From cc697fc292b0a405eaa42c6c8e5f117ba4f7d73b Mon Sep 17 00:00:00 2001
From: Chris Lattner <clattner@nondot.org>
Date: Mon, 4 Oct 2021 21:33:51 -0700
Subject: [PATCH] [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
---
 llvm/lib/Support/APInt.cpp       |  8 ++++++--
 llvm/unittests/ADT/APIntTest.cpp | 11 ++++++++++-
 2 files changed, 16 insertions(+), 3 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;
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index fe7a231..59a8f4c 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1874,6 +1874,10 @@ TEST(APIntTest, insertBits) {
   i63.insertBits(iSrc, 4);
   EXPECT_EQ(static_cast<int64_t>(0x012345600123456Full), i63.getSExtValue());
 
+  // Zero width insert is a noop.
+  i31.insertBits(APInt::getZeroWidth(), 1);
+  EXPECT_EQ(static_cast<int64_t>(0x00123456ull), i31.getSExtValue());
+
   // Insert single word src into one word of dst.
   APInt i120(120, UINT64_MAX, true);
   i120.insertBits(iSrc, 8);
@@ -2587,7 +2591,7 @@ TEST(APIntTest, truncOrSelf) {
   EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(64));
 }
 
-TEST(APIntTest, concatMSB) {
+TEST(APIntTest, concat) {
   APInt Int1(4, 0x1ULL);
   APInt Int3(4, 0x3ULL);
 
@@ -2597,6 +2601,11 @@ TEST(APIntTest, concatMSB) {
 
   APInt I64(64, 0x3ULL);
   EXPECT_EQ(I64, I64.concat(I64).lshr(64).trunc(64));
+
+  APInt I65(65, 0x3ULL);
+  APInt I0 = APInt::getZeroWidth();
+  EXPECT_EQ(I65, I65.concat(I0));
+  EXPECT_EQ(I65, I0.concat(I65));
 }
 
 TEST(APIntTest, multiply) {
-- 
2.7.4