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[CodeGen] Port GlobalMerge to new pass manager #77474
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//===- llvm/CodeGen/GlobalMerge.h -------------------------------*- C++ -*-===// | ||
// | ||
// 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 | ||
// | ||
//===----------------------------------------------------------------------===// | ||
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#ifndef LLVM_CODEGEN_GLOBALMERGE_H | ||
#define LLVM_CODEGEN_GLOBALMERGE_H | ||
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#include "llvm/IR/PassManager.h" | ||
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namespace llvm { | ||
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class TargetMachine; | ||
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struct GlobalMergeOptions { | ||
// FIXME: Infer the maximum possible offset depending on the actual users | ||
// (these max offsets are different for the users inside Thumb or ARM | ||
// functions), see the code that passes in the offset in the ARM backend | ||
// for more information. | ||
unsigned MaxOffset = 0; | ||
bool GroupByUse = true; | ||
bool IgnoreSingleUse = true; | ||
bool MergeConst = false; | ||
/// Whether we should merge global variables that have external linkage. | ||
bool MergeExternal = true; | ||
/// Whether we should try to optimize for size only. | ||
/// Currently, this applies a dead simple heuristic: only consider globals | ||
/// used in minsize functions for merging. | ||
/// FIXME: This could learn about optsize, and be used in the cost model. | ||
bool SizeOnly = false; | ||
}; | ||
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// FIXME: This pass must run before AsmPrinterPass::doInitialization! | ||
class GlobalMergePass : public PassInfoMixin<GlobalMergePass> { | ||
const TargetMachine *TM; | ||
GlobalMergeOptions Options; | ||
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public: | ||
GlobalMergePass(const TargetMachine *TM, GlobalMergeOptions Options) | ||
: TM(TM), Options(Options) {} | ||
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PreservedAnalyses run(Module &M, ModuleAnalysisManager &MAM); | ||
}; | ||
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} // namespace llvm | ||
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#endif // LLVM_CODEGEN_GLOBALMERGE_H |
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Why is this here? Is there an analogous hook in the old PM?
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Some backends, e.g. ARM, use this pass. The legacy version transforms module in
doInitialization
rather thanrunOnFunction
. So, add this hook to add global merge and add this pass in front of the pipeline.There was a problem hiding this comment.
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Something doesn't make sense. Why is this a function pass, that only changes the module and does all the work in doInitalization? Should it be converted to a ModulePass?
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It should be a module pass in theory, will convert it.Uh oh!
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Unfortunately,
GlobalMerge
must run before thedoInitialization
ofAsmPrinter
to keep the dwaf info.May split
AsmPrinter
into multiple passes in future, one reason is that thedoFinalization
inAsmPrinter
is in fact a module IR pass, but now is too early and off-topic.There was a problem hiding this comment.
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Seems to be GlobalMerge should be run earlier than it is now. I would expect it to run before CodeGenPrepare at least (if it even belongs in the codegen pipeline in the first place)
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It sounds like it already is running at the very beginning of the codegen pipeline? doInitialization runs before any passes, I think.
If it did run in the optimization pipeline, it would have to be very late; the target hooks use information about the exact load/store ops that access the global to pick appropriate thresholds. And it messes with alias analysis a bit for mutable variables. So ultimately it wouldn't be much different from running it in early codegen. (Which is really too early for constants, given the pass can't see constant pools, but that's a different issue.)
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It is a CodeGen only IR pass, but some tests run this pass in
opt
.