[GHC] #15104: Update JMP_TBL targets during shortcutting for x86 codegen.

[GHC]

#15104: Update JMP_TBL targets during shortcutting for x86 codegen.
-------------------------------------+-------------------------------------
           Reporter:  AndreasK       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.5
  (NCG)                              |
           Keywords:  CodeGen        |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):  Phab:D4595     |         Wiki Page:
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 Jump tables are generated from Cmm Code in a few steps:

 1. We start with a CmmSwitch at the end of the Cmm pipeline which gets
 turned into a JMP_TBL instruction.
 2. During native codegen we generate static jump tables from JMP_TBL.
 (generateJumpTables)
 3. We shortcut jump targets, for jump tables by updating the static
 tables. (shortcutBranches). This might update the jump targets with
 calculated address not represented by a label.

 After the third step the targets in the JMP_TBL instruction (which are the
 original labels) and in the static data can be out of sync.

 Phab:D4566 went part of the way, updating labels in JMP_TBL. However
 without tables next to code labels can be updated to non-label jump
 destinations which I did not consider at the time.

 Phab:D4595 is intended to fix this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15104>
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