Better import library support for Windows

The import library support added for 7.10.3 was only a partial one.
This support was predicated on using file extensions to determine
whether or not a library was an import library. It also couldn't handle
libraries with multiple dll pointers.

This is a rewrite of that patch and fully integrating it into the normal
archive parsing and loading routines. This solves a host of issues,
among others allowing us to finally use `-lgcc_s`.

This also fixes a problem with our previous implementation, where we
just loaded the DLL and moved on. Doing this had the potential of using
the wrong symbol at resolve time. Say a DLL already loaded (A.dll) has
symbol a exported (dependency of another dll perhaps).

We find an import library `B.lib` explicitly defining an export of `a`.
we load `B.dll` but this gets put after `A.dll`, at resolve time we
would use the value from `A` instead of `B` which is what we wanted.

Test Plan: ./valide and make test TEST=13606

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, RyanGlScott, thomie, #ghc_windows_task_force

GHC Trac Issues: #13606, #12499, #12498

Differential Revision: https://phabricator.haskell.org/D3513

1 files changed, 12 insertions, 2 deletions

diff --git a/rts/LinkerInternals.h b/rts/LinkerInternals.h
index b8c411d22a..48c43ebbbe 100644
--- a/rts/LinkerInternals.h
+++ b/rts/LinkerInternals.h
@@ -32,13 +32,23 @@ typedef enum {
 /* Indication of section kinds for loaded objects.  Needed by
    the GC for deciding whether or not a pointer on the stack
    is a code pointer.
+   See Note [BFD import libraries].
 */
 typedef
-   enum { SECTIONKIND_CODE_OR_RODATA,
+   enum { /* Section is code or readonly. e.g. .text or .r(o)data.  */
+          SECTIONKIND_CODE_OR_RODATA,
+          /* Section contains read/write data. e.g. .data.  */
           SECTIONKIND_RWDATA,
+          /* Static initializer section. e.g. .ctors.  */
           SECTIONKIND_INIT_ARRAY,
+          /* We don't know what the section is and don't care.  */
           SECTIONKIND_OTHER,
-          SECTIONKIND_NOINFOAVAIL }
+          /* Section belongs to an import section group. e.g. .idata$.  */
+          SECTIONKIND_IMPORT,
+          /* Section defines an import library entry, e.g. idata$7.  */
+          SECTIONKIND_IMPORT_LIBRARY,
+          SECTIONKIND_NOINFOAVAIL
+        }
    SectionKind;
 
 typedef