[PATCH] D158223: [clang] Add clang::unnamed_addr attribute that marks globals' address as not significant

[Arthur Eubanks via Phabricator via cfe-commits]

aeubanks created this revision.
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This simply marks the global with the `unnamed_addr` IR attribute so it
be merged with other globals. This can break C++ pointer identity.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D158223

Files:
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/lib/CodeGen/CodeGenModule.cpp
  clang/test/CodeGen/unnamed-addr.c


Index: clang/test/CodeGen/unnamed-addr.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/unnamed-addr.c
@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -O1 -emit-llvm -o - -disable-llvm-passes %s | FileCheck %s
+
+// CHECK: @i = unnamed_addr constant i32 8
+
+[[clang::unnamed_addr]] const int i = 8;
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -5254,6 +5254,9 @@
       GV->setConstant(true);
   }
 
+  if (D->hasAttr<UnnamedAddrAttr>())
+    GV->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
+
   CharUnits AlignVal = getContext().getDeclAlign(D);
   // Check for alignment specifed in an 'omp allocate' directive.
   if (std::optional<CharUnits> AlignValFromAllocate =
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -1408,6 +1408,24 @@
   }];
 }
 
+def UnnamedAddrDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+The ``clang::unnamed_addr`` attribute specifies that the address of a global is
+not significant. This allows global constants with the same contents to be
+merged. This can break global pointer identity, i.e. two different globals have
+the same address.
+
+Example usage:
+
+.. code-block:: c
+
+  // i and j may have the same address.
+  [[clang::unnamed_addr]] const int i = 42;
+  [[clang::unnamed_addr]] const int j = 42;
+  }];
+}
+
 def ObjCRequiresSuperDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -1794,6 +1794,13 @@
   let SimpleHandler = 1;
 }
 
+def UnnamedAddr : InheritableAttr {
+  let Spellings = [Clang<"unnamed_addr">];
+  let Subjects = SubjectList<[GlobalVar], ErrorDiag>;
+  let Documentation = [UnnamedAddrDocs];
+  let SimpleHandler = 1;
+}
+
 def ReturnsTwice : InheritableAttr {
   let Spellings = [GCC<"returns_twice">];
   let Subjects = SubjectList<[Function]>;


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