[commit: ghc] master: Dwarf: Preserve stack pointer register (6fb0ba6)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6fb0ba65d27602ae9a81fe142936b06d54b20b2e/ghc

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

commit 6fb0ba65d27602ae9a81fe142936b06d54b20b2e
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Fri Oct 30 20:49:35 2015 +0100

    Dwarf: Preserve stack pointer register
    
    Here we add a `same_value $sp` instruction to our default unwinding
    rules to ensure that the implicit `$sp = CFA` rule (which `libdw`
    appears to exhibit on x86_64) doesn't overwrite it (necessary since we
    don't use $sp to track our call stack).
    
    See Phab Diff D1189 for details on how we arrived at this resolution.
    
    Reviewers: scpmw, austin, simonmar
    
    Reviewed By: austin, simonmar
    
    Subscribers: thomie, simonmar
    
    Differential Revision: https://phabricator.haskell.org/D1224


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

6fb0ba65d27602ae9a81fe142936b06d54b20b2e
 compiler/nativeGen/Dwarf/Types.hs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/compiler/nativeGen/Dwarf/Types.hs b/compiler/nativeGen/Dwarf/Types.hs
index a2d07b8..d558c2d 100644
--- a/compiler/nativeGen/Dwarf/Types.hs
+++ b/compiler/nativeGen/Dwarf/Types.hs
@@ -231,6 +231,13 @@ pprDwarfFrame DwarfFrame{dwCieLabel=cieLabel,dwCieInit=cieInit,dwCieProcs=procs}
         retReg      = dwarfReturnRegNo plat
         wordSize    = platformWordSize plat
         pprInit (g, uw) = pprSetUnwind plat g (Nothing, uw)
+
+        -- Preserve C stack pointer: This necessary to override that default
+        -- unwinding behavior of setting $sp = CFA.
+        preserveSp = case platformArch plat of
+          ArchX86    -> pprByte dW_CFA_same_value $$ pprLEBWord 4
+          ArchX86_64 -> pprByte dW_CFA_same_value $$ pprLEBWord 7
+          _          -> empty
     in vcat [ ppr cieLabel <> colon
             , pprData4' length -- Length of CIE
             , ppr cieStartLabel <> colon
@@ -250,8 +257,11 @@ pprDwarfFrame DwarfFrame{dwCieLabel=cieLabel,dwCieInit=cieInit,dwCieProcs=procs}
               pprByte (dW_CFA_offset+retReg)
             , pprByte 0
 
+              -- Preserve C stack pointer
+            , preserveSp
+
               -- Sp' = CFA
-              -- (we need to set this manually as our Sp register is
+              -- (we need to set this manually as our (STG) Sp register is
               -- often not the architecture's default stack register)
             , pprByte dW_CFA_val_offset
             , pprLEBWord (fromIntegral spReg)