combineIdenticalAlts is only partially implemented

combineIdenticalAlts commons up branches of case expressions which have the same RHS.

However, it is not fully implemented so opportunities to common up branches are missed. For very big case expressions in might impact compilation time but it could be something which is enabled by -O2.

For example, the C and D case for foo are not commened up but the A and B case in foo2 are.

module Foo where

data T = A | B | C | D


foo x = case x of
          A -> 0
          B -> 1
          C -> 2
          D -> 2

foo2 x = case x of
          A -> 2
          B -> 2
          C -> 0
          D -> 1

See also #18377 for another version of this same issue.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Architecture