<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Jun 16, 2015 at 11:01 AM, Frédéric Riss <span dir="ltr"><<a href="mailto:friss@apple.com" target="_blank">friss@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><div><div class="h5"><blockquote type="cite"><div>On Jun 16, 2015, at 10:54 AM, David Blaikie <<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>> wrote:</div><br><div><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Jun 16, 2015 at 10:43 AM, Duncan P. N. Exon Smith <span dir="ltr"><<a href="mailto:dexonsmith@apple.com" target="_blank">dexonsmith@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span><br>
> On 2015-Jun-11, at 12:26, Frédéric Riss <<a href="mailto:friss@apple.com" target="_blank">friss@apple.com</a>> wrote:<br>
><br>
><br>
>> On Jun 11, 2015, at 12:19 PM, Rafael Espíndola <<a href="mailto:rafael.espindola@gmail.com" target="_blank">rafael.espindola@gmail.com</a>> wrote:<br>
>><br>
>>> When Duncan was working on this I was wondering if it couldn’t be generalized. I thought the ELF linkers relied on the relocations this would generate eg. for strings in the dwarf_str section. I guess all the strings will be in the same fragment. Without relocations, how can a standard ELF linker rewrite the references to the dwarf_str section when combining multiple input files?<br>
>><br>
>> The elf linker needs a relocation if one section refers to another. If<br>
>> two symbols are in different sections, then they are in different<br>
>> fragments.<br>
>><br>
>> So the case of dwarf_str, only if the two symbols are in dwarf_str<br>
>> would this function avoid creating the expression.<br>
><br>
> Sorry for the noise. I thought the logic that did a difference between the string symbol and the start of section symbol was common to all platforms. I see that ELF will emits directly the string symbol which makes it totally irrelevant to this patch :-)<br>
<br>
</span>What a mess. These functions' assumptions are pretty convoluted. I<br>
just spent some time trying to follow this as well (a little behind on<br>
email). I'm not quite convinced this patch is right.<br>
<br>
I hadn't realized that (almost!) all the logic to use<br>
emitLabelDifference() from section starts was Darwin-specific. With<br>
that new knowledge, here are a few useful assumptions we can make in<br>
emitLabelDifference():<br>
<br>
1. It's only used when emitting DWARF. (I have an out-of-tree patch<br>
to rename it to emitDwarfLabelDifference()... I'll commit that soon<br>
I hope.)<br>
2. emitSectionOffset(), which calls into this function only on Mach-O,<br>
is only used to describe DWARF sections. (The same patch fixes its<br>
name.)<br>
3. On ELF, sections can't be split. Since emitLabelDifference() is<br>
only used between two symbols within a section (caveat below for<br>
split-DWARF!!), we can safely emit it absolutely.<br>
4. On Mach-O, DWARF isn't relocatable: the DWARF is always grabbed<br>
from the original object file. The target symbols are always<br>
either (1) DWARF, which won't move, or (2) some offset of an atom,<br>
and atoms can't be split. Safe here too.<br>
<br>
So aside from my caveat, and assuming COFF somehow falls in the ELF<br>
bucket (or doesn't use DWARF), this seems good.<br>
<br>
The one case I can't quite prove to myself is "okay" is split-DWARF.<br>
There's a bunch of logic that calls `emitLabelDifference()` instead of<br>
`emitSectionOffset()` on split-DWARF/DWO. `DIELocList::EmitValue()`<br>
is one suspicious example, since it's emitting a symbol offset into<br>
the TEXT section:<br></blockquote><div><br>Wouldn't this be emitting a relocation into the debug_loc section, not the text section?<br></div></div></div></div></div></blockquote><div><br></div></div></div><div>Yes, I think that’s what Duncan meant. It’s emitting a relocation into the debug_loc section that refers to symbols from the .text section. </div></div></div></blockquote><div><br>Sorry, perhaps an issue with terminology.<br><br>This relocation will be placed in the debug_info section, and will refer to the debug_loc section, right? Neither of these is the text section.<br><br>& in fission, this would be the debug_info.dwo section containing relocations that refer to the debug_loc.dwo section (which is why it doesn't need relocations, just absolute offsets - because .dwo linking is handled by a debug-aware tool, so we don't emit any relocations into the .dwo file)<br> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><div>And —with the new logic — if at some point the MCSymbols that generate this relocation end up in the same fragment, then this relocation is gone too.</div><div><br></div><div>Fred</div><span class=""><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> void DIELocList::EmitValue(const AsmPrinter *AP, dwarf::Form Form) const {<br>
DwarfDebug *DD = AP->getDwarfDebug();<br>
MCSymbol *Label = DD->getDebugLocs().getList(Index).Label;<br>
<br>
if (AP->MAI->doesDwarfUseRelocationsAcrossSections() && !DD->useSplitDwarf())<br>
AP->emitSectionOffset(Label);<br>
else<br>
AP->EmitLabelDifference(Label, Label->getSection().getBeginSymbol(), 4);<br>
}<br>
<br>
I *think* this case is alright, but only because, currently, every<br>
instruction happens to be in a separate RelaxableFragment, so<br>
emitAbsoluteLabelDifference() is going to "fail". What I don't like<br>
is that whoever breaks that assumption is going to have no idea that<br>
the correctness of "emitAbsoluteLabelDifference()" depends on this.<br>
<br>
To rephrase, emitAbsoluteLabelDifference() used to only be called<br>
when it was correct to emit an absolute difference -- if it "failed",<br>
it was only a missed optimization. It looks to me like we're now<br>
*depending* on it "failing" for split-DWARF.<br>
<br>
Or am I wrong that we're depending on that? Does ELF not require<br>
relocations here?<br>
<br>
(There are a couple of other calls to emitLabelDifference() that come<br>
from split-DWARF/DWO logic. I didn't look at them as deeply, but I<br>
imagine they could have similar problems?)</blockquote></div><br></div></div>
</div></blockquote></span></div><br></div></blockquote></div><br></div></div>