On 6/28/21 12:17 AM, Aldy Hernandez wrote:
On 6/25/21 7:19 PM, Martin Sebor wrote:
On 6/25/21 10:20 AM, Aldy Hernandez via Gcc wrote:
Hi folks.
I'm done with benchmarking, testing and cleanups, so I'd like to post
my patchset for review. However, before doing so, I'd like to
address a handful of meta-issues that may affect how I post these
patches.
Trapping on differences
=======================
Originally I wanted to contribute verification code that would trap
if the legacy code threaded any edges the new code couldn't (to be
removed after a week). However, after having tested on various
architectures and only running once into a missing thread, I'm
leaning towards omitting the verification code, since it's fragile,
time consuming, and quite hacky.
For the record, I have tested on x86-64, aarch64, ppc64 and ppc64le.
There is only one case, across bootstrap and regression tests where
the verification code is ever tripped (discussed below).
Performance
===========
I re-ran benchmarks as per our callgrind suite, and the penalty with
the current pipeline is 1.55% of overall compilation time. As is
being discussed, we should be able to mitigate this significantly by
removing other threading passes.
Failing testcases
=================
I have yet to run into incorrect code being generated, but I have had
to tweak a considerable number of tests. I have verified every
single discrepancy and documented my changes in the testsuite when it
merited doing so. However, there are a couple tests that trigger
regressions and I'd like to ask for guidance on how to address them.
1. gcc.c-torture/compile/pr83510.c
I would like to XFAIL this.
What happens here is that thread1 threads a switch statement such
that the various cases have been split into different independent
blocks. One of these blocks exposes an arr[i_27] access which is
later propagated by VRP to be arr[10]. This is an invalid access,
but the array bounds code doesn't know it is an unreachable path.
The test has a bunch of loops that iterate over the 10 array elements.
There have been bug reports about loop unrolling causing false positives
-Warray-bounds (e.g., PR 92539, 92110, or 86341) so this could be
the same issue.
However, it is not until dom2 that we "know" that the value of the
switch index is such that the path to arr[10] is unreachable. For
that matter, it is not until dom3 that we remove the unreachable path.
If you do XFAIL it can you please isolate a small test case and open
a bug and make it a -Warray-bounds blocker?
Will do.
2. -Wfree-nonheap-object
This warning is triggered while cleaning up an auto_vec. I see that
the va_heap::release() inline is wrapped with a pragma ignore
"-Wfree-nonheap-object", but this is not sufficient because jump
threading may alter uses in such a way that may_emit_free_warning()
will warn on the *inlined* location, thus bypassing the pragma.
I worked around this with a mere:
> @@ -13839,6 +13839,7 @@ maybe_emit_free_warning (tree exp)
location_t loc = tree_inlined_location (exp);
+ loc = EXPR_LOCATION (exp);
but this causes a ton of Wfree-nonheap* tests to fail. I think
someone more knowledgeable should address this (msebor??).
This sounds like the same problem as PR 98871. Does the patch below
fix it?
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572515.html
If so, I suggest getting that patch in first to avoid testsuite
failures. If it doesn't fix it I'll look into it before you commit
your changes.
The latest patch in the above thread does not apply. Do you have a more
recent patch?
The first patch in the series applies cleanly for me:
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572516.html
but patch 2/4 does need a few adjustments. With those and your
latest changes applied I don't see any -Wfree-nonheap-object test
failures.
Martin
3. uninit-pred-9_b.c
The uninit code is getting confused with the threading and the bogus
warning in line 24 is back. I looked at the thread, and it is correct.
I'm afraid all these warnings are quite fragile in the presence of
more aggressive optimizations, and I suspect it will only get worse.
From my recent review of open -Wmaybe-uninitialized bugs (and
the code) it does seem to be both fragile and getting worse. I've
only found a few simple problems so far in the code but nothing that
would make a dramatic difference so I can't say if it's possible to
do much better, but I'm not done or ready to give up. If you XFAIL
this too please open a bug for it and make it a blocker for
-Wuninitialized?
Will do.
Good to know these are somewhat known issues.
Thanks.
Aldy
