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
@@ -22,6 +22,8 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Type.h"
 
+#include <algorithm>
+
 using namespace llvm;
 
 #define DEBUG_TYPE "build-libcalls"
@@ -1289,6 +1291,11 @@
     break;
   }
 
+  if (unsigned N = F->getParent()->getNumberRegisterParameters())
+    if ((N = std::min(N, static_cast<unsigned>(F->arg_size()))))
+      while (N--)
+        F->addParamAttr(N, Attribute::InReg);
+
   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 +1,36 @@
+; 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*, ...)
+
+; 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)
+
+@h = constant [2 x i8] c"h\00"
+
+define void @test_simplify2() {
+  %fmt = getelementptr [2 x i8], [2 x i8]* @h, i32 0, i32 0
+  call i32 (i8*, ...) @printf(i8* %fmt)
+  ret void
+}
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"NumRegisterParameters", i32 3}