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, 0 deletions

diff --git a/includes/stg/Types.h b/includes/stg/Types.h
index 0ad3c5c77d..3a08eb2f85 100644
--- a/includes/stg/Types.h
+++ b/includes/stg/Types.h
@@ -124,6 +124,8 @@ typedef uint32_t           StgHalfWord;
 #define FMT_HexWord        FMT_HexWord64
 #define FMT_Int            FMT_Int64
 
+#define strToStgWord       strtoull
+
 #elif SIZEOF_VOID_P == 4
 typedef int32_t            StgInt;
 typedef uint32_t           StgWord;
@@ -139,6 +141,8 @@ typedef uint16_t           StgHalfWord;
 #define FMT_HexWord        FMT_HexWord32
 #define FMT_Int            FMT_Int32
 
+#define strToStgWord       strtoul
+
 #else
 #error GHC untested on this architecture: sizeof(void *) != 4 or 8
 #endif