IR: Introduce local_unnamed_addr attribute.

If a local_unnamed_addr attribute is attached to a global, the address
is known to be insignificant within the module. It is distinct from the
existing unnamed_addr attribute in that it only describes a local property
of the module rather than a global property of the symbol.

This attribute is intended to be used by the code generator and LTO to allow
the linker to decide whether the global needs to be in the symbol table. It is
possible to exclude a global from the symbol table if three things are true:
- This attribute is present on every instance of the global (which means that
  the normal rule that the global must have a unique address can be broken without
  being observable by the program by performing comparisons against the global's
  address)
- The global has linkonce_odr linkage (which means that each linkage unit must have
  its own copy of the global if it requires one, and the copy in each linkage unit
  must be the same)
- It is a constant or a function (which means that the program cannot observe that
  the unique-address rule has been broken by writing to the global)

Although this attribute could in principle be computed from the module
contents, LTO clients (i.e. linkers) will normally need to be able to compute
this property as part of symbol resolution, and it would be inefficient to
materialize every module just to compute it.

See:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160509/356401.html
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160516/356738.html
for earlier discussion.

Part of the fix for PR27553.

Differential Revision: http://reviews.llvm.org/D20348

llvm-svn: 272709

3 files changed, 6 insertions, 18 deletions

diff --git a/llvm/lib/CodeGen/Analysis.cpp b/llvm/lib/CodeGen/Analysis.cpp
index ef405484e84..73bd3d29eae 100644
--- a/llvm/lib/CodeGen/Analysis.cpp
+++ b/llvm/lib/CodeGen/Analysis.cpp
@@ -623,7 +623,9 @@ bool llvm::canBeOmittedFromSymbolTable(const GlobalValue *GV) {
   if (!GV->hasLinkOnceODRLinkage())
     return false;
 
-  if (GV->hasUnnamedAddr())
+  // We assume that anyone who sets global unnamed_addr on a non-constant knows
+  // what they're doing.
+  if (GV->hasGlobalUnnamedAddr())
     return true;
 
   // If it is a non constant variable, it needs to be uniqued across shared
@@ -633,21 +635,7 @@ bool llvm::canBeOmittedFromSymbolTable(const GlobalValue *GV) {
       return false;
   }
 
-  // An alias can point to a variable. We could try to resolve the alias to
-  // decide, but for now just don't hide them.
-  if (isa<GlobalAlias>(GV))
-    return false;
-
-  // If we don't see every use, we have to be conservative and assume the value
-  // address is significant.
-  if (GV->getParent()->getMaterializer())
-    return false;
-
-  GlobalStatus GS;
-  if (GlobalStatus::analyzeGlobal(GV, GS))
-    return false;
-
-  return !GS.IsCompared;
+  return GV->hasAtLeastLocalUnnamedAddr();
 }
 
 // FIXME: make this a proper option
diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index d39881d9ce3..02640f55fa2 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1001,7 +1001,7 @@ static bool isGOTEquivalentCandidate(const GlobalVariable *GV,
   // Global GOT equivalents are unnamed private globals with a constant
   // pointer initializer to another global symbol. They must point to a
   // GlobalVariable or Function, i.e., as GlobalValue.
-  if (!GV->hasUnnamedAddr() || !GV->hasInitializer() || !GV->isConstant() ||
+  if (!GV->hasGlobalUnnamedAddr() || !GV->hasInitializer() || !GV->isConstant() ||
       !GV->isDiscardableIfUnused() || !dyn_cast<GlobalValue>(GV->getOperand(0)))
     return false;
 
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index f7ba62ff231..6d5dd5fa4d5 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -429,7 +429,7 @@ const MCExpr *TargetLoweringObjectFileELF::lowerRelativeReference(
     const TargetMachine &TM) const {
   // We may only use a PLT-relative relocation to refer to unnamed_addr
   // functions.
-  if (!LHS->hasUnnamedAddr() || !LHS->getValueType()->isFunctionTy())
+  if (!LHS->hasGlobalUnnamedAddr() || !LHS->getValueType()->isFunctionTy())
     return nullptr;
 
   // Basic sanity checks.