[GHC] #4012: Compilation results are not deterministic

[GHC]

#4012: Compilation results are not deterministic
-------------------------------------+-------------------------------------
        Reporter:  kili              |                Owner:  niteria
            Type:  bug               |               Status:  patch
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  6.12.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10424            |  Differential Rev(s):  Phab:D910,
                                     |  Phab:D1073, Phab:D1133, Phab:D1192,
       Wiki Page:                    |  Phab:D1268
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"158d2a91581d82dc1690a858b474eaab3a02e43e/ghc" 158d2a9/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="158d2a91581d82dc1690a858b474eaab3a02e43e"
 Make it possible to have different UniqSupply strategies

 To get reproducible/deterministic builds, the way that the Uniques are
 assigned shouldn't matter. This allows to test for that.

 It add 2 new flags:

 * `-dinitial-unique`
 * `-dunique-increment`

 And by varying these you can get interesting effects:

 * `-dinitial-unique=0 -dunique-increment 1` - current sequential
   UniqSupply

 * `-dinitial-unique=16777215 -dunique-increment -1` - UniqSupply that
   generates in decreasing order

 * `-dinitial-unique=1 -dunique-increment PRIME` - where PRIME big enough
   to overflow often - nonsequential order

 I haven't proven the usefullness of the last one yet and it's the reason
 why we have to mask the bits with `0xFFFFFF` in `genSym`, so I can
 remove it if it becomes contentious.

 Test Plan: validate on harbormaster

 Reviewers: simonmar, austin, ezyang, bgamari

 Reviewed By: austin, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1360

 GHC Trac Issues: #4012
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4012#comment:127>
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