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the_o (Ofer Shinaar)
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Jun 24 2018, 12:38 AM (209 w, 5 d)

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Sep 12 2021

the_o added a comment to D109372: [RISCV][RFC] Add Clang support for RISC-V overlay system.

The name "overlay" is ambiguous. Even after I ruled out Gentoo Overlay and overlayfs, I am thinking whether this has anything to do with OVERLAY description in a linker script: https://sourceware.org/binutils/docs/ld/Overlay-Description.html#Overlay-Description

which are used to mark functions or global data as only accessible through the overlay engine

Can you give more descriptions for folks who don't follow the RISC-V side proposal but need to review your changes? :)

Basically hardware-assisted code+rodata banking (I guess either by actually banking ROMs or just paging stuff in and out) that's mostly transparent to software. Functions at the boundary of components (don't know what the granularity is) use a weird indirect calling convention where you instead call into some magic runtime with a unique ID for the callee, it ensures everything's loaded and then tail calls it for you.

Yes it's essentially this. The start of the proposal for this 'overlay' system for RISC-V is a FOSDEM talk 'Cacheable Overlay Manager RISC‐V'. That's also the source of the weird name for the -fcomrv option name.

Would something like -foverlay-manager make more sense? (maybe an -m option would actually be more appropriate given this is still very RISC-V specific?). I'm not sure how to disambiguate from the many overloaded meanings of 'overlay'.

Thank's for the feedback. I'll update this and come back with a tidier patch.

Sep 12 2021, 6:22 AM · Restricted Project