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<div class="moz-cite-prefix">On 09/12/2016 08:51 AM, vivek pandya
via llvm-dev wrote:<br>
</div>
<blockquote
cite="mid:CAHYgpoLt9q5VfJHK9NNubwQOTyY7uvFfpVPk10oPVtdLRc3O1Q@mail.gmail.com"
type="cite">
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<div>Hello Developers,<br>
</div>
<div><br>
</div>
<div>I am working with my other batchmates to improve register
remat in LLVM.</div>
<div>We want to remat live ranges made of multiple instruction.</div>
<div><br>
</div>
<div>Just to support our proposal here is a simple example that
currently remat does</div>
<div>not cover</div>
<div><br>
</div>
<div>$ cat ~/tmp/tl.c</div>
<div>void foo(long);</div>
<div>void bar() {</div>
<div> for (int i = 0; i < 1600; ++i)</div>
<div> foo(3494348345984503943);</div>
<div>}</div>
<div><br>
</div>
<div>$ clang -O3 -S -o - ~/tmp/tl.c -target powerpc64</div>
<div>...</div>
<div># BB#0: # %entry</div>
<div>...</div>
<div> lis 3, 12414</div>
<div> ori 3, 3, 27470</div>
<div> sldi 3, 3, 32</div>
<div> oris 3, 3, 35809</div>
<div> ori 30, 3, 20615</div>
<div>...</div>
<div>.LBB0_1: # %for.body</div>
<div> mr 3, 30</div>
<div> bl foo</div>
<div>...</div>
</div>
</blockquote>
OT: Instead of viewing this as a *single* remat problem, can you
view this as a series of remat problems? If each instruction can be
moved while only increasing the live range of a single register and
shrinking the live range of another, then moving each instruction
one by one and iterating gives the same effect. Note that this only
(obviously) works for single use defs. Extending it to multiple
uses would require a more complicated cost model as you point out.
<br>
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cite="mid:CAHYgpoLt9q5VfJHK9NNubwQOTyY7uvFfpVPk10oPVtdLRc3O1Q@mail.gmail.com"
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<div><br>
</div>
<div>There is a sequence of instructions used to materialize the
constant, the first </div>
<div>one (the lis) is trivially rematerialiable, and the others
depend only on that one,</div>
<div>and have no side effects. If we otherwise needed to spill
the constant, we might</div>
<div>wish to move the entire set of instructions that compute
the value into the loop body.</div>
<div>(Many thanks to Hal Finkel for this example and head start)</div>
<div><br>
</div>
<div>We are following very old but effective paper
"Rematerialization"</div>
<div><a moz-do-not-send="true"
href="http://dl.acm.org/citation.cfm?id=143143">http://dl.acm.org/citation.cfm?id=143143</a>
------------------------------[1]</div>
<div><br>
</div>
<div>This extension will specially improve code quality for RICS
backends like </div>
<div>powerpc, MIPS, ARM, AArch64 etc.</div>
<div><br>
</div>
<div>Here is a tentative apporach ( after reading the above
mentioned paper and current remat code) that I would like to
follow.</div>
<div><br>
</div>
<div>Please share your views because this may be totally wrong
direction. Also I will</div>
<div>be happy if this gets into main line LLVM code but if
community don't want</div>
<div>to make remat heavy than please guide me for my class
project perspective.</div>
<div><br>
</div>
<div>1 ) As LLVM MI is already in SSA form before reg allocation
so for LLVM I think it does not require to build SSA graph and
converting it back after optimization completed as mentioned
in [1]</div>
<div><br>
</div>
<div>2 ) We would like to add a pass similar to SCCP.cpp (Sparse
Conditional Constant</div>
<div>Propagation based on Wegman and Zadeck's work <a
moz-do-not-send="true"
href="http://dl.acm.org/citation.cfm?id=103136">http://dl.acm.org/citation.cfm?id=103136</a>)
as desribed in [1]. This pass will be scheduled to run before
register allocation.</div>
<div><br>
</div>
<div>3 ) Output of the pass added in Step 2 will be a Map of def
to instructions pointers (instructions which can be used to
remat the given live range). The map will contain live ranges
which is due to single instruction and multiple instructions.</div>
</div>
</blockquote>
This sounds overly complex. Can you implement this without needing
the new side structure? Maintaining extra state and keeping it up
to date is expensive. (From a maintenance and code complexity
perspective.)<br>
<blockquote
cite="mid:CAHYgpoLt9q5VfJHK9NNubwQOTyY7uvFfpVPk10oPVtdLRc3O1Q@mail.gmail.com"
type="cite">
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<div><br>
</div>
<div>4 ) The remat APIs defined in LiveRangeEdit.cpp will use
analysis from the Map</div>
<div>when a spill is required for RA.</div>
<div><br>
</div>
<div>5 ) The remat transformation APIs like rematerializeAt()
will be teached to remat</div>
<div>live ranges with multiple instructions too.</div>
<div><br>
</div>
<div>6 ) A cost analysis will be require to decide between remat
and spill. This should be based on at least two factors
register pressure and spill cost</div>
<div><br>
</div>
<div>Few points:</div>
<div>--------------</div>
<div>* The analysis pass to be addes as per (2) will use target
specific information</div>
<div>from TargetInstrInfo.cpp as the current remat
infrastructure uses.</div>
<div><br>
</div>
<div>* This approach will not be on demand as the current
approach is (i.e remat specific</div>
<div>code will be executed only if there is a spill) so the pass
in (2) can be an</div>
<div>overhead so we may want it to enable only for higher level
of optimization.</div>
</div>
</blockquote>
This would be unfortunate. Not fatal, just unfortunate.<br>
<blockquote
cite="mid:CAHYgpoLt9q5VfJHK9NNubwQOTyY7uvFfpVPk10oPVtdLRc3O1Q@mail.gmail.com"
type="cite">
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<div><br>
</div>
<div>* Will it be possible to use existing SCCP.cpp code with
few modification to lattice</div>
<div>and related mathematical operation so that it can serve
both purpose?</div>
<div><br>
</div>
<div>* No changes in current register allocators or spill
framework will be required</div>
<div>because remat entry point will be LiveRangeEdit.</div>
<div><br>
</div>
<div>Any other way with less overhead is always welcomed.</div>
<div>Please help us developing a plan to implement this.</div>
<div><br>
</div>
<div>Hoping for comments!</div>
<div><br>
</div>
<div>Sincerely,</div>
<div>Vivek</div>
<div><br>
</div>
</div>
<br>
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