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perf: eliminate cases with all branches unreachable #11525

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Merged
hargoniX merged 1 commit into from
Dec 5, 2025
Merged

perf: eliminate cases with all branches unreachable #11525

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12 changes: 12 additions & 0 deletions src/Lean/Compiler/LCNF/Simp/Main.lean
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Original file line number Diff line number Diff line change
Expand Up @@ -336,6 +336,18 @@ partial def simp (code : Code) : SimpM Code := withIncRecDepth do
params.forM (eraseParam ·)
markSimplified
return k
if alts.all (·.getCode matches .unreach ..) then
alts.forM (liftM <| ·.getParams.forM eraseParam)
markSimplified
return .unreach resultType
/-
We considered handling a case where we drop a cases if it only has one non-unreachable
branch and doesn't rely on the params of that branch here. However, this has the potential
to hinder reuse in later passes as we loose information about the shape of a variable. We
might be able to reintroduce this at a later point if we track this information different
from cases.
-/

markUsedFVar discr
return code.updateCases! resultType discr alts
end
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