[PATCH] Don't rewind PC for GHC generated frames

[Bartosz Nitka]

GHC - the Haskell compiler generates code that violates one of
GDB's assumptions.

GDB assumes that the address in a frame was generated by the call
instruction and that it is the address of the call instruction
plus 1 (I'm rephrasing the comment in get_frame_address_in_block).
So to get the real address, one has to substract 1. This is doubly
beneficial because some functions are "noreturn" and don't have
further instructions after call, so GDB would be looking at gibberish.
All in all, this is good for C-like languages.

GHC generates completely different code. It uses jumps instead of call
and manages the stack itself. Furthermore every piece of code is preceeded
by some metadata called the Info Table. If we substract from the
program counter it ends up pointing to the metadata, which is undesirable.
GHC has a workaround for this [1] that works most of the time, it basically
lies in the DWARF data and extends the function one byte backwards.
That helps with making unwinding succeed most of the time, but then the
address is also used for looking up symbols and they can't be resolved.

This change disables program counter rewinding for GHC generated compilation
units.

Some additional context can be found here [2].

[1] https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/nativeGen/Dwarf/Types.hs;e9ae0cae9eb6a340473b339b5711ae76c6bdd045$399-417
[2] https://ghc.haskell.org/trac/ghc/wiki/DWARF

gdb/ChangeLog:

       * dwarf2read.c (process_full_comp_unit): Populate producer_is_ghc.
       * frame.c (get_frame_address_in_block): Don't rewind the program
       counter for code generated by GHC.
       * symtab.h (struct compunit_symtab): Add producer_is_ghc.
---
 gdb/ChangeLog    | 7 +++++++
 gdb/dwarf2read.c | 4 ++++
 gdb/frame.c      | 7 +++++++
 gdb/symtab.h     | 3 +++
 4 files changed, 21 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 5c3338f..ff62136 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+2018-02-01  Bartosz Nitka  <niteria@gmail.com>
+
+       * dwarf2read.c (process_full_comp_unit): Populate producer_is_ghc.
+       * frame.c (get_frame_address_in_block): Don't rewind the program
+       counter for code generated by GHC.
+       * symtab.h (struct compunit_symtab): Add producer_is_ghc.
+
 2018-02-01  Yao Qi  <yao.qi@linaro.org>

        * arm-tdep.c (arm_record_extension_space): Change ret to signed.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 51d0f39..2516c48 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -10501,6 +10501,10 @@ process_full_comp_unit (struct
dwarf2_per_cu_data *per_cu,
        cust->epilogue_unwind_valid = 1;

       cust->call_site_htab = cu->call_site_htab;
+
+      if (startswith (cu->producer,
+            "The Glorious Glasgow Haskell Compilation System"))
+        cust->producer_is_ghc = 1;
     }

   if (dwarf2_per_objfile->using_index)
diff --git a/gdb/frame.c b/gdb/frame.c
index 1384ecc..d00df70 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -2458,7 +2458,14 @@ get_frame_address_in_block (struct frame_info
*this_frame)
       && (get_frame_type (this_frame) == NORMAL_FRAME
          || get_frame_type (this_frame) == TAILCALL_FRAME
          || get_frame_type (this_frame) == INLINE_FRAME))
+    {
+      /* GHC intermixes metadata (info tables) with code, going back is
+         guaranteed to land us in the metadata.  */
+      struct compunit_symtab *cust = find_pc_compunit_symtab (pc);
+      if (cust != NULL && cust->producer_is_ghc)
+        return pc;
       return pc - 1;
+    }

   return pc;
 }
diff --git a/gdb/symtab.h b/gdb/symtab.h
index f9d52e7..c164e5b 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1432,6 +1432,9 @@ struct compunit_symtab
      instruction).  This is supported by GCC since 4.5.0.  */
   unsigned int epilogue_unwind_valid : 1;

+  /* This CU was produced by Glasgow Haskell Compiler */
+  unsigned int producer_is_ghc : 1;
+
   /* struct call_site entries for this compilation unit or NULL.  */
   htab_t call_site_htab;