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Factor Expr operand replacement in host IR lowering #5539

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Nov 17, 2025
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Factor Expr operand replacement in host IR lowering #5539

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49 changes: 31 additions & 18 deletions csrc/host_ir/lowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,36 @@ const std::vector<IterDomain*>& findReferenceLoopDomain(
return reference_tv->getLoopDomain();
}

// Returns a new Expr with the inputs and outputs replaced by the replacement
// map. If none of the inputs or outputs are replaced, returns the original
// Expr.
Expr* cloneWithNewOperands(
Expr* e,
const std::unordered_map<Val*, Val*>& replacement_map) {
auto maybe_replace = [&](Val*& x) -> bool {
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@wujingyue wujingyue Nov 15, 2025

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I also tried to use OptOutMutator (#5538) but ran into

[ RUN      ] StreamTest.Matmul
unknown file: Failure
C++ exception with description " INTERNAL ASSERT FAILED at /opt/pytorch/nvfuser/csrc/mutator.cpp:90, please report a bug with repro script to NVFuser at https://github.com/NVIDIA/Fuser/issues. 
Expected !DependencyCheck::isDependencyOf(val, mutation) . Attempted to replace a val, T1_g_float[istreamIdx7{3}, iS11{i2}, iS8{( ceilDiv(i4, 3) )}], with a dependent val, T3_l_float[istreamIdx14{3}, iS12{i2}, iS15{( ceilDiv(i4, 3) )}] (T3_l_float[istreamIdx14{3}, iS12{i2}, iS15{( ceilDiv(i4, 3) )}]), which is not allowed as it would result in a recursive definition of T3_l_float[istreamIdx14{3}, iS12{i2}, iS15{( ceilDiv(i4, 3) )}]
Exception raised from registerMutation at /opt/pytorch/nvfuser/csrc/mutator.cpp:90 (most recent call first):

Let me know if there's an easier way.

Val* new_x = getOrDefault(replacement_map, x);
if (new_x == nullptr) {
return false;
}
x = new_x;
return true;
};

int64_t replaced = 0;

std::vector<Val*> new_ins = e->inputs();
replaced += std::ranges::count_if(new_ins, maybe_replace);

std::vector<Val*> new_outs = e->outputs();
replaced += std::ranges::count_if(new_outs, maybe_replace);

if (replaced == 0) {
return e;
}

return e->newObjectFunc()(e->container(), new_ins, new_outs, e->attributes());
}

void lowerSegment(
const SegmentedGroup& group,
const AliasInfoMap& aliases,
Expand Down Expand Up @@ -219,24 +249,7 @@ void lowerSegment(
}
}

std::vector<Val*> new_inputs;
std::transform(
e->inputs().begin(),
e->inputs().end(),
std::back_inserter(new_inputs),
[&replacement_map](Val* input) {
return getOrDefault(replacement_map, input, input);
});
std::vector<Val*> new_outputs;
std::transform(
e->outputs().begin(),
e->outputs().end(),
std::back_inserter(new_outputs),
[&replacement_map](Val* output) {
return getOrDefault(replacement_map, output, output);
});
Expr* new_e = e->newObjectFunc()(
e->container(), new_inputs, new_outputs, e->attributes());
Expr* new_e = cloneWithNewOperands(e, replacement_map);
for_loop->body().push_back(new_e);
}
break;
Expand Down