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<div class="moz-cite-prefix">Improving lowering for memcmp is
definitely something we should do for all targets. Doing it in a
target specific way is decidedly non-ideal. <br>
<br>
It looks like we already have some code in SelectionDAGBuilder
which tries to optimize the lowering for the memcpy library call.
I am a bit confused by the problem you are trying to solve. Are
you specifically interested in lowering for constant lengths
greater than a legal size? (i.e. do you need the loop?)<br>
<br>
If so, there are two approaches you might consider:<br>
- Expand the memcmp call into the loop form in CodeGenPrep (or a
similar timed pass) where working with multiple basic blocks is
much easier. Long term, the "right place" for this type of thing
is clearly GlobalISEL, but we have a number of other such hacks in
lowering today and continuing to build off of that seems
reasonable.<br>
- Emit the non-early exit form for small constant values (a[0] ==
b[0] && a[1] == b[1] ...). Assuming your backend has
handling for efficiently lowering and chains using branches, you
may very well get the code you want. <br>
<br>
Using the psuedo instruction here feels messy. In particular, I
don't like the fact it basically opts out of all of the combines
which might further improve the lowering.<br>
<br>
Philip<br>
<br>
<br>
On 12/29/2016 11:35 AM, Zaara Syeda via llvm-dev wrote:<br>
</div>
<blockquote
cite="mid:OFAE6A7273.25B9B1C3-ON85258098.006B368C-85258098.006BA763@notes.na.collabserv.com"
type="cite">
<p><font face="Calibri">Currently on PowerPC, calls to memcmp are
not expanded and are left as library calls. In certain
conditions, expansion can improve performance rather than
calling the library function as done for functions like
memcpy, memmove, etc. This patch </font><b><font
face="Calibri">(</font></b><a moz-do-not-send="true"
href="https://reviews.llvm.org/D28163"><b><font face="Calibri">https://reviews.llvm.org/D28163</font></b></a><b><font
face="Calibri">)</font></b><font face="Calibri"> is an
initial implementation for PowerPC to expand memcmp when the
size is an 8 byte multiple.</font></p>
<p><font face="Calibri">The approach currently added for this
expansion tries to use the existing infrastructure by
overriding the virtual function EmitTargetCodeForMemcmp. This
function works on the SelectionDAG, but the expansion requires
control flow for early exit. So, instead of implementing the
expansion within EmitTargetCodeForMemcmp, a new pseudo
instruction is added for memcmp and a SelectionDAG node for
this new pseudo is created in EmitTargetCodeForMemcmp. This
pseudo instruction is then expanded during lowering in
EmitInstrWithCustomInserter.<br>
<br>
The advantage of this approach is that it uses the existing
infrastructure and does not impact other targets. If other
targets would like to expand memcmp, they can also override
EmitTargetCodeForMemcmp and create their own expansion. </font></p>
<p><font face="Calibri">Another option to consider is adding a new
optimization pass for this expansion that isn’t target
specific if other targets would benefit from a more general
infrastructure. </font></p>
<p><font face="Calibri">Please provide feedback if this approach
should be continued to implement the PowerPC specific memcmp
expansions or whether the community is interested in devising
a more general approach</font><font face="Times New Roman">.<br>
<br>
Thanks,</font></p>
<p><font face="Times New Roman">Zaara Syeda</font><br>
<br>
</p>
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