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?
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.
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?
Martin
4. libphobos/src/std/net/isemail.d
This is a D test where we don't actually fail, but we trigger the
verification code. It is the only jump threading edge that the new code
fails to get over the old code, and it only happens on ppc64.
It triggers because a BB4 -> BB5 is too expensive to thread, but a BBn
-> BB3 -> BB4 -> BB5 is considered safe to thread because BB3 is a latch
and it alters the profitability equation. The reason we don't get it,
is that we assume that if a X->Y is unprofitable, it is not worth
looking at W->X->Y and so forth.
Jeff had some fancy ideas on how to attack this. Once such idea was to
stop looking back, but only for things we were absolutely sure would
never yield a profitable path. I tried a subset of this, by allowing
further looks on this latch test, but my 1.55% overall performance
penalty turned into an 8.33% penalty. Personally it looks way too
expensive for this one isolated case. Besides, the test where this
clamping code originally came from still succeeds (commit
eab2541b860c48203115ac6dca3284e982015d2c).
CONCLUSION
==========
That's basically it.
If we agree the above things are not big issues, or can be addressed as
follow-ups, I'd like to start the ball rolling on the new threader. This
would allow more extensive testing of the code, and separate it a bit
from the other big changes coming up :).
Aldy
