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<div class="moz-cite-prefix">On 10/03/2017 04:30 PM, Eric
Christopher wrote:<br>
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<div dir="ltr">On Tue, Oct 3, 2017 at 8:54 AM Hal Finkel via
llvm-dev <<a moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>>
wrote:<br>
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<div class="m_3858040240877242542moz-cite-prefix">On
10/02/2017 10:57 PM, Matthias Braun via llvm-dev wrote:<br>
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<blockquote type="cite"> The distinction between the
LLVMTargetMachine and TargetMachine classes has become
somewhat muddy recently. So I created:
<div><br>
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<div><a moz-do-not-send="true"
href="https://reviews.llvm.org/D38482"
target="_blank">https://reviews.llvm.org/D38482</a></div>
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<div>to clean things up. During review it was noted that
we may rather merge the two instead which looks like
this:</div>
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<div><a moz-do-not-send="true"
href="https://reviews.llvm.org/D38489"
target="_blank">https://reviews.llvm.org/D38489</a></div>
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<div>We really should choose one of the two over the
status quo. Some points for the discussion:</div>
<div><br>
</div>
<div>- I am not aware of any target that is actually
implementing TargetMachine only but not
LLVMTargetMachine (I assume the C backend used to do
it before it was removed)</div>
<div>- Even when merging the two, I believe it is still
possible to implement a target without linking to
lib/CodeGen by returning nullptr for the various
methods related to CodeGen.</div>
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<div bgcolor="#FFFFFF" text="#000000"> If this is true, then
it seems like we don't lose anything from doing this (and
the code gets cleaner). We don't currently have anything
in tree that </div>
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<div>This was my suggestion of yesterday.</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0 0 0
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<div bgcolor="#FFFFFF" text="#000000">motivates the current
split. It does seem useful to retain the ability to have a
direct-from-the-IR backend (mostly for translation into
another IR).<br>
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<div>I'm dubious of this need, but as long as it doesn't add
any overhead to the resultant code I'm good.</div>
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<br>
The other thing that I asked in the other review thread, which I'll
repeat here, is: does GlobalISel further reduce the motivation for
this? The primary reason, as I understand it, for needing this kind
of "direct" translation is to avoid going through type legalization.
This can be important for software targeting FPGAs, for example,
because the hardware can deal with arbitrary bit widths. With
GlobalISel, can you go into CodeGen effectively without any
legalization and then go on from there (if one needed such a thing)?<br>
<br>
The other motivation used to be to avoid register allocation, thus
keeping the infinite set of virtual registers, but we've already
addressed that issue.<br>
<br>
-Hal<br>
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<div>-eric</div>
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<div bgcolor="#FFFFFF" text="#000000"> -Hal</div>
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<div>- The split would give some notion of an internal
CodeGen interface and an external interface visible to
frontend/middleend etc.</div>
<div>- The code gets simpler when merging the two and we
have to document/explain less.</div>
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<div>- Matthias</div>
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<pre class="m_3858040240877242542moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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<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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