[llvm-dev] [RFC] Making .eh_frame more linker-friendly

George Rimar via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 20 07:56:44 PST 2017

>Keeping .eh_frame separated should still simplifies the linker because
>until the last step of building .eh_frame and .eh_frame_hdr, we don't
>really need to parse .eh_frame sections. So, if we have separate .eh_frame
>sections on -ffunction-sections, all we have to do is (1) garbage-collect
>sections and (2) construct .eh_frame and .eh_frame_hdr sections from live
>.eh_frame sections. At step 1, .eh_frame can be handled as a blob of data.
>That is much simpler than (1) parsing all .eh_frame sections beforehand,
>(2) garbage-collect .eh_frame shards, and (3) re-construct .eh_frame and
>.eh_frame_hdr from .eh_frame shards.
>In addition to that, keeping .eh_frame separated should greatly simplifies
>the logic of identical code folding when comparing not only function
>contents but exception handler information.

I tried to investigate how to implement this and suddenly found that Rafael
looks already tried to to the same in 2015: https://marc.info/?l=llvm-commits&m=144683596826489.

Basing on comments, approach worked but had slowdowns for bfd and crashed
gold (or made it slower too).  I can try to investigate this again and either reimplement
or ressurrect that code to check how multiple .eh_frame sections behave
nowadays, but few things are unclear for me.

Rafael, you wrote in description that you would not land that patch that time, do
you also think now we should try this again ? (since we have production ready LLD now).

(Assuming we want to try this again)
What are we going to do with possible slowdowns of GNU linkers ? I guess some option can be introduced
to keep old behavior for them, but than also not sure how LLD should handle 2 different types of
inputs (with single/multiple .eh_frame).

Any considerations of what should we try/check at first ? (I guess perfomance numbers for bfd/gold/LLD
is major point of interest, size of output, anything else ?).


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