[APInt] Add a utility method to change the bit width and storage size of an APInt.

[craig.topper]

Summary

This adds a resize method to APInt that manages deleting/allocating storage for an APInt and changes its bit width. Use this to simplify code in copy assignment and divide.

The assignment code in particular was overly complicated. Treating every possible case as a separate implementation. I'm also pretty sure the clearUnusedBits code at the end was unnecessary. Since we always copying whole words from the source APInt. All unused bits should be clear in the source.

[RKSimon]

include/llvm/ADT/APInt.h

162 ↗

(On Diff #98534)

Make it clear that none of the bits are guaranteed - not just any 'new' bits.

lib/Support/APInt.cpp

127 ↗

(On Diff #98534)

Shouldn't this be == ?

[hans]

include/llvm/ADT/APInt.h

163 ↗

(On Diff #98534)

Since it's nuking the contents of the APInt, I wonder if resize() is the right name; it might confuse users who think it's similar to std::vector's resize() for example.

How about reallocate()?

lib/Support/APInt.cpp

1502 ↗

(On Diff #98534)

A reader might interpret "clear out" as setting to zero, which resize() doesn't do right?

[craig.topper]

Rename to reallocate. Clarify comments.

[RKSimon]

LGTM

lib/Support/APInt.cpp

150 ↗

(On Diff #98650)

Strip the 'this'?

llvm/trunk/include/llvm/ADT/APInt.h

private:
   }
 
   /// \brief Get the word corresponding to a bit position
   /// \returns the corresponding word for the specified bit position.
   uint64_t getWord(unsigned bitPosition) const {
     return isSingleWord() ? U.VAL : U.pVal[whichWord(bitPosition)];
   }
 
+  /// Utility method to change the bit width of this APInt to new bit width,
+  /// allocating and/or deallocating as necessary. There is no guarantee on the
+  /// value of any bits upon return. Caller should populate the bits after.
+  void reallocate(unsigned NewBitWidth);
+
   /// \brief Convert a char array into an APInt
   ///
   /// \param radix 2, 8, 10, 16, or 36
   /// Converts a string into a number.  The string must be non-empty
   /// and well-formed as a number of the given base. The bit-width
   /// must be sufficient to hold the result.
   ///
   /// This is used by the constructors that take string arguments.

llvm/trunk/lib/Support/APInt.cpp

 }
 
 APInt::APInt(unsigned numbits, StringRef Str, uint8_t radix)
   : BitWidth(numbits) {
   assert(BitWidth && "Bitwidth too small");
   fromString(numbits, Str, radix);
 }
 
+void APInt::reallocate(unsigned NewBitWidth) {
+  // If the number of words is the same we can just change the width and stop.
+  if (getNumWords() == getNumWords(NewBitWidth)) {
+    BitWidth = NewBitWidth;
+    return;
+  }
+
+  // If we have an allocation, delete it.
+  if (!isSingleWord())
+    delete [] U.pVal;
+
+  // Update BitWidth.
+  BitWidth = NewBitWidth;
+
+  // If we are supposed to have an allocation, create it.
+  if (!isSingleWord())
+    U.pVal = getMemory(getNumWords());
+}
+
 void APInt::AssignSlowCase(const APInt& RHS) {
   // Don't do anything for X = X
   if (this == &RHS)
     return;
 
-  if (BitWidth == RHS.getBitWidth()) {
-    // assume same bit-width single-word case is already handled
-    assert(!isSingleWord());
-    memcpy(U.pVal, RHS.U.pVal, getNumWords() * APINT_WORD_SIZE);
-    return;
-  }
+  // Adjust the bit width and handle allocations as necessary.
+  reallocate(RHS.getBitWidth());
 
-  if (isSingleWord()) {
-    // assume case where both are single words is already handled
-    assert(!RHS.isSingleWord());
-    U.pVal = getMemory(RHS.getNumWords());
-    memcpy(U.pVal, RHS.U.pVal, RHS.getNumWords() * APINT_WORD_SIZE);
-  } else if (getNumWords() == RHS.getNumWords())
-    memcpy(U.pVal, RHS.U.pVal, RHS.getNumWords() * APINT_WORD_SIZE);
-  else if (RHS.isSingleWord()) {
-    delete [] U.pVal;
+  // Copy the data.
+  if (isSingleWord())
     U.VAL = RHS.U.VAL;
-  } else {
-    delete [] U.pVal;
-    U.pVal = getMemory(RHS.getNumWords());
-    memcpy(U.pVal, RHS.U.pVal, RHS.getNumWords() * APINT_WORD_SIZE);
-  }
-  BitWidth = RHS.BitWidth;
-  clearUnusedBits();
+  else
+    memcpy(U.pVal, RHS.U.pVal, getNumWords() * APINT_WORD_SIZE);
 }
 
 /// This method 'profiles' an APInt for use with FoldingSet.
 void APInt::Profile(FoldingSetNodeID& ID) const {
   ID.AddInteger(BitWidth);
 
   if (isSingleWord()) {
     ID.AddInteger(U.VAL);
if (n == 1) {
     // Now we're ready to invoke the Knuth classical divide algorithm. In this
     // case n > 1.
     KnuthDiv(U, V, Q, R, m, n);
   }
 
   // If the caller wants the quotient
   if (Quotient) {
     // Set up the Quotient value's memory.
-    if (Quotient->BitWidth != LHS.BitWidth) {
-      if (Quotient->isSingleWord())
-        Quotient->U.VAL = 0;
-      else
-        delete [] Quotient->U.pVal;
-      Quotient->BitWidth = LHS.BitWidth;
-      if (!Quotient->isSingleWord())
-        Quotient->U.pVal = getClearedMemory(Quotient->getNumWords());
-    } else
-      Quotient->clearAllBits();
+    Quotient->reallocate(LHS.BitWidth);
+    // Clear out any previous bits.
 
     // The quotient is in Q. Reconstitute the quotient into Quotient's low
     // order words.
     // This case is currently dead as all users of divide() handle trivial cases
     // earlier.
     if (lhsWords == 1) {
       uint64_t tmp = Make_64(Q[1], Q[0]);
       if (Quotient->isSingleWord())
         Quotient->U.VAL = tmp;
       else
         Quotient->U.pVal[0] = tmp;
     } else {
       assert(!Quotient->isSingleWord() && "Quotient APInt not large enough");
       for (unsigned i = 0; i < lhsWords; ++i)
         Quotient->U.pVal[i] = Make_64(Q[i*2+1], Q[i*2]);
     }
   }
 
   // If the caller wants the remainder
   if (Remainder) {
     // Set up the Remainder value's memory.
-    if (Remainder->BitWidth != RHS.BitWidth) {
-      if (Remainder->isSingleWord())
-        Remainder->U.VAL = 0;
-      else
-        delete [] Remainder->U.pVal;
-      Remainder->BitWidth = RHS.BitWidth;
-      if (!Remainder->isSingleWord())
-        Remainder->U.pVal = getClearedMemory(Remainder->getNumWords());
-    } else
-      Remainder->clearAllBits();
+    Remainder->reallocate(RHS.BitWidth);
+    // Clear out any previous bits.
 
     // The remainder is in R. Reconstitute the remainder into Remainder's low
     // order words.
     if (rhsWords == 1) {
       uint64_t tmp = Make_64(R[1], R[0]);
       if (Remainder->isSingleWord())
         Remainder->U.VAL = tmp;
       else