diff --git a/llvm/include/llvm/IR/RuntimeLibcalls.h b/llvm/include/llvm/IR/RuntimeLibcalls.h
index 01359894b0421..ab14ed44fed52 100644
--- a/llvm/include/llvm/IR/RuntimeLibcalls.h
+++ b/llvm/include/llvm/IR/RuntimeLibcalls.h
@@ -186,6 +186,13 @@ struct RuntimeLibcallsInfo {
     return RTLIB::Unsupported;
   }
 
+  /// \returns the function type and attributes for the \p LibcallImpl,
+  /// depending on the target \p TT. If the function has incomplete type
+  /// information, return nullptr for the function type.
+  std::pair<FunctionType *, AttributeList>
+  getFunctionTy(LLVMContext &Ctx, const Triple &TT, const DataLayout &DL,
+                RTLIB::LibcallImpl LibcallImpl) const;
+
 private:
   LLVM_ABI static iota_range<RTLIB::LibcallImpl>
   lookupLibcallImplNameImpl(StringRef Name);
diff --git a/llvm/lib/IR/RuntimeLibcalls.cpp b/llvm/lib/IR/RuntimeLibcalls.cpp
index 77af29b9d70f6..2ce5719228a0d 100644
--- a/llvm/lib/IR/RuntimeLibcalls.cpp
+++ b/llvm/lib/IR/RuntimeLibcalls.cpp
@@ -7,7 +7,9 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/RuntimeLibcalls.h"
+#include "llvm/ADT/FloatingPointMode.h"
 #include "llvm/ADT/StringTable.h"
+#include "llvm/IR/DataLayout.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/xxhash.h"
 #include "llvm/TargetParser/ARMTargetParser.h"
@@ -72,3 +74,80 @@ bool RuntimeLibcallsInfo::darwinHasExp10(const Triple &TT) {
     return false;
   }
 }
+
+std::pair<FunctionType *, AttributeList>
+RuntimeLibcallsInfo::getFunctionTy(LLVMContext &Ctx, const Triple &TT,
+                                   const DataLayout &DL,
+                                   RTLIB::LibcallImpl LibcallImpl) const {
+  static constexpr Attribute::AttrKind CommonFnAttrs[] = {
+      Attribute::NoCallback, Attribute::NoFree, Attribute::NoSync,
+      Attribute::NoUnwind, Attribute::WillReturn};
+
+  switch (LibcallImpl) {
+  case RTLIB::impl___sincos_stret:
+  case RTLIB::impl___sincosf_stret: {
+    if (!darwinHasSinCosStret(TT)) // Non-darwin currently unexpected
+      return {};
+
+    Type *ScalarTy = LibcallImpl == RTLIB::impl___sincosf_stret
+                         ? Type::getFloatTy(Ctx)
+                         : Type::getDoubleTy(Ctx);
+
+    AttrBuilder FuncAttrBuilder(Ctx);
+    for (Attribute::AttrKind Attr : CommonFnAttrs)
+      FuncAttrBuilder.addAttribute(Attr);
+
+    const bool UseSret =
+        TT.isX86_32() || ((TT.isARM() || TT.isThumb()) &&
+                          ARM::computeTargetABI(TT) == ARM::ARM_ABI_APCS);
+
+    FuncAttrBuilder.addMemoryAttr(MemoryEffects::argumentOrErrnoMemOnly(
+        UseSret ? ModRefInfo::Mod : ModRefInfo::NoModRef, ModRefInfo::Mod));
+
+    AttributeList Attrs;
+    Attrs = Attrs.addFnAttributes(Ctx, FuncAttrBuilder);
+
+    if (UseSret) {
+      AttrBuilder AttrBuilder(Ctx);
+      StructType *StructTy = StructType::get(ScalarTy, ScalarTy);
+      AttrBuilder.addStructRetAttr(StructTy);
+      AttrBuilder.addAlignmentAttr(DL.getABITypeAlign(StructTy));
+      FunctionType *FuncTy = FunctionType::get(
+          Type::getVoidTy(Ctx), {DL.getAllocaPtrType(Ctx), ScalarTy}, false);
+
+      return {FuncTy, Attrs.addParamAttributes(Ctx, 0, AttrBuilder)};
+    }
+
+    Type *RetTy =
+        LibcallImpl == RTLIB::impl___sincosf_stret && TT.isX86_64()
+            ? static_cast<Type *>(FixedVectorType::get(ScalarTy, 2))
+            : static_cast<Type *>(StructType::get(ScalarTy, ScalarTy));
+
+    return {FunctionType::get(RetTy, {ScalarTy}, false), Attrs};
+  }
+  case RTLIB::impl_sqrtf:
+  case RTLIB::impl_sqrt: {
+    AttrBuilder FuncAttrBuilder(Ctx);
+
+    for (Attribute::AttrKind Attr : CommonFnAttrs)
+      FuncAttrBuilder.addAttribute(Attr);
+    FuncAttrBuilder.addMemoryAttr(MemoryEffects::errnoMemOnly(ModRefInfo::Mod));
+
+    AttributeList Attrs;
+    Attrs = Attrs.addFnAttributes(Ctx, FuncAttrBuilder);
+
+    Type *ScalarTy = LibcallImpl == RTLIB::impl_sqrtf ? Type::getFloatTy(Ctx)
+                                                      : Type::getDoubleTy(Ctx);
+    FunctionType *FuncTy = FunctionType::get(ScalarTy, {ScalarTy}, false);
+
+    Attrs = Attrs.addRetAttribute(
+        Ctx, Attribute::getWithNoFPClass(Ctx, fcNegInf | fcNegSubnormal |
+                                                  fcNegNormal));
+    return {FuncTy, Attrs};
+  }
+  default:
+    return {};
+  }
+
+  return {};
+}
diff --git a/llvm/lib/Transforms/Utils/DeclareRuntimeLibcalls.cpp b/llvm/lib/Transforms/Utils/DeclareRuntimeLibcalls.cpp
index 0642d51cd2c21..6d4436b92c119 100644
--- a/llvm/lib/Transforms/Utils/DeclareRuntimeLibcalls.cpp
+++ b/llvm/lib/Transforms/Utils/DeclareRuntimeLibcalls.cpp
@@ -16,22 +16,62 @@
 
 using namespace llvm;
 
+static void mergeAttributes(LLVMContext &Ctx, const Module &M,
+                            const DataLayout &DL, const Triple &TT,
+                            Function *Func, FunctionType *FuncTy,
+                            AttributeList FuncAttrs) {
+  AttributeList OldAttrs = Func->getAttributes();
+  AttributeList NewAttrs = OldAttrs;
+
+  {
+    AttrBuilder OldBuilder(Ctx, OldAttrs.getFnAttrs());
+    AttrBuilder NewBuilder(Ctx, FuncAttrs.getFnAttrs());
+    OldBuilder.merge(NewBuilder);
+    NewAttrs = NewAttrs.addFnAttributes(Ctx, OldBuilder);
+  }
+
+  {
+    AttrBuilder OldBuilder(Ctx, OldAttrs.getRetAttrs());
+    AttrBuilder NewBuilder(Ctx, FuncAttrs.getRetAttrs());
+    OldBuilder.merge(NewBuilder);
+    NewAttrs = NewAttrs.addRetAttributes(Ctx, OldBuilder);
+  }
+
+  for (unsigned I = 0, E = FuncTy->getNumParams(); I != E; ++I) {
+    AttrBuilder OldBuilder(Ctx, OldAttrs.getParamAttrs(I));
+    AttrBuilder NewBuilder(Ctx, FuncAttrs.getParamAttrs(I));
+    OldBuilder.merge(NewBuilder);
+    NewAttrs = NewAttrs.addParamAttributes(Ctx, I, OldBuilder);
+  }
+
+  Func->setAttributes(NewAttrs);
+}
+
 PreservedAnalyses DeclareRuntimeLibcallsPass::run(Module &M,
                                                   ModuleAnalysisManager &MAM) {
   RTLIB::RuntimeLibcallsInfo RTLCI(M.getTargetTriple());
   LLVMContext &Ctx = M.getContext();
+  const DataLayout &DL = M.getDataLayout();
+  const Triple &TT = M.getTargetTriple();
 
   for (RTLIB::LibcallImpl Impl : RTLCI.getLibcallImpls()) {
     if (Impl == RTLIB::Unsupported)
       continue;
 
-    // TODO: Declare with correct type, calling convention, and attributes.
+    auto [FuncTy, FuncAttrs] = RTLCI.getFunctionTy(Ctx, TT, DL, Impl);
 
-    FunctionType *FuncTy =
-        FunctionType::get(Type::getVoidTy(Ctx), {}, /*IsVarArgs=*/true);
+    // TODO: Declare with correct type, calling convention, and attributes.
+    if (!FuncTy)
+      FuncTy = FunctionType::get(Type::getVoidTy(Ctx), {}, /*IsVarArgs=*/true);
 
     StringRef FuncName = RTLCI.getLibcallImplName(Impl);
-    M.getOrInsertFunction(FuncName, FuncTy);
+
+    Function *Func =
+        cast<Function>(M.getOrInsertFunction(FuncName, FuncTy).getCallee());
+    if (Func->getFunctionType() == FuncTy) {
+      mergeAttributes(Ctx, M, DL, TT, Func, FuncTy, FuncAttrs);
+      Func->setCallingConv(RTLCI.getLibcallImplCallingConv(Impl));
+    }
   }
 
   return PreservedAnalyses::none();
diff --git a/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/basic.ll b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/basic.ll
index ee3a0539bf300..c005316f07f06 100644
--- a/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/basic.ll
+++ b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/basic.ll
@@ -11,5 +11,9 @@ define float @sinf(float %x) {
 }
 
 ; CHECK: declare void @acosf(...)
+
+; CHECK: declare nofpclass(ninf nsub nnorm) float @sqrtf(float) [[SQRT_ATTRS:#[0-9]+]]
+; CHECK: declare nofpclass(ninf nsub nnorm) double @sqrt(double) [[SQRT_ATTRS:#[0-9]+]]
+
 ; CHECK: declare void @__umodti3(...)
 
diff --git a/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/merge_attributes.ll b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/merge_attributes.ll
new file mode 100644
index 0000000000000..ffbf11d4106dc
--- /dev/null
+++ b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/merge_attributes.ll
@@ -0,0 +1,11 @@
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+
+define noundef nofpclass(nan) float @sqrtf(float %x) "foo" {
+  %ret = call float asm "; $0 = sqrt($1)", "=r,r"(float %x)
+  ret float %ret
+}
+
+; FIXME: Individual fields of nofpclass not merged
+; CHECK: define noundef nofpclass(ninf nsub nnorm) float @sqrtf(float %x) [[SQRT_ATTR:#[0-9]+]] {
+
+; CHECK: attributes [[SQRT_ATTR]] = { nocallback nofree nosync nounwind willreturn memory(errnomem: write) "foo" }
diff --git a/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
new file mode 100644
index 0000000000000..0d0e3da25eea7
--- /dev/null
+++ b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/sincos_stret.ll
@@ -0,0 +1,23 @@
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-apple-macos10.9 < %s | FileCheck -check-prefixes=CHECK,X64 %s
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=arm64-apple-macos10.9 < %s | FileCheck -check-prefixes=CHECK,STRUCT %s
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=thumbv7k-apple-watchos2.0 < %s | FileCheck -check-prefixes=CHECK,STRUCT %s
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=armv7-apple-ios7 < %s | FileCheck -check-prefix=SRET %s
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=thumbv7-apple-ios7 < %s | FileCheck -check-prefix=SRET %s
+
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=armv7-apple-ios6 < %s | FileCheck -check-prefix=NONE %s
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-apple-macos10.8 < %s | FileCheck -check-prefix=NONE %s
+
+; X64: declare <2 x float> @__sincosf_stret(float) [[SINCOS_ATTRS:#[0-9]+]]
+; X64: declare { double, double } @__sincos_stret(double) [[SINCOS_ATTRS:#[0-9]+]]
+
+; STRUCT: declare { float, float } @__sincosf_stret(float) [[SINCOS_ATTRS:#[0-9]+]]
+; STRUCT: declare { double, double } @__sincos_stret(double) [[SINCOS_ATTRS:#[0-9]+]]
+
+; SRET: declare void @__sincosf_stret(ptr sret({ float, float }) align 4, float) [[SINCOS_ATTRS:#[0-9]+]]
+; SRET: declare void @__sincos_stret(ptr sret({ double, double }) align 4, double) [[SINCOS_ATTRS:#[0-9]+]]
+
+; CHECK: attributes [[SINCOS_ATTRS]] = { nocallback nofree nosync nounwind willreturn memory(errnomem: write) }
+; SRET: attributes [[SINCOS_ATTRS]] = { nocallback nofree nosync nounwind willreturn memory(argmem: write, errnomem: write) }
+
+; NONE-NOT: __sincos_stret
+; NONE-NOT: __sincosf_stret
diff --git a/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/wrong_declaration.ll b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/wrong_declaration.ll
new file mode 100644
index 0000000000000..2451010df5b75
--- /dev/null
+++ b/llvm/test/Transforms/Util/DeclareRuntimeLibcalls/wrong_declaration.ll
@@ -0,0 +1,21 @@
+; RUN: opt -S -passes=declare-runtime-libcalls -mtriple=x86_64-apple-macos10.9 < %s | FileCheck %s
+
+; Make sure there is no crash if there are definitions or declarations
+; with the wrong type signature.
+
+; CHECK: define void @sqrtf() {
+define void @sqrtf() {
+  ret void
+}
+
+; CHECK: define float @sqrt(float %0) {
+define float @sqrt(float) {
+  ret float 0.0
+}
+
+; CHECK: declare double @__sincos_stret(double)
+declare double @__sincos_stret(double)
+
+; CHECK: declare { float, float } @__sincosf_stret(float)
+declare { float, float } @__sincosf_stret(float)
+