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A recent change in LLVM causes it to sometimes end up with a thing
with no parent. That is, a debug_line or a debug_loc that has no CU that
refers to it. This is perhaps LLVM DCEing CUs, or something else that
changed - not sure. But it seems like valid DWARF we should handle.
This PR handles that in our code. Two things broke here. First, locs must
be simply ignored when there is no CU. Second, lines are trickier as we
used to compute their position by scanning them, and that list contained
only ones with a CU. So we missed some and ended up with wrong offsets.
To make things simpler and more robust, just track the position of each
line table on itself.
Fixes #3697
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The address size was hard-coded to 4, it now gets this information from .debug_info.
This required changing the parsing order.
Also made failure to parse .debug_loc fail the program, as before this error was easy to ignore.
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The previous code assumed that each compile unit had its own
abbreviation section, and they are all in order. That's normally how
LLVM emits things, but in #2992 there is a testcase in which linking
of object files with IR files somehow ends up with a different order.
The proper fix is to track the binary offsets of abbreviations in the
abbreviation section. That section is comprised of null-terminated
lists, which each CU has an offset to the beginning of. With those
offsets, we can match things properly.
Add a testcase that crashes without this, to prevent regressions.
Fixes #2992
Fixes #3007
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(#2862)
In the .debug_loc section the Start/End address offsets in a location list are
relative to the address of the compilation unit that refers that location list.
There is a problem in function wasm::Debug:: updateLoc(), which compares these
offsets with the actual module addresses of expressions and functions, causing
the generation of invalid location lists.
The fix is not trivial, because the DWARF debug_loc section does not specify
which is the compilation unit associated to each location list entry.
A simple workaround is to store, in LocationUpdater, a map of location list
offsets to the base address of the compilation units referencing them, and that
can be easily calculated in updateDIE().
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Each compilation unit's abbreviations must be terminated by
a zero, so that we use the right abbreviations. This adds that
support to the YAML layer, both adding the zeros and parsing
them to look in the right abbreviation section at the right time.
Also add two large testcases, zlib and cubescript, which
crash without this and the last PR.
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Add support for that section to the YAML layer, and add
code to update it.
The updating is slightly tricky - unlike .debug_ranges, the
size of entries is not fixed. So we can't just skip entries,
as the end marker is smaller than a normal entry. Instead,
replace now-invalid segments with (1, 1) which is of size
0 and so should be ignored by the debugger (we can't use
(0, 0) as that would be an end marker, and (-1, *) is
the special base marker).
In the future we probably do want to do this in a more
sophisticated manner, completely rewriting the indexes
into the section as well. For now though this should be
enough for when binaryen does not optimize (as we
don't move/reorder anything).
Note that this doesn't update the location description
(like where on the wasm expression stack the value is).
Again, that is correct for when binaryen doesn't
optimize, but for fully optimized builds we would need
to track things (which would be hard!).
Also clean up some code that uses "Extra" instead of
"Delimiter" that was missed before, and shorten some
unnecessarily long names.
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Multiple tables appear to be emitted when linking files
together. This fixes our support for that, which did not
update their size properly. This required patching the
YAML emitting code from LLVM in order to measure
the size and then emit it, as that code is apparently
not designed to handle changes in line table
contents.
Other minor fixes:
* Set the flags for our dwarfdump command to emit
the same as llvm-dwarfdump does with -v -all.
* Add support for a few more opcodes,
set_discriminator, set_basic_block, fixed_advance_pc,
set_isa.
* Handle a compile unit without abbreviations in the
YAML code (again, apparently not something this
LLVM code was intended to do).
* Handle a compile unit with zero entries in the
YAML code (ditto).
* Properly set the AddressSize - we use the
DWARFContext in a different way than LLVM expects,
apparently.
With this the emscripten test suite passes with
-gforce_dwarf without crashing.
My overall impression so from the the YAML code is
that it probably isn't a long-term solution for us. Perhaps
it may end up being scaffolding, that is, we can
replace it with our own code eventually that is based
on it, and remove most of the LLVM code. Before
deciding that we should get everything working first,
and this seems like the quickest path there.
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This imports LLVM code for DWARF handling. That code has the
Apache 2 license like us. It's also the same code used to
emit DWARF in the common toolchain, so it seems like a safe choice.
This adds two passes: --dwarfdump which runs the same code LLVM
runs for llvm-dwarfdump. This shows we can parse it ok, and will
be useful for debugging. And --dwarfupdate writes out the DWARF
sections (unchanged from what we read, so it just roundtrips - for
updating we need #2515).
This puts LLVM in thirdparty which is added here.
All the LLVM code is behind USE_LLVM_DWARF, which is on
by default, but off in JS for now, as it increases code size by 20%.
This current approach imports the LLVM files directly. This is not
how they are intended to be used, so it required a bunch of
local changes - more than I expected actually, for the platform-specific
stuff. For now this seems to work, so it may be good enough, but
in the long term we may want to switch to linking against libllvm.
A downside to doing that is that binaryen users would need to
have an LLVM build, and even in the waterfall builds we'd have a
problem - while we ship LLVM there anyhow, we constantly update
it, which means that binaryen would need to be on latest llvm all
the time too (which otherwise, given DWARF is quite stable, we
might not need to constantly update).
An even larger issue is that as I did this work I learned about how
DWARF works in LLVM, and while the reading code is easy to
reuse, the writing code is trickier. The main code path is heavily
integrated with the MC layer, which we don't have - we might want
to create a "fake MC layer" for that, but it sounds hard. Instead,
there is the YAML path which is used mostly for testing, and which
can convert DWARF to and from YAML and from binary. Using
the non-YAML parts there, we can convert binary DWARF to
the YAML layer's nice Info data, then convert that to binary. This
works, however, this is not the path LLVM uses normally, and it
supports only some basic DWARF sections - I had to add ranges
support, in fact. So if we need more complex things, we may end
up needing to use the MC layer approach, or consider some other
DWARF library. However, hopefully that should not affect the core
binaryen code which just calls a library for DWARF stuff.
Helps #2400
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