Repository : ssh://darcs.haskell.org//srv/darcs/ghc On branch : master
http://hackage.haskell.org/trac/ghc/changeset/65fec07bfbf41846689e0b28bc94b168c8cacc84 >--------------------------------------------------------------- commit 65fec07bfbf41846689e0b28bc94b168c8cacc84 Author: Simon Marlow <marlo...@gmail.com> Date: Wed Oct 24 12:03:34 2012 +0100 Comment to explain why we need to split proc points on x86/Darwin with -fPIC >--------------------------------------------------------------- compiler/cmm/CmmPipeline.hs | 32 +++++++++++++++++++++++++++++++- 1 files changed, 31 insertions(+), 1 deletions(-) diff --git a/compiler/cmm/CmmPipeline.hs b/compiler/cmm/CmmPipeline.hs index c523b13..aa8fa2c 100644 --- a/compiler/cmm/CmmPipeline.hs +++ b/compiler/cmm/CmmPipeline.hs @@ -179,11 +179,41 @@ cpsTop hsc_env proc = -- the entry point. splitting_proc_points = hscTarget dflags /= HscAsm || not (tablesNextToCode dflags) - || usingDarwinX86Pic + || usingDarwinX86Pic -- Note [darwin-x86-pic] usingDarwinX86Pic = platformArch platform == ArchX86 && platformOS platform == OSDarwin && gopt Opt_PIC dflags +{- Note [darwin-x86-pic] + +On x86/Darwin, PIC is implemented by inserting a sequence like + + call 1f + 1: popl %reg + +at the proc entry point, and then referring to labels as offsets from +%reg. If we don't split proc points, then we could have many entry +points in a proc that would need this sequence, and each entry point +would then get a different value for %reg. If there are any join +points, then at the join point we don't have a consistent value for +%reg, so we don't know how to refer to labels. + +Hence, on x86/Darwin, we have to split proc points, and then each proc +point will get its own PIC initialisation sequence. + +This isn't an issue on x86/ELF, where the sequence is + + call 1f + 1: popl %reg + addl $_GLOBAL_OFFSET_TABLE_+(.-1b), %reg + +so %reg always has a consistent value: the address of +_GLOBAL_OFFSET_TABLE_, regardless of which entry point we arrived via. + +-} + + + runUniqSM :: UniqSM a -> IO a runUniqSM m = do us <- mkSplitUniqSupply 'u' _______________________________________________ Cvs-ghc mailing list Cvs-ghc@haskell.org http://www.haskell.org/mailman/listinfo/cvs-ghc
