Andrew Jeffery
2009-Sep-20 01:47 UTC
[LLVMdev] Global register variables/custom calling conventions
Hi all, I'm implementing an LLVM backend for qemu-arm (by creating an ARM frontend for LLVM - from what I understand a slightly different approach than the original llvm-qemu project) and I've got to the point where I have to deal with Qemu's use of global register variables. From what I've read LLVM doesn't support GCC style register variables and the workaround was to implement a custom calling convention to ensure they weren't clobbered (at least from a caller's point of view)[1]. I haven't played with the LLVM code base much, just used the interfaces, and so was wondering if anyone had some pointers on what to look at/how to go about this? Cheers, Andrew [1] http://markmail.org/message/ko3rrf5zacoocvg5#query:+page:1+mid:5uygbscyiqmuxqi4+state:results -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 261 bytes Desc: OpenPGP digital signature URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20090920/ec1016d7/attachment.sig>
Anton Korobeynikov
2009-Sep-20 06:45 UTC
[LLVMdev] Global register variables/custom calling conventions
Hello> I'm implementing an LLVM backend for qemu-arm (by creating an ARM frontend > for LLVM - from what I understand a slightly different approach than the > original llvm-qemu project)I don't see the difference so far. Could you please explain?> and I've got to the point where I have to deal with Qemu's use of global register variables.Why? The whole point of llvm-qemu project was that you don't need to go with all that hacks and workarounds - you just emit LLVM IR and let the codegenerator deal with register allocation, etc. For example, you have much available registers on x86-64 (and even on x86-32!). -- With best regards, Anton Korobeynikov Faculty of Mathematics and Mechanics, Saint Petersburg State University
Andrew Jeffery
2009-Sep-20 09:01 UTC
[LLVMdev] Global register variables/custom calling conventions
Anton Korobeynikov wrote:> Hello > >> I'm implementing an LLVM backend for qemu-arm (by creating an ARM frontend >> for LLVM - from what I understand a slightly different approach than the >> original llvm-qemu project) > I don't see the difference so far. Could you please explain?Again, from what I understand, llvm-qemu worked by emitting LLVM IR from QEMU IR API calls. This project goes straight from ARM to LLVM IR, bypassing QEMU's IR, (partially) in the hope that more information about the original intent of the code is retained. I don't have any numbers handy for comparison against the argument "it'd optimise away", however, we have to have an implementation to test this theory anyway ;)> >> and I've got to the point where I have to deal with Qemu's use of global register variables. > Why? The whole point of llvm-qemu project was that you don't need to > go with all that hacks and workaroundsThe point of this is to provide an alternative backend to QEMU that can be run in a separate thread to generate optimised blocks, while working as transparently as possible. A nice property of TCG (QEMU's current JIT, which was dyngen when llvm-qemu was written) is that it's extremely fast at generating reasonable code - this approach keeps it in place while we do extra, possibly more expensive work out of sight. It might not be a pretty idea, but LLVM does generate some very tight code :) It's an experiment - humour me... Sorry if this is somewhat OT. Cheers, Andrew -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 261 bytes Desc: OpenPGP digital signature URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20090920/e52618eb/attachment.sig>
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