Make start address of `osReserveHeapMemory` tunable via command line -xb

Summary:
We stumbled upon a case where an external library (OpenCL) does not work
if a specific address (0x200000000) is taken.

It so happens that `osReserveHeapMemory` starts trying to mmap at 0x200000000:

```
        void *hint = (void*)((W_)8 * (1 << 30) + attempt * BLOCK_SIZE);
        at = osTryReserveHeapMemory(*len, hint);
```

This makes it impossible to use Haskell programs compiled with GHC 8
with C functions that use OpenCL.

See this example ​https://github.com/chpatrick/oclwtf for a repro.

This patch allows the user to work around this kind of behavior outside
our control by letting the user override the starting address through an
RTS command line flag.

Reviewers: bgamari, Phyx, simonmar, erikd, austin

Reviewed By: Phyx, simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D2513

1 files changed, 4 insertions, 1 deletions

diff --git a/rts/sm/OSMem.h b/rts/sm/OSMem.h
index 660942827d..69d87c201e 100644
--- a/rts/sm/OSMem.h
+++ b/rts/sm/OSMem.h
@@ -58,7 +58,10 @@ roundUpToPage (size_t x)
 // to the amount of memory actually reserved.
 //
 // This function is called once when the block allocator is initialized.
-void *osReserveHeapMemory(W_ *len);
+//
+// startAddress must be greater or equal than 8 * (1 << 30), and can be
+// NULL, in which case a default will be picked by the RTS.
+void *osReserveHeapMemory(void *startAddress, W_ *len);
 
 // Commit (allocate memory for) a piece of address space, which must
 // be within the previously reserved space After this call, it is safe