From 9a5f3888505630cea88f8372d3068b2d63cfb381 Mon Sep 17 00:00:00 2001
From: Benson Chu <bensonchu457@gmail.com>
Date: Sun, 15 Aug 2021 13:12:21 -0500
Subject: [PATCH] [AST] Pick last tentative definition as the acting definition

Clang currently picks the second tentative definition when
VarDecl::getActingDefinition is called.

This can lead to attributes being dropped if they are attached to
tentative definitions that appear after the second one. This is
because VarDecl::getActingDefinition loops through VarDecl::redecls
assuming that the last tentative definition is the last element in the
iterator. However, it is the second element that would be the last
tentative definition.

This changeset modifies getActingDefinition to iterate through the
declaration chain in reverse, so that it can immediately return when
it encounters a tentative definition.

Differential Revision: https://reviews.llvm.org/D99732
---
 clang/lib/AST/Decl.cpp                         | 14 +++++++++-----
 clang/test/CodeGen/attr-tentative-definition.c |  7 +++++++
 2 files changed, 16 insertions(+), 5 deletions(-)
 create mode 100644 clang/test/CodeGen/attr-tentative-definition.c

diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index aa9fba519642..835e28c0bc9f 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -2216,14 +2216,18 @@ VarDecl *VarDecl::getActingDefinition() {
     return nullptr;
 
   VarDecl *LastTentative = nullptr;
-  VarDecl *First = getFirstDecl();
-  for (auto I : First->redecls()) {
-    Kind = I->isThisDeclarationADefinition();
+
+  // Loop through the declaration chain, starting with the most recent.
+  for (VarDecl *Decl = getMostRecentDecl(); Decl;
+       Decl = Decl->getPreviousDecl()) {
+    Kind = Decl->isThisDeclarationADefinition();
     if (Kind == Definition)
       return nullptr;
-    if (Kind == TentativeDefinition)
-      LastTentative = I;
+    // Record the first (most recent) TentativeDefinition that is encountered.
+    if (Kind == TentativeDefinition && !LastTentative)
+      LastTentative = Decl;
   }
+
   return LastTentative;
 }
 
diff --git a/clang/test/CodeGen/attr-tentative-definition.c b/clang/test/CodeGen/attr-tentative-definition.c
new file mode 100644
index 000000000000..7405e8079b22
--- /dev/null
+++ b/clang/test/CodeGen/attr-tentative-definition.c
@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -emit-llvm < %s | FileCheck %s
+
+char arr[10];
+char arr[10] __attribute__((section("datadata")));
+char arr[10] __attribute__((aligned(16)));
+
+// CHECK: @arr ={{.*}}section "datadata", align 16