[APInt] Make insertBits and concat work with zero width APInts.

These should both clearly work with our current model for zero width
integers, but don't until now!

Differential Revision: https://reviews.llvm.org/D111113

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
index 9a22ccb..67ea57c 100644
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -345,7 +345,7 @@ void APInt::flipAllBitsSlowCase() {
 /// In the slow case, we know the result is large.
 APInt APInt::concatSlowCase(const APInt &NewLSB) const {
   unsigned NewWidth = getBitWidth() + NewLSB.getBitWidth();
-  APInt Result = NewLSB.zext(NewWidth);
+  APInt Result = NewLSB.zextOrSelf(NewWidth);
   Result.insertBits(*this, NewLSB.getBitWidth());
   return Result;
 }
@@ -360,9 +360,13 @@ void APInt::flipBit(unsigned bitPosition) {
 
 void APInt::insertBits(const APInt &subBits, unsigned bitPosition) {
   unsigned subBitWidth = subBits.getBitWidth();
-  assert(0 < subBitWidth && (subBitWidth + bitPosition) <= BitWidth &&
+  assert(subBitWidth >= 0 && (subBitWidth + bitPosition) <= BitWidth &&
          "Illegal bit insertion");
 
+  // inserting no bits is a noop.
+  if (subBitWidth == 0)
+    return;
+
   // Insertion is a direct copy.
   if (subBitWidth == BitWidth) {
     *this = subBits;