This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH 1/4] x86: Add -mindirect-branch=
> On Sat, Jan 13, 2018 at 7:56 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> >> >> +/* Output a funtion with a call and return thunk for indirect branch.
> >> >> + If BND_P is true, the BND prefix is needed. If REGNO != -1, the
> >> >> + function address is in REGNO. Otherwise, the function address is
> >> >> + on the top of stack. */
> >> >> +
> >> >> +static void
> >> >> +output_indirect_thunk_function (bool need_bnd_p, int regno)
> >> >> +{
> >> >> + char name[32];
> >> >> + tree decl;
> >> >> +
> >> >> + /* Create __x86_indirect_thunk/__x86_indirect_thunk_bnd. */
> >> >> + indirect_thunk_name (name, regno, need_bnd_p);
> >> >> + decl = build_decl (BUILTINS_LOCATION, FUNCTION_DECL,
> >> >> + get_identifier (name),
> >> >> + build_function_type_list (void_type_node, NULL_TREE));
> >> >> + DECL_RESULT (decl) = build_decl (BUILTINS_LOCATION, RESULT_DECL,
> >> >> + NULL_TREE, void_type_node);
> >> >> + TREE_PUBLIC (decl) = 1;
> >> >> + TREE_STATIC (decl) = 1;
> >> >> + DECL_IGNORED_P (decl) = 1;
> >> >
> >> > DECL_ARTIFICIAL as well?
> >>
> >> This is done exactly the same way as PIC thunk. I don't think we
> >> should change it here.
> >> >
> >> > Why do you need to output asm visibility directives by hand when you create
> >> > symbol for the function anyway?
> >>
> >> This is done exactly the same way as PIC thunk. I don't think we
> >> should change it here.
> >
> > I see, it is pretty ancient code. Perhaps you can at least commonize
> > the uglness so we don't duplicate the ifdefs for MACHO?
>
> I don't think we should such surgery at such late stage. I prefer to
> keep it for later cleanup.
OK, lets keep it as it is and clean up next stage1.
Honza
>
> >>
> >> > I would expect that you can just use similar code as cgraph_node::expand_thunk
> >> > when calls output_mi_thunk and get this done in a way that is independent of
> >> > target assembler.
> >>
> >> I took a look. I don't see an easy to do it. I'd like to keep it exactly the
> >> same as PIC thunk. And we change both thunks together later if needed.
> >
> > OK, lets keep it done same was as PIC thunk.
> > Next stage1 we could clean up both.
> >> > I suppose it may make sense to split the insn and at least explicitly represent
> >> > the fact that we (sometimes) push the target to stack.
> >>
> >> Did you mean "split the function"? I will break it into
> >> ix86_output_indirect_branch_via_reg and
> >> ix86_output_indirect_branch_via_push.
> >
> > I mean define_split to insert push instruction into the instruction
> > stream rather then printing it as part of the indirect jump insn.
>
> We don't want anything added between these instructions.
> Split it may lead to trouble.
>
> >>
> >> > Why the memory variant exists at first place?
> >>
> >> When the function address is in memory, we can push it onto stack
> >> to save a register. Also it is needed to cover "call foo" to "call [foo@GOT]"
> >> conversion.
> >> >
> >> > I think you also want to update type to "many" when we do more than just indirect branch.
> >>
> >> Did you mean "multi"? I will change to "multi".
> >
> > Yes.
> >
> >>
> >> > We do not care much about this, but it feels wrong to have attributes off reality.
> >> >> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> >> >> index f3e4a63ab46..ddb6035be96 100644
> >> >> --- a/gcc/doc/extend.texi
> >> >> +++ b/gcc/doc/extend.texi
> >> >> @@ -5754,6 +5754,16 @@ Specify which floating-point unit to use. You must specify the
> >> >> @code{target("fpmath=sse+387")} because the comma would separate
> >> >> different options.
> >> >>
> >> >> +@item indirect_branch("@var{choice}")
> >> >> +@cindex @code{indirect_branch} function attribute, x86
> >> >> +On x86 targets, the @code{indirect_branch} attribute causes the compiler
> >> >> +to convert indirect call and jump with @var{choice}. @samp{keep}
> >> >> +keeps indirect call and jump unmodified. @samp{thunk} converts indirect
> >> >> +call and jump to call and return thunk. @samp{thunk-inline} converts
> >> >> +indirect call and jump to inlined call and return thunk.
> >> >> +@samp{thunk-extern} converts indirect call and jump to external call
> >> >> +and return thunk provided in a separate object file.
> >> >
> >> > Please expand the documentation in a way that random user who is not aware of the
> >> > issue will understand that those are security features that come at a cost.
> >> >
> >> >> +@opindex -mindirect-branch
> >> >> +Convert indirect call and jump with @var{choice}. The default is
> >> >> +@samp{keep}, which keeps indirect call and jump unmodified.
> >> >> +@samp{thunk} converts indirect call and jump to call and return thunk.
> >> >> +@samp{thunk-inline} converts indirect call and jump to inlined call
> >> >> +and return thunk. @samp{thunk-extern} converts indirect call and jump
> >> >> +to external call and return thunk provided in a separate object file.
> >> >> +You can control this behavior for a specific function by using the
> >> >> +function attribute @code{indirect_branch}. @xref{Function Attributes}.
> >> >
> >> > Similarly here.
> >>
> >> I will update documentation with user guide info after Intel white
> >> paper is published.
> >
> > OK,
> > thanks!
> > Honza
> >>
> >> > Rest of the patch seems OK. We may want incrementally represent more of the
> >> > indirect jump/call seqeuence in RTL, but at this point probably keeping things
> >> > simple and localized is not a bad idea. This can be done incrementally.
> >> >
> >> > Please make updated patch and I would like to give others chance to comment today.
> >> >
> >> > Honza
> >>
> >>
> >>
> >> --
> >> H.J.
>
>
>
> --
> H.J.