<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Aug 15, 2018, at 10:57 PM, Hal Finkel via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
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<br class="">
<div class="moz-cite-prefix">On 08/15/2018 02:38 PM, Philip Reames
via llvm-dev wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:7b7d457d-dbf4-d64b-428d-7e47f84c32ef@philipreames.com" class="">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" class=""><p class="">I'm concerned that we're focusing on one side of this. Let me
point out a few concerns w/changing the canonical form here:</p>
<ol class="">
<li class="">LICM does not know how to hoist or sink regions. It does
know how to hoist and sink selects.</li></ol></blockquote></div></div></blockquote>I think we should teach LICM to do this eventually.<br class=""><blockquote type="cite" class=""><div class=""><div text="#000000" bgcolor="#FFFFFF" class=""><blockquote type="cite" cite="mid:7b7d457d-dbf4-d64b-428d-7e47f84c32ef@philipreames.com" class=""><ol class="" start="2">
<li class="">InstCombine has limited support for triangles/diamonds, but
fairly extensive support for selects.</li>
<li class="">EarlyCSE and GVN do not know how to eliminate fully
redundant triangles/diamonds. PRE *may* get some of these
cases, but that is by no means guaranteed or likely to be
robust.</li></ol></blockquote></div></div></blockquote>Agreed, we’ll need a plan to deal with these issues.<br class=""><blockquote type="cite" class=""><div class=""><div text="#000000" bgcolor="#FFFFFF" class=""><blockquote type="cite" cite="mid:7b7d457d-dbf4-d64b-428d-7e47f84c32ef@philipreames.com" class=""><ol class="" start="2">
</ol><p class="">My personal opinion is that selects are the appropriate
canonical form. </p><p class="">For the one of the specific cases mentioned, teaching GVN to do
FRE and PRE for loads from selects of pointers just doesn't seem
that painful. I would be really tempted to do that instead.
Similarly, walking facts back from select uses in CVP seems
generally useful since we have use dependent facts in a bunch of
contexts, not just selects. (Call arguments for instance,
non-null from unconditional deref, etc..)<br class="">
</p><p class="">To be clear, I am raising concerns, not actively objecting to
this. If you want to move forward and commit to work through
all of the issues identified I wont actively stand in the way.</p>
</blockquote>
<br class="">
As I've expressed in the past, I think that not using select as part
of our canonical form is potentially a superior design. However, it
would be a major change. In addition to the issues that Philip
mentions, there's also the fact that we'll just have more basic
blocks and that, in itself, could lead to an increase in compile
time. However, working through these issues will likely leave us
with a more-robust optimizer.<br class="">
<br class="">
See also: <a href="https://bugs.llvm.org/show_bug.cgi?id=34603#c19" class="">https://bugs.llvm.org/show_bug.cgi?id=34603#c19</a><br class=""></div></div></blockquote>Canonicalizing to phis is my preference too.<br class=""><blockquote type="cite" class=""><div class=""><div text="#000000" bgcolor="#FFFFFF" class="">
<br class="">
-Hal<br class="">
<br class="">
<blockquote type="cite" cite="mid:7b7d457d-dbf4-d64b-428d-7e47f84c32ef@philipreames.com" class=""><p class="">Philip<br class="">
</p>
<br class="">
<div class="moz-cite-prefix">On 08/14/2018 12:39 PM, Sanjay Patel
via llvm-dev wrote:<br class="">
</div>
<blockquote type="cite" cite="mid:CA+wODitU1d+Fme24EhhwNkX6tcQzpiRnrKCpeej11ELgt5heig@mail.gmail.com" class="">
<div dir="ltr" class="">
<div class="">I didn't look closely at the new patch, but this appears
to be a small extension to:</div>
<div class=""><a href="https://reviews.llvm.org/D38566" moz-do-not-send="true" class="">https://reviews.llvm.org/D38566</a></div>
<div class="">...and the GVN-based reasons for delaying transformation
to 'select' are discussed in detail in the motivating bug
for that patch:</div>
<div class=""><a href="https://bugs.llvm.org/show_bug.cgi?id=34603" moz-do-not-send="true" class="">https://bugs.llvm.org/show_bug.cgi?id=34603</a></div>
<div class=""><br class="">
</div>
<div class="">So this sounds like the right direction to me. Note that
there was objection to the implementation (a pile of pass
options vs. uniquely named passes).<br class="">
</div>
<div class=""><br class="">
</div>
<div class="">Here's another motivating bug where early transform to
select prevents optimization:</div>
<div class=""><a href="https://bugs.llvm.org/show_bug.cgi?id=36760" moz-do-not-send="true" class="">https://bugs.llvm.org/show_bug.cgi?id=36760</a><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">Is that case affected by this patch?<br class="">
</div>
<div class=""><br class="">
</div>
</div>
<div class="gmail_extra"><br class="">
<div class="gmail_quote">On Tue, Aug 14, 2018 at 11:17 AM,
John Brawn via llvm-dev <span dir="ltr" class=""><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank" moz-do-not-send="true" class="">llvm-dev@lists.llvm.org</a>></span>
wrote:<br class="">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Summary<br class="">
=======<br class="">
<br class="">
I'm planning on adjusting SimplifyCFG so that it doesn't
turn two-entry phi<br class="">
nodes into selects until later in the pass pipeline, to
give passes which can<br class="">
understand phis but not selects more opportunity to
optimize. The thing I'm<br class="">
trying to do which made me think of doing this is
described below, but from the<br class="">
benchmarking I've done it looks like this is overall a
good idea regardless of<br class="">
if I manage to get that done or not.<br class="">
<br class="">
Motivation<br class="">
==========<br class="">
<br class="">
My goal is to get clang to optimize some code containing a
call to<br class="">
std::min_element which is dereferenced, so something like:<br class="">
<br class="">
float min_element_example(float *data, int size)<br class="">
{<br class="">
return *std::min_element(data, data+size);<br class="">
}<br class="">
<br class="">
which, after inlining a specialization, looks like:<br class="">
<br class="">
float min_element_example_inlined(<wbr class="">float *first,
float *last)<br class="">
{<br class="">
for (float *p = first; p != last; ++p)<br class="">
{<br class="">
if (*p < *first)<br class="">
first = p;<br class="">
}<br class="">
return *first;<br class="">
}<br class="">
<br class="">
There are two loads in the loop, *p and *first, but
actually the load *p can be<br class="">
eliminated by using either the previous load *p or the
previous *first,<br class="">
depending on if the if-condition was taken or not. However
the<br class="">
"if (*p < *first) first = p" gets turned by simplifycfg
into a select and this<br class="">
makes optimizing this a lot harder because you no longer
have distinct paths<br class="">
through the CFG.<br class="">
<br class="">
I have some ideas on how to do the optimization (see my
previous RFC "Making GVN<br class="">
able to visit the same block more than once" posted in
April, though I've<br class="">
decided that the specific idea presented there isn't the
right way to do it),<br class="">
but I think the first step is to make sure we don't have a
select when we try<br class="">
to optimise this.<br class="">
<br class="">
Approach<br class="">
========<br class="">
<br class="">
I've posted a patch to <a href="https://reviews.llvm.org/D50723" rel="noreferrer" target="_blank" moz-do-not-send="true" class="">https://reviews.llvm.org/<wbr class="">D50723</a>
showing what I'm<br class="">
intending to do. An extra parameter is added to
SimplifyCFG to control whether<br class="">
two-entry phi nodes are converted into select, and this is
set to false in all<br class="">
instances before the end of module simplification. At the
end of module<br class="">
simplification we do SimplifyCFG, then Instcombine to
optimise the selects that<br class="">
are introduced, then EarlyCSE to eliminate common
subexpressions introduced by<br class="">
instcombine.<br class=""></blockquote></div></div></blockquote></blockquote></div></div></blockquote>Is scheduling another simplifycfg, instcombine + earlycse pass really necessary? I’m concerned about the compile time impact.</div><div><br class=""></div><div>Cheers,</div><div>Amara<br class=""><blockquote type="cite" class=""><div class=""><div text="#000000" bgcolor="#FFFFFF" class=""><blockquote type="cite" cite="mid:7b7d457d-dbf4-d64b-428d-7e47f84c32ef@philipreames.com" class=""><blockquote type="cite" cite="mid:CA+wODitU1d+Fme24EhhwNkX6tcQzpiRnrKCpeej11ELgt5heig@mail.gmail.com" class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<br class="">
Benchmark Results<br class="">
=================<br class="">
<br class="">
These are performance differences reported by LNT when
running llvm-test-suite,<br class="">
spec2000, and spec2006 at -O3 with and without the patch
linked above (using<br class="">
trunk llvm from a week or so ago).<br class="">
<br class="">
AArch64 results on ARM Cortex-A72:<br class="">
<br class="">
Performance Regressions - execution_time
Change<br class="">
SingleSource/Benchmarks/<wbr class="">Shootout/Shootout-ary3
9.48%<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">Packing-flt/Packing-flt
3.79%<br class="">
SingleSource/Benchmarks/<wbr class="">CoyoteBench/huffbench
1.40%<br class="">
<br class="">
Performance Improvements - execution_time
Change<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">Searching-dbl/Searching-dbl
-23.74%<br class="">
External/SPEC/CINT2000/256.<wbr class="">bzip2/256.bzip2
-9.82%<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">Searching-flt/Searching-flt
-9.57%<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">Equivalencing-flt/<wbr class="">Equivalencing-flt
-4.38%<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">LinearDependence-flt/<wbr class="">LinearDependence-flt
-3.94%<br class="">
MultiSource/Benchmarks/TSVC/<wbr class="">Packing-dbl/Packing-dbl
-3.44%<br class="">
External/SPEC/CFP2006/453.<wbr class="">povray/453.povray
-2.50%<br class="">
SingleSource/Benchmarks/Adobe-<wbr class="">C++/stepanov_vector
-1.49%<br class="">
<br class="">
X86_64 results on Intel Xeon E5-2690:<br class="">
<br class="">
Performance Regressions - execution_time Change<br class="">
MultiSource/Benchmarks/<wbr class="">Ptrdist/yacr2/yacr2
5.62%<br class="">
<br class="">
Performance Improvements - execution_time Change<br class="">
SingleSource/Benchmarks/Misc-<wbr class="">C++/Large/sphereflake
-4.43%<br class="">
External/SPEC/CINT2006/456.<wbr class="">hmmer/456.hmmer
-2.50%<br class="">
External/SPEC/CINT2006/464.<wbr class="">h264ref/464.h264ref
-1.60%<br class="">
MultiSource/Benchmarks/nbench/<wbr class="">nbench
-1.19%<br class="">
SingleSource/Benchmarks/Adobe-<wbr class="">C++/functionobjects
-1.07%<br class="">
<br class="">
I had a brief look at the regressions and they all look to
be caused by<br class="">
getting bad luck with branch mispredictions: I looked into
the Shootout-ary3 and<br class="">
yacr2 cases and in both the hot code path was the same
with and without the<br class="">
patch, but with more mispredictions probably caused by
changes elsewhere.<br class="">
<br class="">
John<br class="">
<br class="">
______________________________<wbr class="">_________________<br class="">
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</blockquote>
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<br class="">
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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