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

16 Nov 2018

      #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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