[GHC] #15901: Assert and record that code generation requires distinct uiques for let-binders

[GHC]

#15901: Assert and record that code generation requires distinct uiques for let-
binders
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           Reporter:  sgraf          |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  ⊥
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Simon writes in [https://ghc.haskell.org/trac/ghc/ticket/15754#comment:10
 #15754]:

 Another invariant that the code generator needs is (I believe) that every
 let-binder has a distinct unique.  We can't re-use the same unique, even
 in a different scope, let alone shadowing.

 Why? Because (I believe) that the code generator uses these uniques to
 generate unique top level labels for the entry code and info table for the
 closure.

 Now, it's probably the case that they are unique anyway; but we should
 either
 * Write this down as a invariant of STG, or
 * Establish this invariant in the immediately-before-codegen pass that
 gets the free vars right, or
 * Not assume it in the code generator

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15901>
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