[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
      Resolution:                    |             Keywords:  CodeGen
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * keywords:   => CodeGen


Old description:

> 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

New description:

 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
 1. Write this down as a invariant of STG, or
 2. Establish this invariant in the immediately-before-codegen pass that
 gets the free vars right, or
 3. Not assume it in the code generator

--

Comment:

 I have no strong opinion about 1-3.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15901#comment:1>
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