[CodeGen] Use module flag metadata to enable global-merge.

[ab]

Summary

This lets clang pick, in LTO, whether to enable or not the pass.

Uses the metadata emitted by http://reviews.llvm.org/D7968

Also, the -enable-global-merge flag overrides the module flag.

-Ahmed

[ab]

• Match D7968 changes (i32, "Global Merge")

lib/CodeGen/GlobalMerge.cpp

 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "global-merge"
 
-static cl::opt<bool>
+static cl::opt<cl::boolOrDefault>
 EnableGlobalMerge("enable-global-merge", cl::Hidden,
                   cl::desc("Enable global merge pass"),
-                  cl::init(true));
+                  cl::init(cl::BOU_UNSET));
 
 static cl::opt<bool>
 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
                          cl::desc("Enable global merge pass on constants"),
                          cl::init(false));
 
 // FIXME: this could be a transitional option, and we probably need to remove
 // it if only we are sure this optimization could always benefit all targets.
for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
         if (const GlobalVariable *GV =
             dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
                                      ->stripPointerCasts()))
           MustKeepGlobalVariables.insert(GV);
     }
   }
 }
 
+static bool shouldEnableGlobalMergeForModule(const Module &M) {
+  Metadata *FlagMD = M.getModuleFlag("Global Merge");
+
+  // If not explicitly specified otherwise, enable the pass.
+  if (!FlagMD)
+    return true;
+
+  ConstantAsMetadata *Flag = dyn_cast<ConstantAsMetadata>(FlagMD);
+  if (!Flag || !isa<ConstantInt>(Flag->getValue()))
+    report_fatal_error("Unexpected type for \"Global Merge\" module flag "
+                       "value, should be an integer constant!");
+
+  return !cast<ConstantInt>(Flag->getValue())->isZero();
+}
+
 bool GlobalMerge::doInitialization(Module &M) {
-  if (!EnableGlobalMerge)
+  if (EnableGlobalMerge == cl::BOU_FALSE ||
+      (EnableGlobalMerge == cl::BOU_UNSET &&
+       !shouldEnableGlobalMergeForModule(M)))
     return false;
 
   DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
                                                         BSSGlobals;
   bool Changed = false;
   setMustKeepGlobalVariables(M);
 
   // Grab all non-const globals.

test/CodeGen/AArch64/global-merge-moduleflag-disabled.ll

+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -o - | FileCheck %s --check-prefix=CHECK-NGM
+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -enable-global-merge -o - | FileCheck %s --check-prefix=CHECK-GM
+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -enable-global-merge=false -o - | FileCheck %s --check-prefix=CHECK-NGM
+
+@m = internal global i32 0, align 4
+@n = internal global i32 0, align 4
+
+define void @f1(i32 %a1, i32 %a2) {
+  store i32 %a1, i32* @m, align 4
+  store i32 %a2, i32* @n, align 4
+  ret void
+}
+
+!0 = !{ i32 4, !"Global Merge", i32 0 }
+!llvm.module.flags = !{ !0 }
+
+;CHECK-GM:  .type _MergedGlobals,@object  // @_MergedGlobals
+;CHECK-GM:  .local  _MergedGlobals
+;CHECK-GM:  .comm _MergedGlobals,8,8
+
+;CHECK-NGM-NOT: MergedGlobals

test/CodeGen/AArch64/global-merge-moduleflag-enabled.ll

+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -o - | FileCheck %s --check-prefix=CHECK-GM
+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -enable-global-merge -o - | FileCheck %s --check-prefix=CHECK-GM
+; RUN: llc %s -mtriple=aarch64-none-linux-gnu -enable-global-merge=false -o - | FileCheck %s --check-prefix=CHECK-NGM
+
+@m = internal global i32 0, align 4
+@n = internal global i32 0, align 4
+
+define void @f1(i32 %a1, i32 %a2) {
+  store i32 %a1, i32* @m, align 4
+  store i32 %a2, i32* @n, align 4
+  ret void
+}
+
+!0 = !{ i32 1, !"Global Merge", i32 1 }
+!llvm.module.flags = !{ !0 }
+
+;CHECK-GM:  .type _MergedGlobals,@object  // @_MergedGlobals
+;CHECK-GM:  .local  _MergedGlobals
+;CHECK-GM:  .comm _MergedGlobals,8,8
+
+;CHECK-NGM-NOT: MergedGlobals