Improve access violation reporting on Windows

Summary:
This patch is courtesy of @awson.

Currently, whenever GHC catches a segfault on Windows, it simply reports the
somewhat uninformative message
`Segmentation fault/access violation in generated code`. This patch adds to
the message the type of violation (read/write/dep) and location information,
which should help debugging segfaults in the future.

Fixes #13108.

Test Plan: Build on Windows

Reviewers: austin, erikd, bgamari, simonmar, Phyx

Reviewed By: bgamari, Phyx

Subscribers: awson, thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2969

GHC Trac Issues: #13108

1 files changed, 7 insertions, 1 deletions

diff --git a/rts/win32/veh_excn.c b/rts/win32/veh_excn.c
index bf2151a59a..c94dc5a830 100644
--- a/rts/win32/veh_excn.c
+++ b/rts/win32/veh_excn.c
@@ -32,6 +32,7 @@ PVOID __hs_handle = NULL;
 long WINAPI __hs_exception_handler(struct _EXCEPTION_POINTERS *exception_data)
 {
     long action = EXCEPTION_CONTINUE_SEARCH;
+    ULONG_PTR what;
 
     // When the system unwinds the VEH stack after having handled an excn,
     // return immediately.
@@ -49,7 +50,12 @@ long WINAPI __hs_exception_handler(struct _EXCEPTION_POINTERS *exception_data)
                 action = EXCEPTION_CONTINUE_EXECUTION;
                 break;
             case EXCEPTION_ACCESS_VIOLATION:
-                fprintf(stdout, "Segmentation fault/access violation in generated code\n");
+                what = exception_data->ExceptionRecord->ExceptionInformation[0];
+                fprintf(stdout, "Access violation in generated code"
+                                " when %s %p\n"
+                                , what == 0 ? "reading" : what == 1 ? "writing" : what == 8 ? "executing data at" : "?"
+                                , (void*) exception_data->ExceptionRecord->ExceptionInformation[1]
+                                );
                 action = EXCEPTION_CONTINUE_EXECUTION;
                 break;
             default:;