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, 5 insertions, 4 deletions

diff --git a/rts/win32/OSMem.c b/rts/win32/OSMem.c
index 3450267d9c..3d9a304fda 100644
--- a/rts/win32/OSMem.c
+++ b/rts/win32/OSMem.c
@@ -430,19 +430,20 @@ void setExecutable (void *p, W_ len, rtsBool exec)
 
 static void* heap_base = NULL;
 
-void *osReserveHeapMemory (W_ *len)
+void *osReserveHeapMemory (void *startAddress, W_ *len)
 {
     void *start;
 
-    heap_base = VirtualAlloc(NULL, *len + MBLOCK_SIZE,
+    heap_base = VirtualAlloc(startAddress, *len + MBLOCK_SIZE,
                               MEM_RESERVE, PAGE_READWRITE);
     if (heap_base == NULL) {
         if (GetLastError() == ERROR_NOT_ENOUGH_MEMORY) {
             errorBelch("out of memory");
         } else {
             sysErrorBelch(
-                "osReserveHeapMemory: VirtualAlloc MEM_RESERVE %llu bytes failed",
-                len + MBLOCK_SIZE);
+                "osReserveHeapMemory: VirtualAlloc MEM_RESERVE %llu bytes \
+                at address %p bytes failed",
+                len + MBLOCK_SIZE, startAddress);
         }
         stg_exit(EXIT_FAILURE);
     }