Add a RTS option -xp to load PIC object anywhere in address space

Summary:
This re-applies {D5195} with fixes for i386:
* Fix unused label warnings, see {D5230} or {D5273}
* Fix a silly bug introduced by moving `#if`

{P190}

Add a RTS option -xp to load PIC object anywhere in address space. We do
this by relaxing the requirement of <0x80000000 result of
`mmapForLinker` and implying USE_CONTIGUOUS_MMAP.

We also need to change calls to `ocInit` and `ocGetNames` to avoid
dangling pointers when the address of `oc->image` is changed by
`ocAllocateSymbolExtra`.

Test Plan:
See {D5195}, also test under i386:

```
$ uname -a
Linux watashi-arch32 4.18.5-arch1-1.0-ARCH #1 SMP PREEMPT Tue Aug 28
20:45:30 CEST 2018 i686 GNU/Linux
$ cd testsuite/tests/th/ && make test
...
```

will run `./validate` on stacked diff.

Reviewers: simonmar, bgamari, alpmestan, trommler, hvr, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5289

1 files changed, 12 insertions, 0 deletions

diff --git a/includes/rts/Flags.h b/includes/rts/Flags.h
index 63450d5ece..3582e6abde 100644
--- a/includes/rts/Flags.h
+++ b/includes/rts/Flags.h
@@ -188,6 +188,17 @@ typedef struct _CONCURRENT_FLAGS {
  */
 #define DEFAULT_TICK_INTERVAL USToTime(10000)
 
+/*
+ * When linkerAlwaysPic is true, the runtime linker assume that all object
+ * files were compiled with -fPIC -fexternal-dynamic-refs and load them
+ * anywhere in the address space.
+ */
+#if defined(x86_64_HOST_ARCH) && defined(darwin_HOST_OS)
+#define DEFAULT_LINKER_ALWAYS_PIC true
+#else
+#define DEFAULT_LINKER_ALWAYS_PIC false
+#endif
+
 /* See Note [Synchronization of flags and base APIs] */
 typedef struct _MISC_FLAGS {
     Time    tickInterval;        /* units: TIME_RESOLUTION */
@@ -197,6 +208,7 @@ typedef struct _MISC_FLAGS {
     bool generate_stack_trace;
     bool machineReadable;
     bool internalCounters;       /* See Note [Internal Counter Stats] */
+    bool linkerAlwaysPic;        /* Assume the object code is always PIC */
     StgWord linkerMemBase;       /* address to ask the OS for memory
                                   * for the linker, NULL ==> off */
 } MISC_FLAGS;