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

1 files changed, 2 insertions, 0 deletions

diff --git a/ghc/Main.hs b/ghc/Main.hs
index fc6ab88960..647bbadcf9 100644
--- a/ghc/Main.hs
+++ b/ghc/Main.hs
@@ -44,6 +44,7 @@ import Outputable
 import SrcLoc
 import Util
 import Panic
+import UniqSupply
 import MonadUtils       ( liftIO )
 
 -- Imports for --abi-hash
@@ -236,6 +237,7 @@ main' postLoadMode dflags0 args flagWarnings = do
     printInfoForUser (dflags6 { pprCols = 200 })
                      (pkgQual dflags6) (pprModuleMap dflags6)
 
+  liftIO $ initUniqSupply (initialUnique dflags6) (uniqueIncrement dflags6)
         ---------------- Final sanity checking -----------
   liftIO $ checkOptions postLoadMode dflags6 srcs objs