[GHC] #11337: Unwind information incorrect between Sp adjustment and end of block

2 Jan 2016

      #11337: Unwind information incorrect between Sp adjustment and end of block
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
  (CodeGen)                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 As pointed out in Phab:D1532, the DWARF unwinding information that we
 produce is
 currently a bit oversimplified. Namely, we produce exactly one unwind
 table per
 Cmm block. This works reasonably well in most cases since we most Cmm
 blocks
 have the form,
 {{{
 aProcedure() {
 casl:
     -- we just entered the procedure, so the unwinding is trivial.
     unwind Sp = Sp
     -- we push some values onto the stack...
     I64[Sp - 16] = ...
     I64[Sp - 8] = ...
     -- and before leaving the block we update Sp.
     Sp = Sp - 16;
     -- technically our unwind information is now a lie
     call aFunction() returns to casd;

 casd:
     -- we inherit the unwind information from the state of the stack when
 we
     -- left the preceding block (casl)
     unwind Sp = Sp + 16
     R2 = I64[Sp + 8];
     -- pop off that which we pushed
     Sp = Sp + 16;
     call GHC.List.$wunsafeTake_info(R3, R2) args: 8, res: 0, upd: 8;
 }}}
 Here there is a narrow window where our unwind information is technically
 wrong:
 between updating `Sp` in `casl` and calling into `aFunction`.

 Note that after we arrive in `aFunction` we are safe, since our return
 address
 is `casd`, which has the correct unwinding information.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11337
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