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
@@ -15,12 +15,15 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/IR/Argument.h"
+#include "llvm/IR/CallingConv.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Type.h"
+#include "llvm/Support/TypeSize.h"
 
 using namespace llvm;
 
@@ -1224,6 +1227,41 @@
     F.addParamAttr(ArgNo, ExtAttr);
 }
 
+// Modeled after X86TargetLowering::markLibCallAttributes.
+static void markRegisterParameterAttributes(Function *F) {
+  if (!F->arg_size() || F->isVarArg())
+    return;
+
+  const CallingConv::ID CC = F->getCallingConv();
+  if (CC != CallingConv::C && CC != CallingConv::X86_StdCall)
+    return;
+
+  const Module *M = F->getParent();
+  unsigned N = M->getNumberRegisterParameters();
+  if (!N)
+    return;
+
+  const DataLayout &DL = M->getDataLayout();
+
+  for (Argument &A : F->args()) {
+    Type *T = A.getType();
+    if (!T->isIntOrPtrTy())
+      continue;
+
+    const TypeSize &TS = DL.getTypeAllocSize(T);
+    if (TS > 8)
+      continue;
+
+    assert(TS <= 4 && "Need to account for parameters larger than word size");
+    const unsigned NumRegs = TS > 4 ? 2 : 1;
+    if (N < NumRegs)
+      return;
+
+    N -= NumRegs;
+    F->addParamAttr(A.getArgNo(), Attribute::InReg);
+  }
+}
+
 FunctionCallee llvm::getOrInsertLibFunc(Module *M, const TargetLibraryInfo &TLI,
                                         LibFunc TheLibFunc, FunctionType *T,
                                         AttributeList AttributeList) {
@@ -1289,6 +1327,8 @@
     break;
   }
 
+  markRegisterParameterAttributes(F);
+
   return C;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/simplify-libcalls-inreg.ll b/llvm/test/Transforms/InstCombine/simplify-libcalls-inreg.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/simplify-libcalls-inreg.ll
@@ -0,0 +1,57 @@
+; RUN: opt -passes=instcombine -S %s | FileCheck %s
+
+; The intent of this test is to check that the declarations produces for
+; libcalls retains the inreg parameter attribute.
+
+target datalayout = "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i386-unknown-linux-gnu"
+
+declare ptr @foo()
+declare i32 @memcmp(ptr inreg nocapture noundef, ptr inreg nocapture noundef, i32 inreg noundef)
+declare i32 @printf(i8*, ...)
+declare double @exp2(double)
+declare i32 @__sprintf_chk(i8*, i32, i32, i8*, ...)
+@a = common global [60 x i8] zeroinitializer, align 1
+@b = common global [60 x i8] zeroinitializer, align 1
+@h = constant [2 x i8] c"h\00"
+
+; CHECK:     declare i32 @bcmp(ptr inreg nocapture, ptr inreg nocapture, i32 inreg)
+; CHECK-NOT: declare i32 @bcmp(ptr nocapture, ptr nocapture, i32)
+
+define i32 @baz(ptr inreg noundef %s2, i32 inreg noundef %n){
+  %call = call ptr @foo()
+  %call1 = call i32 @memcmp(ptr inreg noundef %call, ptr inreg noundef %s2, i32 inreg noundef %n)
+  %cmp = icmp eq i32 %call1, 0
+  %conv = zext i1 %cmp to i32
+  ret i32 %conv
+}
+
+; CHECK:     declare noundef i32 @putchar(i32 inreg noundef)
+; CHECK-NOT: declare noundef i32 @putchar(i32 noundef)
+
+define void @test_fewer_params_than_num_register_parameters() {
+  %fmt = getelementptr [2 x i8], [2 x i8]* @h, i32 0, i32 0
+  call i32 (i8*, ...) @printf(i8* %fmt)
+  ret void
+}
+
+; CHECK:     declare double @ldexp(double, i32 inreg)
+; CHECK-NOT: declare double @ldexp(double, i32)
+
+define double @test_non_int_params(i16 signext %x) {
+  %conv = sitofp i16 %x to double
+  %ret = call double @exp2(double %conv)
+  ret double %ret
+}
+
+; CHECK:     declare noundef i32 @sprintf(ptr noalias nocapture noundef writeonly, ptr nocapture noundef readonly, ...)
+; CHECK-NOT: declare noundef i32 @sprintf(ptr inreg noalias nocapture noundef writeonly, ptr inreg nocapture noundef readonly, ...)
+define i32 @test_variadic() {
+  %dst = getelementptr inbounds [60 x i8], [60 x i8]* @a, i32 0, i32 0
+  %fmt = getelementptr inbounds [60 x i8], [60 x i8]* @b, i32 0, i32 0
+  %ret = call i32 (i8*, i32, i32, i8*, ...) @__sprintf_chk(i8* %dst, i32 0, i32 -1, i8* %fmt)
+  ret i32 %ret
+}
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"NumRegisterParameters", i32 3}