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, 8 insertions, 3 deletions

diff --git a/compiler/cbits/genSym.c b/compiler/cbits/genSym.c
index 08d403d849..70ea417c4b 100644
--- a/compiler/cbits/genSym.c
+++ b/compiler/cbits/genSym.c
@@ -2,16 +2,21 @@
 #include "Rts.h"
 
 static HsInt GenSymCounter = 0;
+static HsInt GenSymInc = 1;
 
 HsInt genSym(void) {
 #if defined(THREADED_RTS)
     if (n_capabilities == 1) {
-        return GenSymCounter++;
+        return GenSymCounter = (GenSymCounter + GenSymInc) & 0xFFFFFF;
     } else {
-        return atomic_inc((StgWord *)&GenSymCounter, 1);
+        return atomic_inc((StgWord *)&GenSymCounter, GenSymInc) & 0xFFFFFF;
     }
 #else
-    return GenSymCounter++;
+    return GenSymCounter = (GenSymCounter + GenSymInc) & 0xFFFFFF;
 #endif
 }
 
+void initGenSym(HsInt NewGenSymCounter, HsInt NewGenSymInc) {
+  GenSymCounter = NewGenSymCounter;
+  GenSymInc = NewGenSymInc;
+}