[asan] Protection of stack vars (take 2)

[Jakub Jelinek]

On Tue, Oct 16, 2012 at 01:56:46PM -0700, Xinliang David Li wrote:
> Looks good except for the following:
> 
> 1) I am not sure if the stack slot sharing is handled correctly. If I
> read the code correctly, the redzone var will be only created for the
> representative variable in a partition -- will this lead to false
> negatives? As I asked before, should stack slot sharing even turned
> on?

I thought we don't merge stack slots if the vars have different size,
apparently I've been wrong about that.  So I'll make sure those aren't
shared with flag_asan.  When the size is the same, there should be no false
negatives.

> 2) Performance tuning -- it is probably better to skip those variables
> that are compiler generated -- is there any point guarding them?

In my (admittedly very limited) testing only at -O0 some compiler generated
vars (SSA_NAMEs) got guards.  The trouble here is -fasan -fstack-protector
combination.  I wonder if there can be any DECL_ARTIFICIAL (or non-VAR_DECL/
RESULT_DECL) decls for which stack_protect_decl_phase returns 1 or 2.
If it is unlikely, the best might be to protect all phase 1 and 2 vars and
if flag_asan call expand_stack_vars once more for partitions where any of
the protected vars are user vars, and finally the current expand_stack_vars
(NULL) which would not do any asan protection.

	Jakub