On 6/28/21 10:22 AM, Richard Biener wrote:
On Fri, Jun 25, 2021 at 6:20 PM Aldy Hernandez <al...@redhat.com> 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.
First of all thanks for the detailed analysis and report!
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.
Agreed.
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.
1.55% for the overall compilation looks quite large - is this suite particularly
demanding on VRP/ranger? Is the figure for 'cc1files' similar? (collect
preprocessed sources of gcc/*.{c,cc} compiles, like by appending
-save-temps to CFLAGS in a non-bootstrap build)
The figures are for cc1 files. They are .ii files we run under
callgrind and compare "instruction counts" as per the tool. We've found
it's comparable to -ftime-report, though not exactly. We've been using
this for benchmarking, since evrp times were so small that it was hard
to compare user times.
I'm curious if you tried to look at the worst offenders and if there's
anything new, threading specific. More threading might result in
larger code (how is cc1 size affected by the change?) and more
code to compile by later passes can easily explain >1% differences.
A while ago, I did look at worst offenders on the branch, but there was
nothing obvious. As it stands now, the average cost for the threading
pass is 858%, with 95% of .ii files falling under 1800%. I could look
at the outliers which seem to be c-fold, gcc-ar, gcc-ranlib, gcc-nm, and
c-opts.
cc1 on x86-64 seems to be 0.135% larger for an --enable-checking build.
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.
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.
Sounds reasonable.
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??).
That looks wrong, but see msebors response for maybe some better answer.
Oh, it was just a stop-gap. I have instead opted for the following
temporary measure in Makefile.in:
tree-ssa-loop-im.o-warn = -Wno-free-nonheap-object
I will work with Martin on a more appropriate solution.
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.
Yep. Shouldn't be a blocker, just XFAIL ...
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).
ick - I definitely expect some corner case optimization "regressions"
caused by the change if and only because it might trigger pass
ordering issues with later passes not expecting "optimized" code.
So I'd disregard a single "missed" case for this moment. It might
be interesting to try to distill a C testcase from the case and file it
with bugzilla for reference purposes.
Assuming there's nothing inherently D-ish in the testcase, I can
certainly do this.
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 :).
I'd say go ahead (with the bogus change somehow resolved), I'd still
like to know sth about the compile-time issue but then we can eventually
deal with this as followup as well. If size is the reason we can see taming
down the threader via some --param adjustments for example.
I'm just trying to make sure we're not introducing some algorithmic
quadraticnesses that were not there before.
Agreed. FWIW, there are already various --param knobs in the backwards
threader we could tweak, though in practice the truly problematic cases
we chop right off the bat, as soon as we go down an unprofitable path.
Thanks.
Aldy
