diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -868,8 +868,29 @@
 }
 
 Value *LibCallSimplifier::optimizeMemRChr(CallInst *CI, IRBuilderBase &B) {
-  if (isKnownNonZero(CI->getOperand(2), DL))
-    annotateNonNullNoUndefBasedOnAccess(CI, 0);
+  Value *SrcStr = CI->getArgOperand(0);
+  Value *Size = CI->getArgOperand(2);
+  annotateNonNullAndDereferenceable(CI, 0, Size, DL);
+  Value *CharVal = CI->getArgOperand(1);
+  ConstantInt *LenC = dyn_cast<ConstantInt>(Size);
+
+  if (LenC) {
+    if (LenC->isZero())
+      // Fold memrchr(x, y, 0) --> null.
+      return Constant::getNullValue(CI->getType());
+
+    if (LenC->isOne()) {
+      // Fold memrchr(x, y, 1) --> *x == y ? x : null for any x and y,
+      // constant or otherwise.
+      Value *Val = B.CreateLoad(B.getInt8Ty(), SrcStr, "memrchr.char0");
+      // Slice off the character's high end bits.
+      CharVal = B.CreateTrunc(CharVal, B.getInt8Ty());
+      Value *Cmp = B.CreateICmpEQ(Val, CharVal, "memrchr.char0cmp");
+      Value *NullPtr = Constant::getNullValue(CI->getType());
+      return B.CreateSelect(Cmp, SrcStr, NullPtr, "memrchr.sel");
+    }
+  }
+
   return nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/memrchr-2.ll b/llvm/test/Transforms/InstCombine/memrchr-2.ll
--- a/llvm/test/Transforms/InstCombine/memrchr-2.ll
+++ b/llvm/test/Transforms/InstCombine/memrchr-2.ll
@@ -15,10 +15,8 @@
 
 define i8* @fold_memrchr_a12345_1_ui32max_p1(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_a12345_1_ui32max_p1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967296)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4294967296) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967296)
 ; CHECK-NEXT:    ret i8* [[RET]]
-;
-; CHECK   :      ret i8* null
 
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i64 0, i64 0
   %ret = call i8* @memrchr(i8* %ptr, i32 %0, i64 4294967296)
@@ -30,10 +28,8 @@
 
 define i8* @fold_memrchr_ax1_1_ui32max_p2(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_ax1_1_ui32max_p2(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([1 x i8], [1 x i8]* @ax1, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967297)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4294967297) getelementptr inbounds ([1 x i8], [1 x i8]* @ax1, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967297)
 ; CHECK-NEXT:    ret i8* [[RET]]
-;
-; CHECK   :      ret i8* null
 
   %ptr = getelementptr [1 x i8], [1 x i8]* @ax1, i64 0, i64 0
   %ret = call i8* @memrchr(i8* %ptr, i32 %0, i64 4294967297)
@@ -45,7 +41,7 @@
 
 define i8* @fold_memrchr_ax_1_ui32max_p2(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_ax_1_ui32max_p2(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967297)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4294967297) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4294967297)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -59,10 +55,8 @@
 
 define i8* @fold_memrchr_a12345_c_6(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_a12345_c_6(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 6)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(6) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 6)
 ; CHECK-NEXT:    ret i8* [[RET]]
-;
-; CHECK   :      ret i8* null
 
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i64 0, i64 0
   %ret = call i8* @memrchr(i8* %ptr, i32 %0, i64 6)
@@ -74,10 +68,8 @@
 
 define i8* @fold_memrchr_a12345_c_szmax(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_a12345_c_szmax(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 -1)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(18446744073709551615) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 [[TMP0:%.*]], i64 -1)
 ; CHECK-NEXT:    ret i8* [[RET]]
-;
-; CHECK   :      ret i8* null
 
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i64 0, i64 0
   %ret = call i8* @memrchr(i8* %ptr, i32 %0, i64 18446744073709551615)
diff --git a/llvm/test/Transforms/InstCombine/memrchr-3.ll b/llvm/test/Transforms/InstCombine/memrchr-3.ll
--- a/llvm/test/Transforms/InstCombine/memrchr-3.ll
+++ b/llvm/test/Transforms/InstCombine/memrchr-3.ll
@@ -15,8 +15,7 @@
 
 define i8* @fold_memrchr_ax_c_0(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_ax_c_0(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* getelementptr inbounds ([0 x i8], [0 x i8]* @ax, i64 0, i64 0), i32 [[TMP0:%.*]], i64 0)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* null
 ;
 
   %ptr = getelementptr [0 x i8], [0 x i8]* @ax, i32 0, i32 0
@@ -29,8 +28,7 @@
 
 define i8* @fold_memrchr_a12345_3_0() {
 ; CHECK-LABEL: @fold_memrchr_a12345_3_0(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 3, i64 0)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* null
 ;
 
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i32 0, i32 0
@@ -43,8 +41,7 @@
 
 define i8* @fold_memrchr_a12345_1_1() {
 ; CHECK-LABEL: @fold_memrchr_a12345_1_1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 1, i64 1)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0)
 ;
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i32 0, i32 0
   %ret = call i8* @memrchr(i8* %ptr, i32 1, i64 1)
@@ -56,8 +53,7 @@
 
 define i8* @fold_memrchr_a12345_5_1() {
 ; CHECK-LABEL: @fold_memrchr_a12345_5_1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 5, i64 1)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* null
 ;
   %ptr = getelementptr [5 x i8], [5 x i8]* @a12345, i32 0, i32 0
   %ret = call i8* @memrchr(i8* %ptr, i32 5, i64 1)
@@ -69,8 +65,7 @@
 
 define i8* @fold_memrchr_a123123_1_1() {
 ; CHECK-LABEL: @fold_memrchr_a123123_1_1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 1, i64 1)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0)
 ;
   %ptr = getelementptr [6 x i8], [6 x i8]* @a123123, i32 0, i32 0
   %ret = call i8* @memrchr(i8* %ptr, i32 1, i64 1)
@@ -82,8 +77,7 @@
 
 define i8* @fold_memrchr_a123123_3_1() {
 ; CHECK-LABEL: @fold_memrchr_a123123_3_1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 3, i64 1)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    ret i8* null
 ;
   %ptr = getelementptr [6 x i8], [6 x i8]* @a123123, i32 0, i32 0
   %ret = call i8* @memrchr(i8* %ptr, i32 3, i64 1)
@@ -95,8 +89,11 @@
 
 define i8* @fold_memrchr_ax_c_1(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_ax_c_1(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([0 x i8], [0 x i8]* @ax, i64 0, i64 0), i32 [[TMP0:%.*]], i64 1)
-; CHECK-NEXT:    ret i8* [[RET]]
+; CHECK-NEXT:    [[MEMRCHR_CHAR0:%.*]] = load i8, i8* getelementptr inbounds ([0 x i8], [0 x i8]* @ax, i64 0, i64 0), align 1
+; CHECK-NEXT:    [[TMP2:%.*]] = trunc i32 [[TMP0:%.*]] to i8
+; CHECK-NEXT:    [[MEMRCHR_CHAR0CMP:%.*]] = icmp eq i8 [[MEMRCHR_CHAR0]], [[TMP2]]
+; CHECK-NEXT:    [[MEMRCHR_SEL:%.*]] = select i1 [[MEMRCHR_CHAR0CMP]], i8* getelementptr inbounds ([0 x i8], [0 x i8]* @ax, i64 0, i64 0), i8* null
+; CHECK-NEXT:    ret i8* [[MEMRCHR_SEL]]
 ;
   %ptr = getelementptr [0 x i8], [0 x i8]* @ax, i32 0, i32 0
   %ret = call i8* @memrchr(i8* %ptr, i32 %0, i64 1)
@@ -108,7 +105,7 @@
 
 define i8* @fold_memrchr_a12345_5_5() {
 ; CHECK-LABEL: @fold_memrchr_a12345_5_5(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 5, i64 5)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(5) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 5, i64 5)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -122,7 +119,7 @@
 
 define i8* @fold_memrchr_a12345_5_4() {
 ; CHECK-LABEL: @fold_memrchr_a12345_5_4(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 5, i64 4)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 5, i64 4)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -136,7 +133,7 @@
 
 define i8* @fold_memrchr_a12345_4_5() {
 ; CHECK-LABEL: @fold_memrchr_a12345_4_5(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 4, i64 5)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(5) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 4, i64 5)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -150,7 +147,7 @@
 
 define i8* @fold_memrchr_a12345p1_1_4() {
 ; CHECK-LABEL: @fold_memrchr_a12345p1_1_4(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 1), i32 5, i64 4)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 1), i32 5, i64 4)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -164,7 +161,7 @@
 
 define i8* @fold_memrchr_a12345_2_5() {
 ; CHECK-LABEL: @fold_memrchr_a12345_2_5(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 2, i64 5)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(5) getelementptr inbounds ([5 x i8], [5 x i8]* @a12345, i64 0, i64 0), i32 2, i64 5)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -192,7 +189,7 @@
 
 define i8* @fold_memrchr_a123123_3_5() {
 ; CHECK-LABEL: @fold_memrchr_a123123_3_5(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 3, i64 5)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(5) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 3, i64 5)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -206,7 +203,7 @@
 
 define i8* @fold_memrchr_a123123_3_6() {
 ; CHECK-LABEL: @fold_memrchr_a123123_3_6(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 3, i64 6)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(6) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 3, i64 6)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -219,7 +216,7 @@
 
 define i8* @fold_memrchr_a123123_2_6() {
 ; CHECK-LABEL: @fold_memrchr_a123123_2_6(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 2, i64 6)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(6) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 2, i64 6)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -232,7 +229,7 @@
 
 define i8* @fold_memrchr_a123123_1_6() {
 ; CHECK-LABEL: @fold_memrchr_a123123_1_6(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 1, i64 6)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(6) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 1, i64 6)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -246,7 +243,7 @@
 
 define i8* @fold_memrchr_a123123_0_6() {
 ; CHECK-LABEL: @fold_memrchr_a123123_0_6(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 0, i64 6)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(6) getelementptr inbounds ([6 x i8], [6 x i8]* @a123123, i64 0, i64 0), i32 0, i64 6)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
diff --git a/llvm/test/Transforms/InstCombine/memrchr-4.ll b/llvm/test/Transforms/InstCombine/memrchr-4.ll
--- a/llvm/test/Transforms/InstCombine/memrchr-4.ll
+++ b/llvm/test/Transforms/InstCombine/memrchr-4.ll
@@ -14,7 +14,7 @@
 
 define i8* @fold_memrchr_a11111_c_5(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_a11111_c_5(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @a11111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 5)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(5) getelementptr inbounds ([5 x i8], [5 x i8]* @a11111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 5)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -28,7 +28,7 @@
 
 define i8* @fold_memrchr_a1110111_c_3(i32 %0) {
 ; CHECK-LABEL: @fold_memrchr_a1110111_c_3(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 3)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(3) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 3)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -42,7 +42,7 @@
 
 define i8* @call_memrchr_a1110111_c_4(i32 %0) {
 ; CHECK-LABEL: @call_memrchr_a1110111_c_4(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(4) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 4)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;
 
@@ -56,7 +56,7 @@
 
 define i8* @call_memrchr_a11111_c_7(i32 %0) {
 ; CHECK-LABEL: @call_memrchr_a11111_c_7(
-; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 7)
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @memrchr(i8* noundef nonnull dereferenceable(7) getelementptr inbounds ([7 x i8], [7 x i8]* @a1110111, i64 0, i64 0), i32 [[TMP0:%.*]], i64 7)
 ; CHECK-NEXT:    ret i8* [[RET]]
 ;