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[CodeGen][NPM] Update BranchFolderLegacy make tail merge configurable via flag #135277

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[CodeGen][NPM] Update BranchFolderLegacy make tail merge configurable via flag #135277

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1439764
[CodeGen][NPM] Create method to add the branch folding pass with the …
mikhailramalho Apr 10, 2025
1129f40
Add printPipeline
mikhailramalho Apr 10, 2025
7d8d9df
Remove unused include
mikhailramalho Apr 10, 2025
9b95974
Pass global flag via argument
mikhailramalho Apr 11, 2025
17fcc45
Removed the default arg
mikhailramalho Apr 11, 2025
73b228a
PassConfig is no longer needed
mikhailramalho Apr 11, 2025
f2dd02c
Implemented suggested changes
mikhailramalho Apr 14, 2025
fc156b5
Default to true
mikhailramalho Apr 15, 2025
b49d6d3
Revert "Implemented suggested changes"
mikhailramalho Apr 15, 2025
1293477
Check if the pass was disabled when adding the pass via the pass' poi…
mikhailramalho Apr 15, 2025
756e1a5
Moved requiresStructuredCFG check to pass creation
mikhailramalho Apr 18, 2025
501a1cf
Use isPassSubstitutedOrOverridden to check if the pass was disabled
mikhailramalho Apr 21, 2025
61251d3
Removed unnecessary local variable
mikhailramalho Apr 21, 2025
ec4571a
clang-format
mikhailramalho Apr 21, 2025
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3 changes: 3 additions & 0 deletions llvm/include/llvm/CodeGen/BranchFoldingPass.h
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Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,9 @@ class BranchFolderPass : public PassInfoMixin<BranchFolderPass> {
return MachineFunctionProperties().set(
MachineFunctionProperties::Property::NoPHIs);
}

void printPipeline(raw_ostream &OS,
function_ref<StringRef(StringRef)> MapClassName2PassName);
};

} // namespace llvm
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2 changes: 2 additions & 0 deletions llvm/include/llvm/CodeGen/Passes.h
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Expand Up @@ -257,6 +257,8 @@ namespace llvm {
/// branches.
extern char &BranchFolderPassID;

MachineFunctionPass *createBranchFolderPass(bool EnableTailMerge);

/// BranchRelaxation - This pass replaces branches that need to jump further
/// than is supported by a branch instruction.
extern char &BranchRelaxationPassID;
Expand Down
19 changes: 17 additions & 2 deletions llvm/lib/CodeGen/BranchFolding.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -90,10 +90,13 @@ namespace {

/// BranchFolderPass - Wrap branch folder in a machine function pass.
class BranchFolderLegacy : public MachineFunctionPass {
bool EnableTailMerge;

public:
static char ID;

explicit BranchFolderLegacy() : MachineFunctionPass(ID) {}
explicit BranchFolderLegacy(bool EnableTailMerge = true)
: MachineFunctionPass(ID), EnableTailMerge(EnableTailMerge) {}
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@mikhailramalho mikhailramalho Apr 10, 2025

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That added the initial port to the new pm. This is now changing the pass arguments in the old PM, without the matching new PM change

Are you talking about the default argument? I see that BranchFolderPass already takes a bool EnableTailMerge:

llvm-project/llvm/include/llvm/CodeGen/BranchFoldingPass.h

Line 19 in b39ab7a

BranchFolderPass(bool EnableTailMerge) : EnableTailMerge(EnableTailMerge) {}


bool runOnMachineFunction(MachineFunction &MF) override;

Expand Down Expand Up @@ -144,6 +147,13 @@ PreservedAnalyses BranchFolderPass::run(MachineFunction &MF,
return getMachineFunctionPassPreservedAnalyses();
}

void BranchFolderPass::printPipeline(
raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
OS << MapClassName2PassName(name());
if (EnableTailMerge)
OS << "<enable-tail-merge>";
}

bool BranchFolderLegacy::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
Expand All @@ -152,7 +162,8 @@ bool BranchFolderLegacy::runOnMachineFunction(MachineFunction &MF) {
// TailMerge can create jump into if branches that make CFG irreducible for
// HW that requires structurized CFG.
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
PassConfig->getEnableTailMerge();
PassConfig->getEnableTailMerge() &&
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@mikhailramalho mikhailramalho Apr 10, 2025

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This global state should be moved to only change the pass parameter

Do you mean I should remove the PassConfig->getEnableTailMerge()?

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@optimisan optimisan Apr 11, 2025

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Yes, it can be passed as the EnableTailMerge parameter when TargetPassConfig creates this pass instance (where currently this is done through its ID).

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@mikhailramalho mikhailramalho Apr 11, 2025

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Hey, so I did 9b95974, but I'm getting a weird error: when the branch folding pass runs now, I get that the pre-conditions are not valid:

MachineFunctionProperties required by Control Flow Optimizer pass are not met by function RWBufferStore_Vec4_I32.
Required properties: NoPHIs
Current properties: IsSSA, TracksLiveness, Legalized, Selected

This seems to only happen because of this change:

   // Branch folding must be run after regalloc and prolog/epilog insertion.
-  addPass(&BranchFolderPassID);
+  addPass(createBranchFolderPass(getEnableTailMerge()));

The calling addPass with the BranchFolderPassID, it doesn't seem to be added to the list of passes because char BranchFolderLegacy::ID = 0, whereas when we call createBranchFolderPass, the pass is added and fails.

Maybe I'm missing some detail here, but the pass doesn't seem to run at all before my changes.

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@mikhailramalho mikhailramalho Apr 11, 2025

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In the failing tests, the branch folding pass is disabled here:

llvm-project/llvm/lib/Target/SPIRV/SPIRVTargetMachine.cpp

Line 174 in 32c3909

disablePass(&BranchFolderPassID);

So by calling createBranchFolderPass, we don't check the disabled passed

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@mikhailramalho mikhailramalho Apr 11, 2025

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This also seems to bypass the -disable-branch-fold.

Any suggestions on how to proceed?

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@arsenm arsenm Apr 18, 2025

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I think the function properties error is from somewhere else. Most passes probably are not appropriately reporting cleared properties

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@arsenm arsenm Apr 18, 2025

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What test hit the property error? I think SPIRV needs to cear NoPHIs property after its selection. It's bypassing the usual place it's cleared (though I'm not sure why it's cleared here)

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@mikhailramalho mikhailramalho Apr 18, 2025

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All tests are passing with the latest version of this PR

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@arsenm arsenm Apr 25, 2025

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Is this avoided after the new version of #136327?

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@mikhailramalho mikhailramalho Apr 26, 2025

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This piece of code is no longer part of the PR.

this->EnableTailMerge;
MBFIWrapper MBBFreqInfo(
getAnalysis<MachineBlockFrequencyInfoWrapperPass>().getMBFI());
BranchFolder Folder(
Expand Down Expand Up @@ -2080,3 +2091,7 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
++NumHoist;
return true;
}

MachineFunctionPass *llvm::createBranchFolderPass(bool EnableTailMerge = true) {
return new BranchFolderLegacy(EnableTailMerge);
}