<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Thu, May 28, 2015 at 6:25 PM, Nick Kledzik <span dir="ltr"><<a href="mailto:kledzik@apple.com" target="_blank">kledzik@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><span class=""><div>On May 28, 2015, at 5:42 PM, Sean Silva <<a href="mailto:chisophugis@gmail.com" target="_blank">chisophugis@gmail.com</a>> wrote:</div><br><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>I guess, looking back at Nick's comment:</div><div><br></div><div>"<span style="font-size:12.8000001907349px">The atom model is a good fit for the llvm compiler model for all architectures. There is a one-to-one mapping between llvm::GlobalObject (e.g. function or global variable) and lld:DefinedAtom."</span></div><div><span style="font-size:12.8000001907349px"><br></span></div><div><span style="font-size:12.8000001907349px">it seems that the primary issue on the ELF/COFF side is that currently the LLVM backends are taking a finer-grained atomicity that is present inside LLVM, and losing information by converting that to a coarser-grained atomicity that is the typical "section" in ELF/COFF.</span></div><div><span style="font-size:12.8000001907349px">But doesn't -ffunction-sections -fdata-sections already fix this, basically?</span></div><div><span style="font-size:12.8000001907349px"><br></span></div><div><span style="font-size:12.8000001907349px">On the Mach-O side, the issue seems to be that Mach-O's notion of section carries more hard-coded meaning than e.g. ELF, so at the very least another layer of subdivision below what Mach-O calls "section" would be needed to preserve this information; currently symbols are used as a bit of a hack as this "sub-section" layer.</span></div></div></div></div></blockquote></span><div>I’m not sure what you mean here.</div><span class=""><br><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><span style="font-size:12.8000001907349px"><br></span></div><div><span style="font-size:12.8000001907349px">So the problem seems to be that the transport format between the compiler and linker varies by platform, and each one has a different way to represent things, some can't represent everything we want to do, apparently.</span></div></div></div></div></blockquote></span><div>Yes!</div><span class=""><div><br></div><br><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><span style="font-size:12.8000001907349px">BUT it sounds like at least relocatable ELF semantics can, in principle, represent everything that we can imagine an "atom-based file format"/"native format" to want to represent. </span><span style="font-size:12.8000001907349px">Just to play devil's advocate here, let's start out with the "native format" being relocatable ELF - on *all platforms*. Relocatable object files are just a transport format between compiler and linker, after all; who cares what we use? If the alternative is a completely new format, then bootstrapping from relocatable ELF is strictly less churn/tooling cost.</span></div><div><br></div><div><span style="font-size:12.8000001907349px">People on the "atom side of the fence", what do you think? Is there anything that we cannot achieve by saying "native"="relocatable ELF"?</span><span style="font-size:12.8000001907349px"><br></span></div></div></div></div></blockquote></span></div><div>1) Turns out .o files are written once but read many times by the linker. Therefore, the design goal of .o files should be that they are as fast to read/parse in the linker as possible. Slowing down the compiler to make a .o file that is faster for the linker to read is a good trade off. This is the motivation for the native format - not that it is a universal format.</div></div></blockquote><div><br class="">I don't think that switching from ELF to something new can make linkers significantly faster. We need to handle ELF files carefully not to waste time on initial load, but if you do, reading data required for symbol resolution from ELF file should be satisfactory fast (I did that for COFF -- the current "atom-based ELF" linker is doing too much things in an initial load, like read all relocation tables, splitting indivisble chunk of data and connect them with "indivisible" edges, etc.) Looks like we read symbol table pretty quickly in the new implementation, and the bottleneck of it is now the time to insert symbols into the symbol hash table -- which you cannot make faster by changing object file format.</div><div><br></div><div>Speaking of the performance, if I want to make a significant difference, I'd focus on introducing new symbol resolution semantics. Especially, the Unix linker semantics is pretty bad for performance because we have to visit files one by one serially and possibly repeatedly. It's not only bad for parallelism but also for a single-thread case because it increase size of data to be processed. This is I believe the true bottleneck of Unix linkers. Tackling that problem seems to be most important to me, and "ELF as a file format is slow" is still an unproved thing to me.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br></div><div>2) I think the ELF camp still thinks that linkers are “dumb”. That they just collate .o files into executable files. The darwin linker does a lot of processing/optimizing the content (e.g. Objective-C optimizing, dead stripping, function/data re-ordering). This is why atom level granularity is needed.</div></div></blockquote><div><br></div><div>I think that all these things are doable (and are being done) using -ffunction-sections.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br></div><div>For darwin, ELF based .o files is not interesting. It won’t be faster, and it will take a bunch of effort to figure out how to encode all the mach-o info into ELF. We’d rather wait for a new native format.</div></div></blockquote><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><span class=""><font color="#888888"><div><br></div><div>-Nick</div><div><br></div></font></span></div><br>_______________________________________________<br>
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