[llvm] r239552 - Generalize emitAbsoluteSymbolDiff.

[Frédéric Riss]

> On Jun 16, 2015, at 10:54 AM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Tue, Jun 16, 2015 at 10:43 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> 
> > On 2015-Jun-11, at 12:26, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> >
> >
> >> On Jun 11, 2015, at 12:19 PM, Rafael Espíndola <rafael.espindola at gmail.com <mailto:rafael.espindola at gmail.com>> wrote:
> >>
> >>> 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?
> >>
> >> The elf linker needs a relocation if one section refers to another. If
> >> two symbols are in different sections, then they are in different
> >> fragments.
> >>
> >> So the case of dwarf_str, only if the two symbols are in dwarf_str
> >> would this function avoid creating the expression.
> >
> > 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 :-)
> 
> What a mess.  These functions' assumptions are pretty convoluted.  I
> just spent some time trying to follow this as well (a little behind on
> email).  I'm not quite convinced this patch is right.
> 
> I hadn't realized that (almost!) all the logic to use
> emitLabelDifference() from section starts was Darwin-specific.  With
> that new knowledge, here are a few useful assumptions we can make in
> emitLabelDifference():
> 
>  1. It's only used when emitting DWARF.  (I have an out-of-tree patch
>     to rename it to emitDwarfLabelDifference()... I'll commit that soon
>     I hope.)
>  2. emitSectionOffset(), which calls into this function only on Mach-O,
>     is only used to describe DWARF sections.  (The same patch fixes its
>     name.)
>  3. On ELF, sections can't be split.  Since emitLabelDifference() is
>     only used between two symbols within a section (caveat below for
>     split-DWARF!!), we can safely emit it absolutely.
>  4. On Mach-O, DWARF isn't relocatable: the DWARF is always grabbed
>     from the original object file.  The target symbols are always
>     either (1) DWARF, which won't move, or (2) some offset of an atom,
>     and atoms can't be split.  Safe here too.
> 
> So aside from my caveat, and assuming COFF somehow falls in the ELF
> bucket (or doesn't use DWARF), this seems good.
> 
> The one case I can't quite prove to myself is "okay" is split-DWARF.
> There's a bunch of logic that calls `emitLabelDifference()` instead of
> `emitSectionOffset()` on split-DWARF/DWO.  `DIELocList::EmitValue()`
> is one suspicious example, since it's emitting a symbol offset into
> the TEXT section:
> 
> Wouldn't this be emitting a relocation into the debug_loc section, not the text section?

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. 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.

Fred

>     void DIELocList::EmitValue(const AsmPrinter *AP, dwarf::Form Form) const {
>       DwarfDebug *DD = AP->getDwarfDebug();
>       MCSymbol *Label = DD->getDebugLocs().getList(Index).Label;
> 
>       if (AP->MAI->doesDwarfUseRelocationsAcrossSections() && !DD->useSplitDwarf())
>         AP->emitSectionOffset(Label);
>       else
>         AP->EmitLabelDifference(Label, Label->getSection().getBeginSymbol(), 4);
>     }
> 
> I *think* this case is alright, but only because, currently, every
> instruction happens to be in a separate RelaxableFragment, so
> emitAbsoluteLabelDifference() is going to "fail".  What I don't like
> is that whoever breaks that assumption is going to have no idea that
> the correctness of "emitAbsoluteLabelDifference()" depends on this.
> 
> To rephrase, emitAbsoluteLabelDifference() used to only be called
> when it was correct to emit an absolute difference -- if it "failed",
> it was only a missed optimization.  It looks to me like we're now
> *depending* on it "failing" for split-DWARF.
> 
> Or am I wrong that we're depending on that?  Does ELF not require
> relocations here?
> 
> (There are a couple of other calls to emitLabelDifference() that come
> from split-DWARF/DWO logic.  I didn't look at them as deeply, but I
> imagine they could have similar problems?)
> 

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