Opened 10 months ago

Last modified 10 months ago

#15901 new task

Assert and record that code generation requires distinct uiques for let-binders

Reported by: sgraf Owned by:
Priority: normal Milestone:
Component: Compiler Version: 8.6.2
Keywords: CodeGen Cc: simonpj, simonmar, osa1, sgraf
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:

Description (last modified by simonpj)

Simon writes in #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

Change History (2)

comment:1 Changed 10 months ago by simonpj

Description: modified (diff)
Keywords: CodeGen added

I have no strong opinion about 1-3.

comment:2 Changed 10 months ago by simonmar

This is currently done by CorePrep, and noted explicitly in the comments (CorePrep.hs):

6.  Clone all local Ids.
    This means that all such Ids are unique, rather than the
    weaker guarantee of no clashes which the simplifier provides.
    And that is what the code generator needs.

    We don't clone TyVars or CoVars. The code gen doesn't need that,
    and doing so would be tiresome because then we'd need
    to substitute in types and coercions.
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