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From 358b508051333882d4099acca8f269e6fb2b7d65 Mon Sep 17 00:00:00 2001
From: Bertram Felgenhauer <int-e@gmx.de>
Date: Thu, 3 May 2018 18:03:53 +0300
Subject: [PATCH] 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
---
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 23a2c920e7..579ed0d256 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@
--
2.17.0
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