Re: [PATCH v5] Introduce strub: machine-independent stack scrubbing

[Alexandre Oliva <oliva@adacore.com>]

On Dec  6, 2023, Jan Hubicka <hubicka@ucw.cz> wrote:

> I am sorry for sending this late.

No need to be sorry.  Thank you very much for taking the time to review
and comment on it.

> I think the ipa changes are generally fine.

Phew :-)

>> +static inline bool
>> +strub_always_inline_p (cgraph_node *node)
>> +{
>> +  return lookup_attribute ("always_inline", DECL_ATTRIBUTES (node->decl));
>> +}
> We may want to ahve this as cgraph_node::always_inline_p since there are
> now quite many places we look up this attribute.

Can do.  Would such a global refactoring still be welcome at this stage,
or should it be saved for stage1?  I guess it could still go in, so
simple it is...


>> +/* The strub pass proper adjusts types, signatures, and at-calls calls, and
>> +   splits internal-strub functions.  */
>> +
>> +unsigned int
>> +pass_ipa_strub::execute (function *)
>> +{
>> +  cgraph_node *onode;
>> +
>> +  ipa_strub_set_mode_for_new_functions ();
>> +
>> +  /* First, adjust the signature of at-calls functions.  We adjust types of
>> +     at-calls functions first, so that we don't modify types in place unless
>> +     strub is explicitly requested.  */

> I think Martin ma have more specific opinion on this, but since this is
> not running as the ipa pass during WPA stage, I think the param
> modification infrastructure is not really that much hepful here. 

Hmm...  I wonder if this is indeed what Martin refers to.  There are two
separate pieces of logic for parm-tweaking, one for "at-calls" strub
functions, that get the signature and the type of the function itself
modified (akin to adding the implicit "this" parameter to a C++
nonstatic member-function), and is implemented under the comment above,
and there's the splitting-out of "internal" strub function bodies into a
clone with a modified signature, that is implemented elsewhere.  The
latter uses cloning and thus (some, but not much) IPA param modification
infrastructure, but the former doesn't IIRC.

>> +	/* ??? Maybe we could adjust it instead.  */
>> +	if (drop_fnspec)
>> +	  remove_named_attribute_unsharing ("fn spec",
>> +					    &TYPE_ATTRIBUTES (nftype));

> ipa param modification also doesn't know how to update fn spec, this is
> something we should look into next stage1...
> There is also access attribute which speaks directly about individual
> arugments, perhaps you want to drop this one too?

Hmm, I can't recall whether I've come across it before (it sounds
vaguely familiar, but unless they become "fn spec" (ISTR synthetic "fn
spec"s), I think I'd have dealt with them already.

I'll dig it a little further.

> Are variadic thunks working with scrubbing?

Yeah, the wrapper is rewritten to call va_start itself, and the
split-out wrapped body is modified to call va_copy instead of va_start.

-- 
Alexandre Oliva, happy hacker            https://FSFLA.org/blogs/lxo/
   Free Software Activist                   GNU Toolchain Engineer
More tolerance and less prejudice are key for inclusion and diversity
Excluding neuro-others for not behaving ""normal"" is *not* inclusive