[PATCH] D42856: [ThinLTO] Convert dead alias to declarations

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson created this revision.
tejohnson added reviewers: grimar, rafael.
Herald added subscribers: inglorion, mehdi_amini.

This complements the fixes in r323633 and r324075 which drop the
definitions of dead functions and variables, respectively.

Fixes PR36208.


Repository:
  rL LLVM

https://reviews.llvm.org/D42856

Files:
  lib/LTO/LTOBackend.cpp
  lib/Transforms/IPO/FunctionImport.cpp
  test/LTO/Resolution/X86/not-prevailing-alias.ll


Index: test/LTO/Resolution/X86/not-prevailing-alias.ll
===================================================================
--- /dev/null
+++ test/LTO/Resolution/X86/not-prevailing-alias.ll
@@ -0,0 +1,34 @@
+; Test to ensure that dead alias are dropped by converting to a declaration
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: llvm-lto2 run %t1.bc -r %t1.bc,barAlias,x \
+; RUN:   -r %t1.bc,bar,x -r %t1.bc,zed,px \
+; RUN:   -o %t2.o -save-temps
+
+; Check that bar and barAlias were dropped to declarations
+; RUN: llvm-dis %t2.o.1.1.promote.bc -o - | FileCheck %s --check-prefix=DROP
+; DROP-DAG: declare void @bar()
+; DROP-DAG: declare void @barAlias()
+
+; Check that 'barAlias' was not inlined.
+; RUN: llvm-objdump -d %t2.o.1 | FileCheck %s
+; CHECK:      zed:
+; CHECK-NEXT:  {{.*}}  pushq
+; CHECK-NEXT:  {{.*}}  callq   0
+
+; Check that 'bar' produced as undefined.
+; RUN: llvm-readelf --symbols %t2.o.1 | FileCheck %s --check-prefix=SYMBOLS
+; SYMBOLS: NOTYPE  GLOBAL DEFAULT  UND barAlias
+; SYMBOLS: FUNC    GLOBAL DEFAULT    2 zed
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+ at barAlias = alias void(), void()* @bar
+define void @bar() {
+  ret void
+}
+
+define i32 @zed() {
+  call void @barAlias()
+  ret i32 1
+}
Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- lib/Transforms/IPO/FunctionImport.cpp
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -624,14 +624,23 @@
     V->setLinkage(GlobalValue::ExternalLinkage);
     V->clearMetadata();
     V->setComdat(nullptr);
-  } else
-    // For now we don't resolve or drop aliases. Once we do we'll
-    // need to add support here for creating either a function or
-    // variable declaration, and return the new GlobalValue* for
-    // the caller to use.
-    // Support of dropping aliases is required for correct dead code
-    // elimination performed in thin LTO backends (see 'dropDeadSymbols').
-    llvm_unreachable("Expected function or variable");
+  } else {
+    GlobalValue *NewGV;
+    if (GV.getValueType()->isFunctionTy())
+      NewGV =
+          Function::Create(cast<FunctionType>(GV.getValueType()),
+                           GlobalValue::ExternalLinkage, "", GV.getParent());
+    else
+      NewGV =
+          new GlobalVariable(*GV.getParent(), GV.getValueType(),
+                             /*isConstant*/ false, GlobalValue::ExternalLinkage,
+                             /*init*/ nullptr, "",
+                             /*insertbefore*/ nullptr, GV.getThreadLocalMode(),
+                             GV.getType()->getAddressSpace());
+    NewGV->takeName(&GV);
+    GV.replaceAllUsesWith(NewGV);
+    GV.eraseFromParent();
+  }
 }
 
 /// Fixup WeakForLinker linkages in \p TheModule based on summary analysis.
Index: lib/LTO/LTOBackend.cpp
===================================================================
--- lib/LTO/LTOBackend.cpp
+++ lib/LTO/LTOBackend.cpp
@@ -407,12 +407,11 @@
         convertToDeclaration(GV);
   };
 
-  // Process functions and global now.
-  // FIXME: add support for aliases (needs support in convertToDeclaration).
-  for (auto &GV : Mod)
-    MaybeDrop(GV);
-  for (auto &GV : Mod.globals())
-    MaybeDrop(GV);
+  for (Module::global_value_iterator I = Mod.global_value_begin(),
+                                     E = Mod.global_value_end();
+       I != E;)
+    // Increment iterator here since MaybeDrop can delete aliases.
+    MaybeDrop(*I++);
 }
 
 Error lto::thinBackend(Config &Conf, unsigned Task, AddStreamFn AddStream,


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