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Simplify HLS loop dead node elimination #726

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Simplify HLS loop dead node elimination #726

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e35c965
Simplify HLS loop dead node elimination
caleridas Jan 7, 2025
9da1c2c
Merge branch 'master' into theta-api-hls-dns
sjalander Jan 12, 2025
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33 changes: 12 additions & 21 deletions jlm/hls/backend/rvsdg2rhls/UnusedStateRemoval.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,12 @@
namespace jlm::hls
{

static bool
IsPassthroughLoopVar(const rvsdg::ThetaNode::LoopVar & loopvar)
{
return loopvar.pre->nusers() == 1 && loopvar.post->origin() == loopvar.pre;
}

static bool
IsPassthroughArgument(const rvsdg::output & argument)
{
Expand Down Expand Up @@ -115,22 +121,6 @@ RemoveUnusedStatesFromLambda(llvm::lambda::node & lambdaNode)
remove(&lambdaNode);
}

static void
RemovePassthroughArgument(const rvsdg::RegionArgument & argument)
{
auto origin = argument.input()->origin();
auto result = dynamic_cast<rvsdg::RegionResult *>(*argument.begin());
argument.region()->node()->output(result->output()->index())->divert_users(origin);

auto inputIndex = argument.input()->index();
auto outputIndex = result->output()->index();
auto region = argument.region();
region->RemoveResult(result->index());
region->RemoveArgument(argument.index());
region->node()->RemoveInput(inputIndex);
region->node()->RemoveOutput(outputIndex);
}

static void
RemoveUnusedStatesFromGammaNode(rvsdg::GammaNode & gammaNode)
{
Expand Down Expand Up @@ -178,15 +168,16 @@ RemoveUnusedStatesFromGammaNode(rvsdg::GammaNode & gammaNode)
static void
RemoveUnusedStatesFromThetaNode(rvsdg::ThetaNode & thetaNode)
{
auto thetaSubregion = thetaNode.subregion();
for (int i = thetaSubregion->narguments() - 1; i >= 0; --i)
std::vector<rvsdg::ThetaNode::LoopVar> loopvars;
for (const auto & loopvar : thetaNode.GetLoopVars())
{
auto & argument = *thetaSubregion->argument(i);
if (IsPassthroughArgument(argument))
if (IsPassthroughLoopVar(loopvar))
{
RemovePassthroughArgument(argument);
loopvar.output->divert_users(loopvar.input->origin());
loopvars.push_back(loopvar);
}
}
thetaNode.RemoveLoopVars(std::move(loopvars));
}

static void
Expand Down
6 changes: 2 additions & 4 deletions jlm/hls/backend/rvsdg2rhls/distribute-constants.cpp
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Expand Up @@ -36,10 +36,8 @@ distribute_constant(const rvsdg::SimpleOperation & op, rvsdg::simple_output * ou
loopvar.output->divert_users(
rvsdg::SimpleNode::create_normalized(out->region(), op, {})[0]);
distribute_constant(op, arg_replacement);
theta->subregion()->RemoveResult(loopvar.post->index());
theta->subregion()->RemoveArgument(loopvar.pre->index());
theta->RemoveInput(loopvar.input->index());
theta->RemoveOutput(loopvar.output->index());
loopvar.post->divert_to(loopvar.pre);
theta->RemoveLoopVars({ loopvar });
changed = true;
break;
}
Expand Down
7 changes: 1 addition & 6 deletions tests/jlm/hls/backend/rvsdg2rhls/UnusedStateRemovalTests.cpp
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Expand Up @@ -112,12 +112,7 @@ TestTheta()
jlm::hls::RemoveUnusedStates(*rvsdgModule);

// Assert
// This assert is only here so that we do not forget this test when we refactor the code
assert(thetaNode->ninputs() == 1);

// FIXME: This transformation is broken for theta nodes. For the setup above, it
// removes all inputs/outputs, except the predicate. However, the only
// input and output it should remove are input 1 and output 0, respectively.
assert(thetaNode->ninputs() == 3);
}

static void
Expand Down
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