How inline helps devirtualization in ModuleInlinerWrapperPass?

zxl · February 10, 2022, 1:31am

Hello all:
I’m interested in inline and devirtualization implementation in llvm. Then I read code below:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/IPO/Inliner.cpp#:~:text=PreservedAnalyses%20ModuleInlinerWrapperPass%3A%3Arun,ModuleAnalysisManager%20%26MAM)
I found code below:

if (MaxDevirtIterations == 0)
    MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(std::move(PM)));
  else
    MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(
        createDevirtSCCRepeatedPass(std::move(PM), MaxDevirtIterations)));

In ModuleInlinerWrapperPass constructor, I found PM only add InlinerPass. So in DevirtSCCRepeatedPass::run function,during each interation, we first run inline pass, then do devirtualization? The code location is below:

github.com

llvm/llvm-project/blob/main/llvm/lib/Analysis/CGSCCPassManager.cpp#:~:text=DevirtSCCRepeatedPass

//===- CGSCCPassManager.cpp - Managing & running CGSCC passes -------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/CGSCCPassManager.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/LazyCallGraph.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"

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Do we need some alias analysis or other pass between inline and devirtualization which can help us do devirtualization better?
Thank you very much!
ZXL

mtrofin · February 12, 2022, 3:15am

Take a look at PassBuilder::buildInlinerPipeline and how it sets up the module inliner (L707 onward)