[llvm-dev] Range lists, zero-length functions, linker gc

Thu May 28 15:55:20 PDT 2020

On Thu, May 28, 2020 at 6:03 AM Alexey Lapshin <alapshin at accesssoftek.com>
wrote:

> Hi David,
>
>
> >So there have been several recent discussions about the issues around
>
> >DWARF-agnostic linking and gc-sections, linkonce function definitions
> being
>
> >dropped, etc - and just how much DWARF-awareness would be suitable
>
> >in a linker to help with this situation.
>
> > I'd like to discuss a narrower instance of this issue: Zero length
> gc'd/deduplicated functions.
>
> > LLVM seems to at least produce zero length functions in a few cases:
> > * non-void function without a return statement
> > * function definition containing only llvm_unreachable
> > (both of these trap at -O0, but at higher optimization levels even the
> trap
> > instruction is removed & you get the full power UB of control
> flowing off
> > the end of the function into whatever other bytes are after that
> function)
>
> > So, for context, debug_ranges (this whole issue doesn't exist in
> DWARFv5,
> > FWIW) is a list of address pairs, terminated by a pair of zeros.
> > With function sections, or even just with normal C++ inline functions,
> > the CU will have a range entry for that function that consists of two
> relocations
> > - to the start and end of the function. Generally the start of the
> function is the
> > start of the section, and the end is "start of function + length of
> function (aka addend)".
>
> >  Usually any relocation to the section would keep that section "alive"
> during linking -
> > but that would cause debug info to defeat linker GC and deduplication.
> So there's
> > special rules for how linkers handle these relocations in debug info to
> allow the
> > sections to be dropped - what do you write in the bytes that requested
> the relocation?
>
> > Binutils ld: Special cases only debug_ranges, resolving all relocations
> to dead
> > code to 1. In other debug sections, these values are all resolved to
> zero.
> > Gold and lld: Special cases all debug info sections - resolving all
> relocations
> > to "addend" (so begin usually goes to zero, end goes to "size of
> function")
>
> > These special rules are designed to ensure omitted/gc'd/deduplicated
> functions
> > don't cause the range list to terminate prematurely (which would happen
> if begin/end
> > were both resolved to zero).
>
> >But with an empty function, gold and lld's strategy here fails to avoid
> terminating a
> >range list by accident.
>
> > What should we do about it?
>
> >  1) Ensure no zero-length functions exist? (doesn't address backwards
> > compatibility/existing functions/other compilers)
> > 2) adopt the binutils approach to this (at least in debug_ranges - maybe
> in all
> > debug sections? (doing it in other sections could break )
> >  3) Revisit the discussion about using an even more 'blessed' value,
> > like int max-1? ( https://reviews.llvm.org/D59553 )
>
> >  (I don't have links to all the recent threads about this discussion - I
> think D59553
> > might've spawned a separate broader discussion/non-review - oh, Alexey
> wrote a
> > good summary with links to other discussions here:
> >  http://lists.llvm.org/pipermail/llvm-dev/2019-September/135068.html )
>
> > Thoughts?
>
> I think for the problem of "zero length functions and .debug_ranges"
> binutils approach looks good:
>
> >Special cases only debug_ranges, resolving all relocations to
> >dead code to 1. In other debug sections, these values are all resolved to
> >zero.
>
> But, this would not completely solve the problem from
> https://reviews.llvm.org/D59553 - Overlapped address ranges. Binutils
> approach will solve the problem if the address range specified as
> start_address:end_address. While resolving relocations, it would replace
> such a range with 1:1.
> However, It would not work if address ranges were specified as
> start_address:length since the length is not relocated.
>
This case could be additionally fixed by fast scan debug_info for High_PC
> defined as length and changing it to 1. Something which you suggested here:
> http://lists.llvm.org/pipermail/llvm-dev/2020-May/141599.html.
>

Hmm, I don't /think/ I intended to suggest anything that would have to
parse all the debug_info, even if just to fixup high_pc. I meant that
debug_rnglist for the CU at least (rnglist has fewer problems - you can't
accidentally terminate it early, but still has the "large functions in
programs that use relatively low code addresses can't just be resolved to
"addend" because then [0, length) of the large function might overlap into
that code address range") could be modified by a DWARF-aware linker to
remove the unused chunks. The DWARF that describes a specific function
using low_pc/high_pc - it may be split into a .dwo file and unreachable by
the linker - so it /needs/ a magic value for the address referenced by the
low_pc to indicate that it is invalid.

Which all comes back to "we probably need to pick a value that's explicitly
invalid" and -2 (max - 1) seems to be about the right thing.

>
> So it looks like following solution could fix both problems and be
> relatively fast:
>
> "Resolve all relocations from debug sections into dead code to 1. Parse
> debug sections and replace HighPc of an address range pointing to dead code
> and specified as length to 1".
>

That second part seems pretty expensive compared to anything else the
linker is doing with debug info. I'd try to avoid it if at all possible.

> As the result all address ranges pointing into dead code would be marked
> as zero length.
>
> There still exist another problem:
>
> DWARF4: "A range list entry (but not a base address selection or end of
> list entry) whose beginning and
> ending addresses are equal has no effect because the size of the range
> covered by such an
> entry is zero."
>
> DWARF5: "A bounded range entry whose beginning and ending address offsets
> are equal
> (including zero) indicates an empty range and may be ignored."
>
> These rules allow us to ignore zero-length address ranges. I.e., some tool
> reading DWARF is permitted to ignore related DWARF entries.
>

I agree it allows consumers to ignore that entry in the range list because
that entry is zero-length/equivalent to not being present at all - I don't
think that means consumers can ignore the DIE that refers to this range
list. I think it's valid DWARF to have a CU that only describes types,
without any code attached to it at all. Or for a subprogram that's been
eliminated to still be used by a consumer for name lookup purposes - so the
consumer can tell the user there is a function called "f1" and tell the
user what parameter types, return type it has, etc - not ignore it entirely.

> In that case, there could be ignored essential descriptions. That problem
> could happen with -flto=thin example
> https://reviews.llvm.org/D54747#1503720 . In this example, all type
> definitions except one were replaced with declarations by thinlto. The
> definition, which was left, is in a piece of debug info related to deleted
> code. According to zero-length rule, that definition could be ignored, and
> finally, incomplete debug info could be used.
>

Yeah, I think the bug there is the linker dropping object files just
because they have no exxecutable code in them - I think the patch that did
that was reverted, if I'm remembering correctly.

>
> So, it probably should be forbidden to generate debug_info, which could
> become incomplete after removing pieces related to zero length address
> ranges. Otherwise, creating zero-length address ranges could lead to
> incomplete debug info.
>
> Thank you, Alexey.
>
>
>
>
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