Index: clang/include/clang/AST/Decl.h
===================================================================
--- clang/include/clang/AST/Decl.h
+++ clang/include/clang/AST/Decl.h
@@ -2604,6 +2604,16 @@
   /// that requires constrained FP intrinsics
   void setUsesFPIntrin(bool I) { FunctionDeclBits.UsesFPIntrin = I; }
 
+  /// Determine whether the function shadows an inline builtin definition.
+  bool shadowsGNUInlineIntrinsic() const {
+    return FunctionDeclBits.ShadowsGNUInlineIntrinsic;
+  }
+
+  /// Set whether the function shadows an inline builtin definition.
+  void setShadowsGNUInlineIntrinsic(bool I) {
+    FunctionDeclBits.ShadowsGNUInlineIntrinsic = I;
+  }
+
   /// Flag that this function is implicitly inline.
   void setImplicitlyInline(bool I = true) { FunctionDeclBits.IsInline = I; }
 
Index: clang/include/clang/AST/DeclBase.h
===================================================================
--- clang/include/clang/AST/DeclBase.h
+++ clang/include/clang/AST/DeclBase.h
@@ -1612,10 +1612,13 @@
 
     /// Indicates if the function uses Floating Point Constrained Intrinsics
     uint64_t UsesFPIntrin : 1;
+
+    /// Indicates that this function shadows a gnu_inline intrinsic redefinition
+    uint64_t ShadowsGNUInlineIntrinsic : 1;
   };
 
   /// Number of non-inherited bits in FunctionDeclBitfields.
-  enum { NumFunctionDeclBits = 27 };
+  enum { NumFunctionDeclBits = 28 };
 
   /// Stores the bits used by CXXConstructorDecl. If modified
   /// NumCXXConstructorDeclBits and the accessor
@@ -1632,7 +1635,7 @@
     /// exactly 64 bits and thus the width of NumCtorInitializers
     /// will need to be shrunk if some bit is added to NumDeclContextBitfields,
     /// NumFunctionDeclBitfields or CXXConstructorDeclBitfields.
-    uint64_t NumCtorInitializers : 21;
+    uint64_t NumCtorInitializers : 20;
     uint64_t IsInheritingConstructor : 1;
 
     /// Whether this constructor has a trail-allocated explicit specifier.
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -2908,6 +2908,7 @@
   FunctionDeclBits.IsMultiVersion = false;
   FunctionDeclBits.IsCopyDeductionCandidate = false;
   FunctionDeclBits.HasODRHash = false;
+  FunctionDeclBits.ShadowsGNUInlineIntrinsic = false;
   if (TrailingRequiresClause)
     setTrailingRequiresClause(TrailingRequiresClause);
 }
Index: clang/lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenFunction.cpp
+++ clang/lib/CodeGen/CodeGenFunction.cpp
@@ -1299,18 +1299,34 @@
   // When generating code for a builtin with an inline declaration, use a
   // mangled name to hold the actual body, while keeping an external definition
   // in case the function pointer is referenced somewhere.
-  if (FD->isInlineBuiltinDeclaration() && Fn) {
-    std::string FDInlineName = (Fn->getName() + ".inline").str();
-    llvm::Module *M = Fn->getParent();
-    llvm::Function *Clone = M->getFunction(FDInlineName);
-    if (!Clone) {
-      Clone = llvm::Function::Create(Fn->getFunctionType(),
-                                     llvm::GlobalValue::InternalLinkage,
-                                     Fn->getAddressSpace(), FDInlineName, M);
-      Clone->addFnAttr(llvm::Attribute::AlwaysInline);
+  if (Fn) {
+    if (FD->isInlineBuiltinDeclaration()) {
+      std::string FDInlineName = (Fn->getName() + ".inline").str();
+      llvm::Module *M = Fn->getParent();
+      llvm::Function *Clone = M->getFunction(FDInlineName);
+      if (!Clone) {
+        Clone = llvm::Function::Create(Fn->getFunctionType(),
+                                       llvm::GlobalValue::InternalLinkage,
+                                       Fn->getAddressSpace(), FDInlineName, M);
+        Clone->addFnAttr(llvm::Attribute::AlwaysInline);
+      }
+      Fn->setLinkage(llvm::GlobalValue::ExternalLinkage);
+      Fn = Clone;
+    }
+
+    // Detect the unusual situation where an inline version is shadowed by a
+    // non-inline version. In that case we should pick the external one
+    // everywhere. That's GCC behavior too. Unfortunately, I cannot find a way
+    // to detect that situation before we reach codegen, so do some late
+    // replacement.
+    else if (FD->shadowsGNUInlineIntrinsic()) {
+      std::string FDInlineName = (Fn->getName() + ".inline").str();
+      llvm::Module *M = Fn->getParent();
+      if (llvm::Function *Clone = M->getFunction(FDInlineName)) {
+        Clone->replaceAllUsesWith(Fn);
+        Clone->eraseFromParent();
+      }
     }
-    Fn->setLinkage(llvm::GlobalValue::ExternalLinkage);
-    Fn = Clone;
   }
 
   // Check if we should generate debug info for this function.
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -10927,6 +10927,11 @@
   }
 
   if (Redeclaration) {
+    FunctionDecl *OldFDecl = dyn_cast<FunctionDecl>(OldDecl);
+    if (OldFDecl && OldFDecl->isInlineBuiltinDeclaration() &&
+        !NewFD->isInlineBuiltinDeclaration())
+      NewFD->setShadowsGNUInlineIntrinsic(true);
+
     // NewFD and OldDecl represent declarations that need to be
     // merged.
     if (MergeFunctionDecl(NewFD, OldDecl, S, MergeTypeWithPrevious)) {
Index: clang/test/CodeGen/gnu-inline-redecl.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/gnu-inline-redecl.c
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -triple x86_64 -S -emit-llvm -O1 -o - %s | FileCheck %s
+//
+// Verifies that the gnu_inline version is ignored in favor of the redecl
+
+extern inline __attribute__((gnu_inline)) unsigned long some_size(int c) {
+  return 1;
+}
+unsigned long mycall(int s) {
+  // CHECK-LABEL: i64 @mycall
+  // CHECK: ret i64 2
+  return some_size(s);
+}
+unsigned long some_size(int c) {
+  return 2;
+}
+unsigned long yourcall(int s) {
+  // CHECK-LABEL: i64 @yourcall
+  // CHECK: ret i64 2
+  return some_size(s);
+}
Index: clang/test/CodeGen/strlen-inline-builtin-redecl.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/strlen-inline-builtin-redecl.c
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -triple x86_64 -S -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s
+//
+// Verifies that clang-generated *.inline are removed when shadowed by an external definition
+
+// CHECK-NOT: strlen.inline
+
+unsigned long strnlen(const char *, unsigned long);
+
+extern inline __attribute__((always_inline)) __attribute__((gnu_inline)) unsigned long strlen(const char *p) {
+  return 1;
+}
+unsigned long mystrlen(char const *s) {
+  return strlen(s);
+}
+unsigned long strlen(const char *s) {
+  return 2;
+}
+unsigned long yourstrlen(char const *s) {
+  return strlen(s);
+}