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Re: [RFC][AArch64] function prologue analyzer in linux kernel


On Fri, Jan 08, 2016 at 02:36:32PM +0900, AKASHI Takahiro wrote:
> On 01/07/2016 11:56 PM, Richard Earnshaw (lists) wrote:
> >On 07/01/16 14:22, Will Deacon wrote:
> >>On Thu, Dec 24, 2015 at 04:57:54PM +0900, AKASHI Takahiro wrote:
> >>>So I'd like to introduce a function prologue analyzer to determine
> >>>a size allocated by a function's prologue and deduce it from "Depth".
> >>>My implementation of this analyzer has been submitted to
> >>>linux-arm-kernel mailing list[1].
> >>>I borrowed some ideas from gdb's analyzer[2], especially a loop of
> >>>instruction decoding as well as stop of decoding at exiting a basic block,
> >>>but implemented my own simplified one because gdb version seems to do
> >>>a bit more than what we expect here.
> >>>Anyhow, since it is somewhat heuristic (and may not be maintainable for
> >>>a long term), could you review it from a broader viewpoint of toolchain,
> >>>please?
> >>>
> >>My main issue with this is that we cannot rely on the frame layout
> >>generated by the compiler and there's little point in asking for
> >>commitment here. Therefore, the heuristics will need updating as and
> >>when we identify new frames that we can't handle. That's pretty fragile
> >>and puts us on the back foot when faced with newer compilers. This might
> >>be sustainable if we don't expect to encounter much variation, but even
> >>that would require some sort of "buy-in" from the various toolchain
> >>communities.
> >>
> >>GCC already has an option (-fstack-usage) to determine the stack usage
> >>on a per-function basis and produce a report at build time. Why can't
> >>we use that to provide the information we need, rather than attempt to
> >>compute it at runtime based on your analyser?
> >>
> >>If -fstack-usage is not sufficient, understanding why might allow us to
> >>propose a better option.
> >
> >Can you not use the dwarf frame unwind data?  That's always sufficient
> >to recover the CFA (canonical frame address - the value in SP when
> >executing the first instruction in a function).  It seems to me it's
> >unlikely you're going to need something that's an exceedingly high
> >performance operation.
> 
> Thank you for your comment.
> Yeah, but we need some utility routines to handle unwind data(.debug_frame).
> In fact, some guy has already attempted to merge (part of) libunwind into
> the kernel[1], but it was rejected by the kernel community (including Linus
> if I correctly remember). It seems that they thought the code was still buggy.

The ARC guys seem to have sneaked something in for their architecture:

  http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arc/kernel/unwind.c

so it might not be impossible if we don't require all the bells and
whistles of libunwind.

> That is one of reasons that I wanted to implement my own analyzer.

I still don't understand why you can't use fstack-usage. Can you please
tell me why that doesn't work? Am I missing something?

Will


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