[llvm-dev] DWARF: Ranges base address specifier entries & Gold's gdb-index 32 bit bug

[David Blaikie via llvm-dev]

On Tue, Aug 8, 2017 at 7:43 AM Nico Weber <thakis at chromium.org> wrote:

> On Tue, Aug 8, 2017 at 10:41 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Tue, Aug 8, 2017 at 6:56 AM Nico Weber <thakis at chromium.org> wrote:
>>
>>> On Aug 7, 2017 6:58 PM, "David Blaikie" <dblaikie at gmail.com> wrote:
>>>
>>> Context:
>>>
>>> In r309526 (with a followup fix in r309529) I implemented the use of
>>> DWARF's debug_ranges base address specifier entries to reduce the number of
>>> object file relocations needed for debug_ranges*.
>>>
>>> * in a particular binary internally, an optimized build had a 70%
>>> reduction in debug_ranges.reloc, a 16% decrease in total object size (with
>>> compressed debug info and fission)
>>>
>>> Nico noted that this broke the Chromium build (
>>> https://bugs.llvm.org/show_bug.cgi?id=34007 ). Turns out GNU Binutils
>>> Gold has a bug (I've reported it as:
>>> https://sourceware.org/bugzilla/show_bug.cgi?id=21894 - wouldn't expect
>>> much action on it, though) where it fails to parse a base address specifier
>>> entry when trying to build gdb-index (-Wl,-gdb-index) for a 32 bit build.
>>>
>>> To address this in the short term, I added a flag that disables this
>>> feature by default for now.
>>>
>>> So, two questions:
>>>
>>> 1) What does everyone reckon the best solution to this is?
>>>   * Keep the feature disabled by default - with a flag to enable it for
>>> those who want it/can use it
>>>   * Enable the feature everywhere - with a flag to disable it
>>>   * Conditionally disable (or conditionally enable) the feature, with a
>>> flag to enable/disable it
>>>     * best condition we probably have is "disabled if 32 bit and
>>> -ggnu-pubnames" (gnu-pubnames are necessary for efficient building of
>>> gdb-index, and isn't the default)
>>>
>>>
>>> FWIW, we use -Wl,--gdb-index but had never heard of -ggnu-pubnames
>>> before last week and don't use it (yet?).
>>>
>>
>> PR34007 about the gold failure shows -ggnu-pubnames in the repro, though?
>>
>
> 'cause you asked me to test with that on IRC :-) We don't use it in our
> "real" build config.
>

Ah, OK!

:/


>
>
>>
>>
>>> So maybe just "32-bit".
>>>
>>> Either of these options works for us. From a usability perspective
>>> independent of chromium, making your linker not crash by default seems kind
>>> of nice to me. But either of these options works for me. Disabling the
>>> feature everywhere by default seems a bit strange since it has no known
>>> drawbacks on 64-bit (yet? :-) ).
>>>
>>
>>> 2) How to implement the flag:
>>>   * backend option as it is currently (-mllvm
>>> -use-dwarf-ranges-base-address-specifier (maybe drop "-specifier" to make
>>> it a bit more terse))
>>>   * clang option that maps to
>>>     * backend option (as currently)
>>>     * MCOption (programmatic/C++ API rather than command line of a
>>> backend option)
>>>     * LLVM IR attribute on the CU (this feature could be supported on a
>>> per-CU basis easiyl enough, thus survive LTO, etc, exactly as the user
>>> requested (but really if the user requested this on at least one CU, they
>>> probably might as well have it on all their CUs))
>>>     * LLVM IR module metadata
>>>
>>>
>>> Any ideas/thoughts/other aspects?
>>>
>>>
>>>
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