[DebugInfo][Split DWARF][LTO]: Ensure only a single CU is emitted

Split DWARF doesn't handle LTO of any form (roughly there's an
assumption that each dwo file will have one CU - it's not explicitly

[DebugInfo][Split DWARF][LTO]: Ensure only a single CU is emitted

Split DWARF doesn't handle LTO of any form (roughly there's an
assumption that each dwo file will have one CU - it's not explicitly
documented, nor explicitly handled, so the ecosystem isn't really well
understood/tested/etc).

This had previously been handled by implementing (& disabling by
default) the `-split-dwarf-cross-cu-references` flag, which would
disable use of ref_addr across two dwo CUs.

This worked for a while, at least in LTO (it didn't address Split
DWARF+Full LTO, but that's an unlikely combination, as the benefits of
Split DWARF are more limited in a full LTO build) - because the only
source of cross-CU references was inlined functions, so by making those
non-cross-CU (by moving the referenced inlined function DWARF
description into the referencing CU) the result was one CU per dwo.

But recently the Function Specialization pass was added to the ThinLTO
pipeline, which caused imported functions that may not be inlined to be
emitted by a backend compile. This meant foreign CU entities (not just
abstract origins/cross-CU referenced entities)/standalone foreign CUs
could be emitted by a backend compile.

The end result was, due to a bug* in binutils dwp (I think basically
it saw two CUs in a single dwo and reprocessed the offsets in the shared
debug_str_offsets.dwo section) this situation lead to corrupted strings.

So to make this more robust, I've generalized the definition of the
`-split-dwarf-cross-cu-references` flag (perhaps it should be renamed at
this point, but it's /really/ niche, doubt anyone's using it - more or
less there for experimentation when we get around to figuring out
spec'ing LTO+Split DWARF) to mean "single CU in a dwo file" and added
more general handling for this.

There's certainly some weird corner cases that could come up in terms of
"how do we choose which CU to put everything in" - for now it's "first
come, first served" which is probably going to be OK for ThinLTO - the
base module will have the first functions and first CU, imported
fragments will come after that. For LTO the choice will be fairly
arbitrary - but, again, essentially whichever module comes first.

* Arguably a bug in binutils dwp, but since the feature isn't well
specified, I'd rather avoid dabbling in this uncertain area and ensure
LLVM doesn't produce especially novel DWARF (dwos with multiple CUs)
regardless of whether binutils dwp would/should be fixed. I'm not
confident debuggers could read such a dwo file well, etc.

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[test] Re-enable accidentally disabled X86 tests

A number of X86 tests were accidentally disabled in
https://reviews.llvm.org/D73568. This commit re-enables those tests.

```
$ for x86_test in $(gg

[test] Re-enable accidentally disabled X86 tests

A number of X86 tests were accidentally disabled in
https://reviews.llvm.org/D73568. This commit re-enables those tests.

```
$ for x86_test in $(gg 'REQUIRES: x86$' llvm/test | fst); do sed -i "" '/REQUIRES: x86/d' $x86_test; done
```

(Note that 'x86' is not an available feature, that's what caused the
tests to be disabled.)

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Remove lit feature object-emission

Summary: The lit feature object-emission was added because Hexagon did not support the integrated assembler, so some tests needed to be turned off with a Hexagon t

Remove lit feature object-emission

Summary: The lit feature object-emission was added because Hexagon did not support the integrated assembler, so some tests needed to be turned off with a Hexagon target. Hexagon now supports the integrated assembler, so this feature can be removed.

Reviewers: bcain, kparzysz, jverma, whitequark, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: mehdi_amini, hiraditya, steven_wu, dexonsmith, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73568

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[DWARF5][Debuginfo] Compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) do not match.

That patch fixes incompatible compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_

[DWARF5][Debuginfo] Compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) do not match.

That patch fixes incompatible compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) error.

cat split-dwarf.cpp
int main()
{
int a = 1;
return 0;
}

clang++ -O -g -gsplit-dwarf -gdwarf-5 split-dwarf.cpp; llvm-dwarfdump --verify ./a.out | grep skeleton
error: Compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) do not match.

The fix is to change DW_TAG_compile_unit into DW_TAG_skeleton_unit when skeleton file is generated.

Differential Revision: https://reviews.llvm.org/D70880

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llvm-dwarfdump: Remove extraneous space between '(' and 'indexed'

When dumping string or address indexes

llvm-svn: 349997

DebugInfo: Avoid using split DWARF when the split unit would be empty.

In ThinLTO many split CUs may be effectively empty because of the lack
of support for cross-unit references in split DWARF.

Us

DebugInfo: Avoid using split DWARF when the split unit would be empty.

In ThinLTO many split CUs may be effectively empty because of the lack
of support for cross-unit references in split DWARF.

Using a split unit in those cases is just a waste/overhead - and turned
out to be one contributor to a significant symbolizer performance issue
when global variable debug info was being imported (see r348416 for the
primary fix) due to symbolizers seeing CUs with no ranges, assuming
there might still be addresses covered and walking into the split CU to
see if there are any ranges (when that split CU was in a DWP file, that
meant loading the DWP and its index, the index was extra large because
of all these fractured/empty CUs... and so was very expensive to load).

(the 3rd fix which will follow, is to assume that a CU with no ranges is
empty rather than merely missing its CU level range data - and to not
walk into its DIEs (split or otherwise) in search of address information
that is generally not present)

llvm-svn: 349207

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Clarify comment in the string-offsets-table-order.ll test

llvm-svn: 340826

[DebugInfo] Reduce debug_str_offsets section size

Summary:
The accelerator tables use the debug_str section to store their strings.
However, they do not support the indirect method of access that is

[DebugInfo] Reduce debug_str_offsets section size

Summary:
The accelerator tables use the debug_str section to store their strings.
However, they do not support the indirect method of access that is
available for the debug_info section (DW_FORM_strx et al.).

Currently our code is assuming that all strings can/will be referenced
indirectly, and puts all of them into the debug_str_offsets section.
This is generally true for regular (unsplit) dwarf, but in the DWO case,
most of the strings in the debug_str section will only be used from the
accelerator tables. Therefore the contents of the debug_str_offsets
section will be largely unused and bloating the main executable.

This patch rectifies this by teaching the DwarfStringPool to
differentiate between strings accessed directly and indirectly. When a
user inserts a string into the pool it has to declare whether that
string will be referenced directly or not. If at least one user requsts
indirect access, that string will be assigned an index ID and put into
debug_str_offsets table. Otherwise, the offset table is skipped.

This approach reduces the overall binary size (when compiled with
-gdwarf-5 -gsplit-dwarf) in my tests by about 2% (debug_str_offsets is
shrunk by 99%).

Reviewers: probinson, dblaikie, JDevlieghere

Subscribers: aprantl, mgrang, llvm-commits

Differential Revision: https://reviews.llvm.org/D49493

llvm-svn: 339122

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