Re: How to traverse all the local variables that declared in the current routine?

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Nov 24, 2020 at 4:47 PM Qing Zhao <QING.ZHAO@oracle.com> wrote:
>
>
>
> > On Nov 24, 2020, at 1:32 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> >
> > On Tue, Nov 24, 2020 at 12:05 AM Qing Zhao via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >>
> >> Hi,
> >>
> >> Does gcc provide an iterator to traverse all the local variables that are declared in the current routine?
> >>
> >> If not, what’s the best way to traverse the local variables?
> >
> > Depends on what for.  There's the source level view you get by walking
> > BLOCK_VARS of the
> > scope tree, theres cfun->local_variables (FOR_EACH_LOCAL_DECL) and
> > there's SSA names
> > (FOR_EACH_SSA_NAME).
>
> I am planing to add a new phase immediately after “pass_late_warn_uninitialized” to initialize all auto-variables that are
> not explicitly initialized in the declaration, the basic idea is following:
>
> ** The proposal:
>
> A. add a new GCC option: (same name and meaning as CLANG)
> -ftrivial-auto-var-init=[pattern|zero], similar pattern init as CLANG;
>
> B. add a new attribute for variable:
> __attribute((uninitialized)
> the marked variable is uninitialized intentionaly for performance purpose.
>
> C. The implementation needs to keep the current static warning on uninitialized
> variables untouched in order to avoid "forking the language".
>
>
> ** The implementation:
>
> There are two major requirements for the implementation:
>
> 1. all auto-variables that do not have an explicit initializer should be initialized to
> zero by this option.  (Same behavior as CLANG)
>
> 2. keep the current static warning on uninitialized variables untouched.
>
> In order to satisfy 1, we should check whether an auto-variable has initializer
> or not;
> In order to satisfy 2, we should add this new transformation after
> "pass_late_warn_uninitialized".
>
> So, we should be able to check whether an auto-variable has initializer or not after “pass_late_warn_uninitialized”,
> If Not, then insert an initialization for it.
>
> For this purpose, I guess that “FOR_EACH_LOCAL_DECL” might be better?

Yes, but do you want to catch variables promoted to register as well
or just variables
on the stack?

> Another issue is, in order to check whether an auto-variable has initializer, I plan to add a new bit in “decl_common” as:
>   /* In a VAR_DECL, this is DECL_IS_INITIALIZED.  */
>   unsigned decl_is_initialized :1;
>
> /* IN VAR_DECL, set when the decl is initialized at the declaration.  */
> #define DECL_IS_INITIALIZED(NODE) \
>   (DECL_COMMON_CHECK (NODE)->decl_common.decl_is_initialized)
>
> set this bit when setting DECL_INITIAL for the variables in FE. then keep it
> even though DECL_INITIAL might be NULLed.

For locals it would be more reliable to set this flag during gimplification.

> Do you have any comment and suggestions?

As said above - do you want to cover registers as well as locals?  I'd do
the actual zeroing during RTL expansion instead since otherwise you
have to figure youself whether a local is actually used (see expand_stack_vars)

Note that optimization will already made have use of "uninitialized" state
of locals so depending on what the actual goal is here "late" may be too late.

Richard.

>
> Thanks a lot for the help.
>
> Qing
>
> > Richard.
> >
> >>
> >> Thanks.
> >>
> >> Qing
>