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<p class="MsoNormal">If I’m following correctly, there are two size-limited areas. One area, limited to 2GB, is the “text” area. This contains all the code. Then there’s a “global” area, limited to 4GB, which is pointed to by the global pointer. This contains
the GOT, plus a flexible area that the object file can stick small bits of data into. And then outside of both of those, additional data is unlimited.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It took me multiple times reading through the proposal to parse that out; it might be a good idea to reorganize the proposal so that’s explained somewhere explicitly.<o:p></o:p></p>
<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal">My big question here is, how much benefit are you really getting from having a global pointer? If you eliminate it and combine the two size-limited areas into one, you end up with essentially the small-PIC code model. The small-PIC code
model supports everything your proposed “compact” model does, with a couple minor differences:<o:p></o:p></p>
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<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2">The size-limited areas are limited to 2GB combined, instead of 6GB combined.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo2">The relaxations are a little different. Small-PIC always takes two instructions to access a GOT entry; the compact model can do it in one instruction for the first 500 (?) entries
in the GOT. Not sure what would end up with smaller codesize in practice.<o:p></o:p></li></ol>
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<p class="MsoNormal">-Eli<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Evandro Menezes via llvm-dev<br>
<b>Sent:</b> Thursday, March 19, 2020 9:19 AM<br>
<b>To:</b> Alex Bradbury <asb@lowrisc.org><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] Re: [llvm-dev] RISC-V LLVM sync-up call 19 Mar 2020<o:p></o:p></p>
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<p class="MsoNormal">Here's the draft proposal for the compact code model on RV. I'd appreciate your feedback before I propose it to the foundation and go about updating the psABI.
<o:p></o:p></p>
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<p class="MsoNormal">Thank you,<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:black">__ <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:black">Evandro Menezes ◊ SiFive ◊ Austin, TX<o:p></o:p></span></p>
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<p class="MsoNormal">On Mar 19, 2020, at 8:58, Alex Bradbury via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">For background on these calls, see<br>
<<a href="http://lists.llvm.org/pipermail/llvm-dev/2019-September/135087.html">http://lists.llvm.org/pipermail/llvm-dev/2019-September/135087.html</a>>.<br>
<br>
Reminder: the purpose is to co-ordinate between active contributors.<br>
If you have support questions etc then it's best to post to llvm-dev.<br>
<br>
We have a call each Thursday at 4pm GMT, via<br>
<<a href="https://meet.google.com/ske-zcog-spp">https://meet.google.com/ske-zcog-spp</a>>.<br>
<br>
I've created a shared calendar which may help in keeping track, which<br>
is accessible at:<br>
* <<a href="https://calendar.google.com/calendar/b/1?cid=bG93cmlzYy5vcmdfMG41cGtlc2ZqY25wMGJoNWhwczFwMGJkODBAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ">https://calendar.google.com/calendar/b/1?cid=bG93cmlzYy5vcmdfMG41cGtlc2ZqY25wMGJoNWhwczFwMGJkODBAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ</a>><br>
* <<a href="https://calendar.google.com/calendar/ical/lowrisc.org_0n5pkesfjcnp0bh5hps1p0bd80%40group.calendar.google.com/public/basic.ics">https://calendar.google.com/calendar/ical/lowrisc.org_0n5pkesfjcnp0bh5hps1p0bd80%40group.calendar.google.com/public/basic.ics</a>><br>
<br>
Issues to discuss today include the following:<br>
* Improving rust code size by not forcing frame pointers<br>
<<a href="https://github.com/rust-lang/rust/pull/69890">https://github.com/rust-lang/rust/pull/69890</a>><br>
* Compact code model (Evandro)<br>
* Update on embedded PIC discussions<br>
* Small data limit <<a href="https://reviews.llvm.org/D57497">https://reviews.llvm.org/D57497</a>><br>
* Bitmanip / experimental extension status<br>
* ELF attribute support close to merging<br>
<<a href="https://reviews.llvm.org/D75833">https://reviews.llvm.org/D75833</a>> <<a href="https://reviews.llvm.org/D74023">https://reviews.llvm.org/D74023</a>><br>
* No other topics were submitted, as always, please do submit things<br>
you'd like to discuss<br>
<br>
Best,<br>
<br>
Alex<br>
_______________________________________________<br>
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