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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 8 07:41:45 PDT 2017

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 debugranges base address specifier entries to reduce the number of object file relocations needed for debugranges*. * in a particular binary internally, an optimized build had a 70% reduction in debugranges.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/showbug.cgi?id=34007 ). Turns out GNU Binutils Gold has a bug (I've reported it as: https://sourceware.org/bugzilla/showbug.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?

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? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170808/a2111b97/attachment.html>

More information about the llvm-dev mailing list