWords(); ++i)
       pVal[i] = -1ULL;
+  clearUnusedBits();
 }
 
 void APInt::initSlowCase(const APInt& that) {
   pVal = getMemory(getNumWords());
   memcpy(pVal, that.pVal, getNumWords() * APINT_WORD_SIZE);
 }
 
 void APInt::initFromArray(ArrayRef<uint64_t> bigVal) {