[GHC] #15068: Small number of test case failures on Ubuntu Bionic (GCC 7.3)

[GHC]

#15068: Small number of test case failures on Ubuntu Bionic (GCC 7.3)
-------------------------------------+-------------------------------------
        Reporter:  jrp               |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.1
  (CodeGen)                          |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  Incorrect result  |            Test Case:  T13658
  at runtime                         |  T14779a T14779b T14868 debug
                                     |  parsing001
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4654
       Wiki Page:                    |
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Comment (by Ömer Sinan Ağacan <omeragacan@…>):

 In [changeset:"358b508051333882d4099acca8f269e6fb2b7d65/ghc"
 358b5080/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="358b508051333882d4099acca8f269e6fb2b7d65"
 Compute DW_FORM_block length correctly; also fixes #15068

 Before this patch, the pprUnwindwExpr function computed the length of
 by the following assembly fragment:

         .uleb128 1f-.-1
         <expression data>
 1:

 That is, to compute the length, it takes the difference of the label 1
 and the address of the .uleb128 directive, and subtracts 1.

 In #15068 it was reported that `as` from binutils 4.30 has trouble with
 evaluating the `.` part of the expression. However, there is actually a
 problem with the expression, if the length of the data ever becomes
 larger than 128: In that case, the .uleb128 directive will emit more
 than 1 byte, and the computed length will be wrong.

 The present patch changes the assembly fragment to use two labels,
 which fixes both these problems.

         .uleb128 2f-1f
 1:
         <expression data>
 2:

 Test Plan: validate

 Reviewers: bgamari, osa1

 Reviewed By: bgamari

 Subscribers: thomie, carter

 GHC Trac Issues: #15068

 Differential Revision: https://phabricator.haskell.org/D4654
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15068#comment:16>
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