diff --git a/llvm/include/llvm/Analysis/TargetLibraryInfo.h b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
--- a/llvm/include/llvm/Analysis/TargetLibraryInfo.h
+++ b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
@@ -52,7 +52,7 @@
   unsigned char AvailableArray[(NumLibFuncs+3)/4];
   DenseMap<unsigned, std::string> CustomNames;
   static StringLiteral const StandardNames[NumLibFuncs];
-  bool ShouldExtI32Param, ShouldExtI32Return, ShouldSignExtI32Param;
+  bool ShouldExtI32Param, ShouldExtI32Return, ShouldSignExtI32Param, ShouldSignExtI32Return;
   unsigned SizeOfInt;
 
   enum AvailabilityState {
@@ -189,6 +189,12 @@
     ShouldSignExtI32Param = Val;
   }
 
+  /// Set to true iff i32 results from library functions should have signext
+  /// attribute if they correspond to C-level int or unsigned int.
+  void setShouldSignExtI32Return(bool Val) {
+    ShouldSignExtI32Return = Val;
+  }
+
   /// Returns the size of the wchar_t type in bytes or 0 if the size is unknown.
   /// This queries the 'wchar_size' metadata.
   unsigned getWCharSize(const Module &M) const;
@@ -401,6 +407,8 @@
   Attribute::AttrKind getExtAttrForI32Return(bool Signed = true) const {
     if (Impl->ShouldExtI32Return)
       return Signed ? Attribute::SExt : Attribute::ZExt;
+    if (Impl->ShouldSignExtI32Return)
+      return Attribute::SExt;
     return Attribute::None;
   }
 
diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -169,7 +169,7 @@
   TLI.setUnavailable(LibFunc_fgets_unlocked);
 
   bool ShouldExtI32Param = false, ShouldExtI32Return = false,
-       ShouldSignExtI32Param = false;
+       ShouldSignExtI32Param = false, ShouldSignExtI32Return = false;
   // PowerPC64, Sparc64, SystemZ need signext/zeroext on i32 parameters and
   // returns corresponding to C-level ints and unsigned ints.
   if (T.isPPC64() || T.getArch() == Triple::sparcv9 ||
@@ -182,9 +182,15 @@
   if (T.isMIPS() || T.isRISCV64()) {
     ShouldSignExtI32Param = true;
   }
+  // riscv64 needs signext on i32 returns corresponding to both signed and
+  // unsigned ints.
+  if (T.isRISCV64()) {
+    ShouldSignExtI32Return = true;
+  }
   TLI.setShouldExtI32Param(ShouldExtI32Param);
   TLI.setShouldExtI32Return(ShouldExtI32Return);
   TLI.setShouldSignExtI32Param(ShouldSignExtI32Param);
+  TLI.setShouldSignExtI32Return(ShouldSignExtI32Return);
 
   // Let's assume by default that the size of int is 32 bits, unless the target
   // is a 16-bit architecture because then it most likely is 16 bits. If that
@@ -882,6 +888,7 @@
     : CustomNames(TLI.CustomNames), ShouldExtI32Param(TLI.ShouldExtI32Param),
       ShouldExtI32Return(TLI.ShouldExtI32Return),
       ShouldSignExtI32Param(TLI.ShouldSignExtI32Param),
+      ShouldSignExtI32Return(TLI.ShouldSignExtI32Return),
       SizeOfInt(TLI.SizeOfInt) {
   memcpy(AvailableArray, TLI.AvailableArray, sizeof(AvailableArray));
   VectorDescs = TLI.VectorDescs;
@@ -893,6 +900,7 @@
       ShouldExtI32Param(TLI.ShouldExtI32Param),
       ShouldExtI32Return(TLI.ShouldExtI32Return),
       ShouldSignExtI32Param(TLI.ShouldSignExtI32Param),
+      ShouldSignExtI32Return(TLI.ShouldSignExtI32Return),
       SizeOfInt(TLI.SizeOfInt) {
   std::move(std::begin(TLI.AvailableArray), std::end(TLI.AvailableArray),
             AvailableArray);
@@ -905,6 +913,7 @@
   ShouldExtI32Param = TLI.ShouldExtI32Param;
   ShouldExtI32Return = TLI.ShouldExtI32Return;
   ShouldSignExtI32Param = TLI.ShouldSignExtI32Param;
+  ShouldSignExtI32Return = TLI.ShouldSignExtI32Return;
   SizeOfInt = TLI.SizeOfInt;
   memcpy(AvailableArray, TLI.AvailableArray, sizeof(AvailableArray));
   return *this;
@@ -915,6 +924,7 @@
   ShouldExtI32Param = TLI.ShouldExtI32Param;
   ShouldExtI32Return = TLI.ShouldExtI32Return;
   ShouldSignExtI32Param = TLI.ShouldSignExtI32Param;
+  ShouldSignExtI32Return = TLI.ShouldSignExtI32Return;
   SizeOfInt = TLI.SizeOfInt;
   std::move(std::begin(TLI.AvailableArray), std::end(TLI.AvailableArray),
             AvailableArray);
diff --git a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/BuildLibCalls.cpp
@@ -1240,6 +1240,13 @@
     F.addParamAttr(ArgNo, ExtAttr);
 }
 
+static void setRetExtAttr(Function &F,
+                          const TargetLibraryInfo &TLI, bool Signed = true) {
+  Attribute::AttrKind ExtAttr = TLI.getExtAttrForI32Return(Signed);
+  if (ExtAttr != Attribute::None && !F.hasRetAttribute(ExtAttr))
+    F.addRetAttr(ExtAttr);
+}
+
 // Modeled after X86TargetLowering::markLibCallAttributes.
 static void markRegisterParameterAttributes(Function *F) {
   if (!F->arg_size() || F->isVarArg())
@@ -1315,6 +1322,8 @@
     // on any target: A size_t argument (which may be an i32 on some targets)
     // should not trigger the assert below.
   case LibFunc_bcmp:
+    setRetExtAttr(*F, TLI);
+    break;
   case LibFunc_calloc:
   case LibFunc_fwrite:
   case LibFunc_malloc:
diff --git a/llvm/test/Transforms/InstCombine/RISCV/memcmp.ll b/llvm/test/Transforms/InstCombine/RISCV/memcmp.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/RISCV/memcmp.ll
@@ -0,0 +1,19 @@
+; RUN: opt %s -passes=instcombine -mtriple=riscv64-unknown-linux-gnu -S | FileCheck %s
+
+declare signext i32 @memcmp(ptr, ptr, i64)
+
+; Make sure we use signext attribute for the bcmp result.
+define signext i32 @test_bcmp(ptr %mem1, ptr %mem2, i64 %size) {
+; CHECK-LABEL: define {{[^@]+}}@test_bcmp(
+; CHECK-NEXT:    [[BCMP:%.*]] = call i32 @bcmp(ptr [[MEM1:%.*]], ptr [[MEM2:%.*]], i64 [[SIZE:%.*]])
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[BCMP]], 0
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i1 [[CMP]] to i32
+; CHECK-NEXT:    ret i32 [[ZEXT]]
+;
+  %call = call signext i32 @memcmp(ptr %mem1, ptr %mem2, i64 %size)
+  %cmp = icmp eq i32 %call, 0
+  %zext = zext i1 %cmp to i32
+  ret i32 %zext
+}
+
+; CHECK: declare signext i32 @bcmp(ptr nocapture, ptr nocapture, i64)