diff --git a/llvm/include/llvm/IR/Attributes.h b/llvm/include/llvm/IR/Attributes.h
--- a/llvm/include/llvm/IR/Attributes.h
+++ b/llvm/include/llvm/IR/Attributes.h
@@ -604,6 +604,10 @@
                                                   unsigned MinValue,
                                                   unsigned MaxValue);
 
+  /// Merge two AttributeLists. If either is empty, the other one is returned.
+  LLVM_NODISCARD AttributeList merge(LLVMContext &C,
+                                     const AttributeList &Other);
+
   //===--------------------------------------------------------------------===//
   // AttributeList Accessors
   //===--------------------------------------------------------------------===//
diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -1550,6 +1550,25 @@
   return addAttributes(C, Index, B);
 }
 
+AttributeList AttributeList::merge(LLVMContext &C, const AttributeList &Other) {
+  if (isEmpty())
+    return Other;
+  if (Other.isEmpty())
+    return *this;
+
+  auto IndexEnd = std::max(this->index_end(), Other.index_end());
+  if (index_begin() == IndexEnd)
+    return {};
+
+  SmallVector<AttributeSet, 4> AttrSets;
+  for (unsigned I = index_begin(); I != IndexEnd; ++I) {
+    AttrBuilder B1(*this, I);
+    AttrBuilder B2(Other, I);
+    AttrSets.push_back(AttributeSet::get(C, B1.merge(B2)));
+  }
+  return getImpl(C, AttrSets);
+}
+
 //===----------------------------------------------------------------------===//
 // AttributeList Accessor Methods
 //===----------------------------------------------------------------------===//
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
@@ -1498,9 +1498,11 @@
   // The incoming attribute set may have come from a speculatable intrinsic, but
   // is being replaced with a library call which is not allowed to be
   // speculatable.
-  CI->setAttributes(Attrs.removeAttribute(B.getContext(),
-                                          AttributeList::FunctionIndex,
-                                          Attribute::Speculatable));
+  CI->setAttributes(
+      Attrs
+          .removeAttribute(B.getContext(), AttributeList::FunctionIndex,
+                           Attribute::Speculatable)
+          .merge(B.getContext(), CI->getCalledFunction()->getAttributes()));
   if (const Function *F =
           dyn_cast<Function>(Callee.getCallee()->stripPointerCasts()))
     CI->setCallingConv(F->getCallingConv());
diff --git a/llvm/test/Transforms/InstCombine/exp2-1.ll b/llvm/test/Transforms/InstCombine/exp2-1.ll
--- a/llvm/test/Transforms/InstCombine/exp2-1.ll
+++ b/llvm/test/Transforms/InstCombine/exp2-1.ll
@@ -15,7 +15,7 @@
 
 define double @test_simplify1(i32 %x) {
 ; LDEXP32-LABEL: @test_simplify1(
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[X:%.*]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[X:%.*]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify1(
@@ -24,7 +24,7 @@
 ; LDEXP16-NEXT:    ret double [[RET]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify1(
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[X:%.*]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[X:%.*]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify1(
@@ -40,16 +40,16 @@
 define double @test_simplify2(i16 signext %x) {
 ; LDEXP32-LABEL: @test_simplify2(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = sext i16 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify2(
-; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 [[X:%.*]])
+; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 signext [[X:%.*]])
 ; LDEXP16-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify2(
 ; NOLDEXPF-NEXT:    [[TMP1:%.*]] = sext i16 [[X:%.*]] to i32
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify2(
@@ -65,17 +65,17 @@
 define double @test_simplify3(i8 signext %x) {
 ; LDEXP32-LABEL: @test_simplify3(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = sext i8 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify3(
 ; LDEXP16-NEXT:    [[TMP1:%.*]] = sext i8 [[X:%.*]] to i16
-; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 [[TMP1]])
+; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 signext [[TMP1]])
 ; LDEXP16-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify3(
 ; NOLDEXPF-NEXT:    [[TMP1:%.*]] = sext i8 [[X:%.*]] to i32
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify3(
@@ -90,7 +90,7 @@
 
 define float @test_simplify4(i32 %x) {
 ; LDEXP32-LABEL: @test_simplify4(
-; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 [[X:%.*]])
+; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 signext [[X:%.*]])
 ; LDEXP32-NEXT:    ret float [[LDEXPF]]
 ;
 ; LDEXP16-LABEL: @test_simplify4(
@@ -144,7 +144,7 @@
 define double @test_simplify6(i16 zeroext %x) {
 ; LDEXP32-LABEL: @test_simplify6(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = zext i16 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify6(
@@ -154,7 +154,7 @@
 ;
 ; NOLDEXPF-LABEL: @test_simplify6(
 ; NOLDEXPF-NEXT:    [[TMP1:%.*]] = zext i16 [[X:%.*]] to i32
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify6(
@@ -170,17 +170,17 @@
 define double @test_simplify7(i8 zeroext %x) {
 ; LDEXP32-LABEL: @test_simplify7(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify7(
 ; LDEXP16-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i16
-; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 [[TMP1]])
+; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 signext [[TMP1]])
 ; LDEXP16-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify7(
 ; NOLDEXPF-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify7(
@@ -196,12 +196,12 @@
 define float @test_simplify8(i8 zeroext %x) {
 ; LDEXP32-LABEL: @test_simplify8(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret float [[LDEXPF]]
 ;
 ; LDEXP16-LABEL: @test_simplify8(
 ; LDEXP16-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i16
-; LDEXP16-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 [[TMP1]])
+; LDEXP16-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 signext [[TMP1]])
 ; LDEXP16-NEXT:    ret float [[LDEXPF]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify8(
@@ -225,17 +225,17 @@
 define double @test_simplify9(i8 zeroext %x) {
 ; LDEXP32-LABEL: @test_simplify9(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret double [[LDEXP]]
 ;
 ; LDEXP16-LABEL: @test_simplify9(
 ; LDEXP16-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i16
-; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 [[TMP1]])
+; LDEXP16-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i16 signext [[TMP1]])
 ; LDEXP16-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify9(
 ; NOLDEXPF-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 [[TMP1]])
+; NOLDEXPF-NEXT:    [[LDEXP:%.*]] = call double @ldexp(double 1.000000e+00, i32 signext [[TMP1]])
 ; NOLDEXPF-NEXT:    ret double [[LDEXP]]
 ;
 ; NOLDEXP-LABEL: @test_simplify9(
@@ -251,12 +251,12 @@
 define float @test_simplify10(i8 zeroext %x) {
 ; LDEXP32-LABEL: @test_simplify10(
 ; LDEXP32-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i32
-; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 [[TMP1]])
+; LDEXP32-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 signext [[TMP1]])
 ; LDEXP32-NEXT:    ret float [[LDEXPF]]
 ;
 ; LDEXP16-LABEL: @test_simplify10(
 ; LDEXP16-NEXT:    [[TMP1:%.*]] = zext i8 [[X:%.*]] to i16
-; LDEXP16-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 [[TMP1]])
+; LDEXP16-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 signext [[TMP1]])
 ; LDEXP16-NEXT:    ret float [[LDEXPF]]
 ;
 ; NOLDEXPF-LABEL: @test_simplify10(
diff --git a/llvm/test/Transforms/InstCombine/pow_fp_int.ll b/llvm/test/Transforms/InstCombine/pow_fp_int.ll
--- a/llvm/test/Transforms/InstCombine/pow_fp_int.ll
+++ b/llvm/test/Transforms/InstCombine/pow_fp_int.ll
@@ -51,7 +51,7 @@
 
 define double @pow_sitofp_double_const_base_2_fast(i32 %x) {
 ; CHECK-LABEL: @pow_sitofp_double_const_base_2_fast(
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i32 [[X:%.*]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i32 signext [[X:%.*]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
@@ -78,7 +78,7 @@
 define double @pow_uitofp_const_base_2_fast(i31 %x) {
 ; CHECK-LABEL: @pow_uitofp_const_base_2_fast(
 ; CHECK-NEXT:    [[TMP1:%.*]] = zext i31 [[X:%.*]] to i32
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i32 [[TMP1]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i32 signext [[TMP1]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
@@ -343,7 +343,7 @@
 
 define double @pow_sitofp_const_base_2_no_fast(i32 %x) {
 ; CHECK-LABEL: @pow_sitofp_const_base_2_no_fast(
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 [[X:%.*]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i32 signext [[X:%.*]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
diff --git a/llvm/test/Transforms/InstCombine/pow_fp_int16.ll b/llvm/test/Transforms/InstCombine/pow_fp_int16.ll
--- a/llvm/test/Transforms/InstCombine/pow_fp_int16.ll
+++ b/llvm/test/Transforms/InstCombine/pow_fp_int16.ll
@@ -57,7 +57,7 @@
 
 define double @pow_sitofp_double_const_base_2_fast(i16 %x) {
 ; CHECK-LABEL: @pow_sitofp_double_const_base_2_fast(
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i16 [[X:%.*]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i16 signext [[X:%.*]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
@@ -84,7 +84,7 @@
 define double @pow_uitofp_const_base_2_fast(i15 %x) {
 ; CHECK-LABEL: @pow_uitofp_const_base_2_fast(
 ; CHECK-NEXT:    [[TMP1:%.*]] = zext i15 [[X:%.*]] to i16
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i16 [[TMP1]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call afn float @ldexpf(float 1.000000e+00, i16 signext [[TMP1]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
@@ -322,7 +322,7 @@
 
 define double @pow_sitofp_const_base_2_no_fast(i16 %x) {
 ; CHECK-LABEL: @pow_sitofp_const_base_2_no_fast(
-; CHECK-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 [[X:%.*]])
+; CHECK-NEXT:    [[LDEXPF:%.*]] = call float @ldexpf(float 1.000000e+00, i16 signext [[X:%.*]])
 ; CHECK-NEXT:    [[RES:%.*]] = fpext float [[LDEXPF]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
diff --git a/llvm/test/Transforms/InstCombine/simplify-libcalls.ll b/llvm/test/Transforms/InstCombine/simplify-libcalls.ll
--- a/llvm/test/Transforms/InstCombine/simplify-libcalls.ll
+++ b/llvm/test/Transforms/InstCombine/simplify-libcalls.ll
@@ -190,7 +190,7 @@
 }
 define double @fake_ldexp(i32 %x) {
 ; CHECK32-LABEL: @fake_ldexp(
-; CHECK32-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i32 %x)
+; CHECK32-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i32 signext %x)
 ; CHECK32-NEXT:    ret double [[Z]]
 
 ; CHECK16-LABEL: @fake_ldexp(
@@ -205,11 +205,11 @@
 define double @fake_ldexp_16(i16 %x) {
 ; CHECK32-LABEL: @fake_ldexp_16(
 ; CHECK32-NEXT:    [[Y:%.*]] = sext i16 %x to i32
-; CHECK32-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i32 [[Y]])
+; CHECK32-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i32 signext [[Y]])
 ; CHECK32-NEXT:    ret double [[Z]]
 
 ; CHECK16-LABEL: @fake_ldexp_16(
-; CHECK16-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i16 %x)
+; CHECK16-NEXT:    [[Z:%.*]] = call double @ldexp(double 1.0{{.*}}, i16 signext %x)
 ; CHECK16-NEXT:    ret double [[Z]]
 
   %y = sitofp i16 %x to double
@@ -217,6 +217,8 @@
   ret double %z
 }
 
+; CHECK: declare double @ldexp(double, i{{16|32}} signext)
+
 
 attributes #0 = { nobuiltin }
 attributes #1 = { builtin }
diff --git a/llvm/unittests/IR/AttributesTest.cpp b/llvm/unittests/IR/AttributesTest.cpp
--- a/llvm/unittests/IR/AttributesTest.cpp
+++ b/llvm/unittests/IR/AttributesTest.cpp
@@ -68,6 +68,33 @@
   EXPECT_TRUE(AL.hasFnAttribute(Attribute::NoReturn));
 }
 
+TEST(Attributes, MergeAttributes) {
+  LLVMContext C;
+  AttributeList AL1;
+  AttributeList AL2;
+  EXPECT_TRUE(AL1.merge(C, AL2).isEmpty());
+
+  AttrBuilder B;
+  B.addAttribute(Attribute::NoReturn);
+  AL1 = AL1.addAttributes(C, AttributeList::FunctionIndex, AttributeSet::get(C, B));
+  AttributeList Merged = AL1.merge(C, AL2);
+  EXPECT_EQ(Merged, AL1);
+
+  B.clear();
+  B.addAttribute(Attribute::Hot);
+  AL2 = AL2.addParamAttributes(C, 1, B);
+  B.addAttribute(Attribute::Convergent);
+  AL1 = AL1.addParamAttributes(C, 1, B);
+  Merged = AL1.merge(C, AL2);
+
+  EXPECT_TRUE(Merged.hasFnAttribute(Attribute::NoReturn));
+  EXPECT_FALSE(Merged.hasParamAttribute(0, Attribute::NoReturn));
+  EXPECT_TRUE(Merged.hasParamAttribute(1, Attribute::Hot));
+  EXPECT_TRUE(Merged.hasParamAttribute(1, Attribute::Convergent));
+  EXPECT_FALSE(Merged.hasAttrSomewhere(Attribute::Cold));
+  EXPECT_EQ(Merged, AL2.merge(C, AL1));
+}
+
 TEST(Attributes, RemoveAlign) {
   LLVMContext C;