On 11/30/20 9:23 AM, Qing Zhao wrote:
Hi, Martin,
Thanks a lot for your suggestion.
On Nov 25, 2020, at 6:08 PM, Martin Sebor <mse...@gmail.com
<mailto:mse...@gmail.com>> wrote:
On 11/24/20 9:54 AM, Qing Zhao via Gcc-patches wrote:
On Nov 24, 2020, at 9:55 AM, Richard Biener
<richard.guent...@gmail.com <mailto:richard.guent...@gmail.com>> wrote:
On Tue, Nov 24, 2020 at 4:47 PM Qing Zhao <qing.z...@oracle.com
<mailto:qing.z...@oracle.com>> wrote:
On Nov 24, 2020, at 1:32 AM, Richard Biener
<richard.guent...@gmail.com <mailto:richard.guent...@gmail.com>>
wrote:
On Tue, Nov 24, 2020 at 12:05 AM Qing Zhao via Gcc-patches
<gcc-patches@gcc.gnu.org <mailto: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?
I think both as long as they are source-level auto-variables. Then
which one is better?
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.
You mean I can set the flag “DECL_IS_INITIALIZED (decl)” inside the
routine “gimpley_decl_expr” (gimplify.c) as following:
if (VAR_P (decl) && !DECL_EXTERNAL (decl))
{
tree init = DECL_INITIAL (decl);
...
if (init && init != error_mark_node)
{
if (!TREE_STATIC (decl))
{
DECL_IS_INITIALIZED(decl) = 1;
}
Is this enough for all Frontends? Are there other places that I need
to maintain this bit?
Do you have any comment and suggestions?
As said above - do you want to cover registers as well as locals?
All the locals from the source-code point of view should be covered.
(From my study so far, looks like that Clang adds that phase in FE).
If GCC adds this phase in FE, then the following design requirement
C. The implementation needs to keep the current static warning on
uninitialized
variables untouched in order to avoid "forking the language”.
cannot be satisfied. Since gcc’s uninitialized variables analysis is
applied quite late.
So, we have to add this new phase after “pass_late_warn_uninitialized”.
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)
Adding this new transformation during RTL expansion is okay. I will
check on this in more details to see how to add it to RTL expansion
phase.
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.
This is a really good point…
In order to avoid optimization to use the “uninitialized” state of
locals, we should add the zeroing phase as early as possible (adding
it in FE might be best
for this issue). However, if we have to met the following requirement:
C. The implementation needs to keep the current static warning on
uninitialized
variables untouched in order to avoid "forking the language”.
We have to move the new phase after all the uninitialized analysis is
done in order to avoid “forking the language”.
So, this is a problem that is not easy to resolve.
Do you have suggestion on this?
Not having thought about it very long or hard I'd be tempted to do
it the other way around. For each use of an uninitialized variable
found, first either issue or queue up a -Wuninitialized for it and
then initialize it. Then (if queued) at some later point, issue
the queued up -Wuninitialized. The last part would be done in
tree-ssa-uninit.c where the remaining uses of uninitialized
variables would trigger warnings and induce their initialization
(if there were any left).
The major issue with this approach is:
There are two passes for uninitialized variable analysis:
pass_early_warn_uninitialized
pass_late_warn_uninitialized
The early pass is placed at the very beginning of the tree optimizer.
But the late pass is placed at the very late stage of the tree optimizer.
If we add the initializations at the early pass, the result of the late
pass will be changed by the new added initializations. This does not meet
the requirement.
Do I miss anything here?
I'm not sure. As I said, I'd consider issuing (or queuing up for
issuing later) -Wuninitialized at the same time as initializing
the uninitialized variables. With that approach I'd expect to
diagnose all the same instances of uninitialized uses as the two
passes do today (actually, I'd expect to diagnose more of them,
including those Richard referred to above whose uninitialized
state may have been made use of for optimization decisions(*)).
Also with this approach the two existing warning passes would
cease to serve their current purpose of hunting down uninitialized
variables because by the time they ran all their uses would have
been initialized (and warnings issued).
One question in my mind is what to do with -Wmaybe-uninitialized.
Should those also be initialized, even though they're not necessarily
used? Or are you only hoping to tackle -Wuninitialized?
Martin
[*] With the initialization approach I'd expect concerns about
the cost of losing those optimization opportunities. Although
those could be addressed by making the initialization optional
(i.e., opt-in).
