Revert "Allow system header to provide their own implementation of some builtin"

This reverts commit 921f871ac4 because it
causes libc++ code to trigger __warn_memset_zero_len.

See https://reviews.llvm.org/D71082.

clang/include/clang/AST/Decl.h

@@ -2309,9 +2309,6 @@ public:
   /// true through IsAligned.
   bool isReplaceableGlobalAllocationFunction(bool *IsAligned = nullptr) const;
 
-  /// Determine if this function provides an inline implementation of a builtin.
-  bool isInlineBuiltinDeclaration() const;
-
   /// Determine whether this is a destroying operator delete.
   bool isDestroyingOperatorDelete() const;
 

clang/lib/AST/Decl.cpp

@@ -3046,14 +3046,6 @@ bool FunctionDecl::isReplaceableGlobalAllocationFunction(bool *IsAligned) const
   return Params == FPT->getNumParams();
 }
 
-bool FunctionDecl::isInlineBuiltinDeclaration() const {
-  if (!getBuiltinID())
-    return false;
-
-  const FunctionDecl *Definition;
-  return hasBody(Definition) && Definition->isInlineSpecified();
-}
-
 bool FunctionDecl::isDestroyingOperatorDelete() const {
   // C++ P0722:
   //   Within a class C, a single object deallocation function with signature

clang/lib/CodeGen/CGExpr.cpp

@@ -4621,15 +4621,8 @@ RValue CodeGenFunction::EmitSimpleCallExpr(const CallExpr *E,
 }
 
 static CGCallee EmitDirectCallee(CodeGenFunction &CGF, const FunctionDecl *FD) {
-
   if (auto builtinID = FD->getBuiltinID()) {
-    // Replaceable builtin provide their own implementation of a builtin. Unless
-    // we are in the builtin implementation itself, don't call the actual
-    // builtin. If we are in the builtin implementation, avoid trivial infinite
-    // recursion.
-    if (!FD->isInlineBuiltinDeclaration() ||
-        CGF.CurFn->getName() == FD->getName())
-      return CGCallee::forBuiltin(builtinID, FD);
+    return CGCallee::forBuiltin(builtinID, FD);
   }
 
   llvm::Constant *calleePtr = EmitFunctionDeclPointer(CGF.CGM, FD);

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1840,11 +1840,6 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD, llvm::Function *F,
   else if (const auto *SA = FD->getAttr<SectionAttr>())
      F->setSection(SA->getName());
 
-  if (FD->isInlineBuiltinDeclaration()) {
-    F->addAttribute(llvm::AttributeList::FunctionIndex,
-                    llvm::Attribute::NoBuiltin);
-  }
-
   if (FD->isReplaceableGlobalAllocationFunction()) {
     // A replaceable global allocation function does not act like a builtin by
     // default, only if it is invoked by a new-expression or delete-expression.

clang/test/CodeGen/memcpy-nobuiltin.c

@@ -1,15 +0,0 @@
-// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -DWITH_DECL | FileCheck --check-prefix=CHECK-WITH-DECL %s
-// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -UWITH_DECL | FileCheck --check-prefix=CHECK-NO-DECL %s
-// RUN: %clang_cc1 -verify -S -emit-llvm -o- %s -isystem %S -DWITH_SELF_REFERENCE_DECL | FileCheck --check-prefix=CHECK-SELF-REF-DECL %s
-//
-// CHECK-WITH-DECL-NOT: @llvm.memcpy
-// CHECK-NO-DECL: @llvm.memcpy
-// CHECK-SELF-REF-DECL: @llvm.memcpy
-//
-#include <memcpy-nobuiltin.inc>
-void test(void *dest, void const *from, size_t n) {
-  memcpy(dest, from, n);
-
-  static char buffer[1];
-  memcpy(buffer, from, 2); // expected-warning {{'memcpy' will always overflow; destination buffer has size 1, but size argument is 2}}
-}

clang/test/CodeGen/memcpy-nobuiltin.inc

@@ -1,19 +0,0 @@
-#include <stddef.h>
-extern void *memcpy(void *dest, void const *from, size_t n);
-
-#ifdef WITH_DECL
-inline void *memcpy(void *dest, void const *from, size_t n) {
-  char const *ifrom = from;
-  char *idest = dest;
-  while (n--)
-    *idest++ = *ifrom++;
-  return dest;
-}
-#endif
-#ifdef WITH_SELF_REFERENCE_DECL
-inline void *memcpy(void *dest, void const *from, size_t n) {
-  if (n != 0)
-    memcpy(dest, from, n);
-  return dest;
-}
-#endif