diff --git a/llvm/include/llvm/ADT/APInt.h b/llvm/include/llvm/ADT/APInt.h
--- a/llvm/include/llvm/ADT/APInt.h
+++ b/llvm/include/llvm/ADT/APInt.h
@@ -1218,7 +1218,7 @@
   /// Truncate to new width.
   ///
   /// Truncate the APInt to a specified width. It is an error to specify a width
-  /// that is greater than or equal to the current width.
+  /// that is greater than the current width.
   APInt trunc(unsigned width) const;
 
   /// Truncate to new width with unsigned saturation.
@@ -1238,7 +1238,7 @@
   ///
   /// This operation sign extends the APInt to a new width. If the high order
   /// bit is set, the fill on the left will be done with 1 bits, otherwise zero.
-  /// It is an error to specify a width that is less than or equal to the
+  /// It is an error to specify a width that is less than the
   /// current width.
   APInt sext(unsigned width) const;
 
@@ -1246,7 +1246,7 @@
   ///
   /// This operation zero extends the APInt to a new width. The high order bits
   /// are filled with 0 bits.  It is an error to specify a width that is less
-  /// than or equal to the current width.
+  /// than the current width.
   APInt zext(unsigned width) const;
 
   /// Sign extend or truncate to width
diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -896,11 +896,14 @@
 
 // Truncate to new width.
 APInt APInt::trunc(unsigned width) const {
-  assert(width < BitWidth && "Invalid APInt Truncate request");
+  assert(width <= BitWidth && "Invalid APInt Truncate request");
 
   if (width <= APINT_BITS_PER_WORD)
     return APInt(width, getRawData()[0]);
 
+  if (width == BitWidth)
+    return *this;
+
   APInt Result(getMemory(getNumWords(width)), width);
 
   // Copy full words.
@@ -918,7 +921,7 @@
 
 // Truncate to new width with unsigned saturation.
 APInt APInt::truncUSat(unsigned width) const {
-  assert(width < BitWidth && "Invalid APInt Truncate request");
+  assert(width <= BitWidth && "Invalid APInt Truncate request");
 
   // Can we just losslessly truncate it?
   if (isIntN(width))
@@ -929,7 +932,7 @@
 
 // Truncate to new width with signed saturation.
 APInt APInt::truncSSat(unsigned width) const {
-  assert(width < BitWidth && "Invalid APInt Truncate request");
+  assert(width <= BitWidth && "Invalid APInt Truncate request");
 
   // Can we just losslessly truncate it?
   if (isSignedIntN(width))
@@ -941,11 +944,14 @@
 
 // Sign extend to a new width.
 APInt APInt::sext(unsigned Width) const {
-  assert(Width > BitWidth && "Invalid APInt SignExtend request");
+  assert(Width >= BitWidth && "Invalid APInt SignExtend request");
 
   if (Width <= APINT_BITS_PER_WORD)
     return APInt(Width, SignExtend64(U.VAL, BitWidth));
 
+  if (Width == BitWidth)
+    return *this;
+
   APInt Result(getMemory(getNumWords(Width)), Width);
 
   // Copy words.
@@ -965,11 +971,14 @@
 
 //  Zero extend to a new width.
 APInt APInt::zext(unsigned width) const {
-  assert(width > BitWidth && "Invalid APInt ZeroExtend request");
+  assert(width >= BitWidth && "Invalid APInt ZeroExtend request");
 
   if (width <= APINT_BITS_PER_WORD)
     return APInt(width, U.VAL);
 
+  if (width == BitWidth)
+    return *this;
+
   APInt Result(getMemory(getNumWords(width)), width);
 
   // Copy words.
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1181,18 +1181,22 @@
   APInt AP_100 = APInt(8, 100);
   APInt AP_200 = APInt(8, 200);
 
+  EXPECT_EQ(APInt(8, 100), AP_100.truncUSat(8));
   EXPECT_EQ(APInt(7, 100), AP_100.truncUSat(7));
   EXPECT_EQ(APInt(6, 63), AP_100.truncUSat(6));
   EXPECT_EQ(APInt(5, 31), AP_100.truncUSat(5));
 
+  EXPECT_EQ(APInt(8, 200), AP_200.truncUSat(8));
   EXPECT_EQ(APInt(7, 127), AP_200.truncUSat(7));
   EXPECT_EQ(APInt(6, 63), AP_200.truncUSat(6));
   EXPECT_EQ(APInt(5, 31), AP_200.truncUSat(5));
 
+  EXPECT_EQ(APInt(8, 42), AP_42.truncSSat(8));
   EXPECT_EQ(APInt(7, 42), AP_42.truncSSat(7));
   EXPECT_EQ(APInt(6, 31), AP_42.truncSSat(6));
   EXPECT_EQ(APInt(5, 15), AP_42.truncSSat(5));
 
+  EXPECT_EQ(APInt(8, -56), AP_200.truncSSat(8));
   EXPECT_EQ(APInt(7, -56), AP_200.truncSSat(7));
   EXPECT_EQ(APInt(6, -32), AP_200.truncSSat(6));
   EXPECT_EQ(APInt(5, -16), AP_200.truncSSat(5));
@@ -2637,23 +2641,28 @@
   EXPECT_EQ(~uint64_t(0), APInt(1, 1).sext(64));
 
   APInt i32_max(APInt::getSignedMaxValue(32).sext(63));
+  EXPECT_EQ(i32_max, i32_max.sext(63));
   EXPECT_EQ(32U, i32_max.countLeadingZeros());
   EXPECT_EQ(0U, i32_max.countTrailingZeros());
   EXPECT_EQ(31U, i32_max.countPopulation());
 
   APInt i32_min(APInt::getSignedMinValue(32).sext(63));
+  EXPECT_EQ(i32_min, i32_min.sext(63));
   EXPECT_EQ(32U, i32_min.countLeadingOnes());
   EXPECT_EQ(31U, i32_min.countTrailingZeros());
   EXPECT_EQ(32U, i32_min.countPopulation());
 
   APInt i32_neg1(APInt(32, ~uint64_t(0)).sext(63));
+  EXPECT_EQ(i32_neg1, i32_neg1.sext(63));
   EXPECT_EQ(63U, i32_neg1.countLeadingOnes());
   EXPECT_EQ(0U, i32_neg1.countTrailingZeros());
   EXPECT_EQ(63U, i32_neg1.countPopulation());
 }
 
-TEST(APIntTest, truncOrSelf) {
+TEST(APIntTest, trunc) {
   APInt val(32, 0xFFFFFFFF);
+  EXPECT_EQ(0xFFFF, val.trunc(16));
+  EXPECT_EQ(0xFFFFFFFF, val.trunc(32));
   EXPECT_EQ(0xFFFF, val.truncOrSelf(16));
   EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(32));
   EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(64));