[commit: ghc] master: Compute DW_FORM_block length correctly; also fixes #15068 (358b508)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/358b508051333882d4099acca8f269e6fb2b7d65/ghc

>---------------------------------------------------------------

commit 358b508051333882d4099acca8f269e6fb2b7d65
Author: Bertram Felgenhauer <int-e at gmx.de>
Date:   Thu May 3 18:03:53 2018 +0300

    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


>---------------------------------------------------------------

358b508051333882d4099acca8f269e6fb2b7d65
 compiler/nativeGen/Dwarf/Types.hs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/compiler/nativeGen/Dwarf/Types.hs b/compiler/nativeGen/Dwarf/Types.hs
index 23a2c92..579ed0d 100644
--- a/compiler/nativeGen/Dwarf/Types.hs
+++ b/compiler/nativeGen/Dwarf/Types.hs
@@ -492,9 +492,11 @@ pprUnwindExpr spIsCFA expr
         pprE (UwPlus u1 u2)   = pprE u1 $$ pprE u2 $$ pprByte dW_OP_plus
         pprE (UwMinus u1 u2)  = pprE u1 $$ pprE u2 $$ pprByte dW_OP_minus
         pprE (UwTimes u1 u2)  = pprE u1 $$ pprE u2 $$ pprByte dW_OP_mul
-    in text "\t.uleb128 1f-.-1" $$ -- DW_FORM_block length
+    in text "\t.uleb128 2f-1f" $$ -- DW_FORM_block length
+       -- computed as the difference of the following local labels 2: and 1:
+       text "1:" $$
        pprE expr $$
-       text "1:"
+       text "2:"
 
 -- | Generate code for re-setting the unwind information for a
 -- register to @undefined@