SystemZAlignGlobals.cpp

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//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Try to increase the alignment of globals to at least 2, to avoid the need for
// GOT indirection to load an unaligned address.
//
//===----------------------------------------------------------------------===//
 
#include "SystemZ.h"
#include "llvm/IR/Analysis.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Pass.h"
 
using namespace llvm;
 
namespace {
 
class SystemZAlignGlobalsLegacy : public ModulePass {
public:
  static char ID;
 
  explicit SystemZAlignGlobalsLegacy() : ModulePass(ID) {}
 
  StringRef getPassName() const override { return "SystemZ Align Globals"; }
 
  bool runOnModule(Module &M) override;
};
 
} // end anonymous namespace
 
char SystemZAlignGlobalsLegacy::ID = 0;
 
INITIALIZE_PASS(SystemZAlignGlobalsLegacy, "systemz-align-globals",
                "Try aligning globals to 2 bytes", false, false)
 
ModulePass *llvm::createSystemZAlignGlobalsLegacyPass() {
  return new SystemZAlignGlobalsLegacy();
}
 
static bool alignGlobalsInModule(Module &M) {
  bool Changed = false;
 
  for (GlobalVariable &GV : M.globals()) {
    MaybeAlign GVAlign = GV.getAlign();
    if (!GVAlign || GVAlign.value() > 1)
      continue;
 
    if (!GV.canIncreaseAlignment())
      continue;
 
    GV.setAlignment(Align(2));
    Changed = true;
  }
 
  return Changed;
}
 
bool SystemZAlignGlobalsLegacy::runOnModule(Module &M) {
  return alignGlobalsInModule(M);
}
 
PreservedAnalyses SystemZAlignGlobalsPass::run(Module &M,
                                               ModuleAnalysisManager &MAM) {
  return alignGlobalsInModule(M) ? PreservedAnalyses::none()
                                 : PreservedAnalyses::all();
}