[llvm-dev] [RFC] Displaying source variable locations in llvm-objdump

Evgenii Stepanov via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 26 10:38:39 PST 2019


I like this a lot! I think the dwarf expression syntax is spot-on.

On Tue, Nov 26, 2019 at 9:12 AM Eric Christopher <echristo at gmail.com> wrote:

> Hi Oliver,
> This is really cool. I absolutely support this for llvm-objdump. As
> far as output I don't have any strong opinions other than it might be
> good to separate out the "drawing" code as much as possible from the
> variable collection and range code to make it a little easier, but
> that's about it from here.
> Thanks for the work, can't wait to use it.
> -eric
> On Tue, Nov 26, 2019 at 8:50 AM Oliver Stannard via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi llvm-dev,
> >
> > I've uploaded a prototype patch at https://reviews.llvm.org/D70720
> which adds a new feature to llvm-objdump: displaying the location (in
> registers/memory/etc) of source-level variables alongside the disassembly
> display. I've put a demo of the output at https://reviews.llvm.org/M2.
> >
> > I have two use-cases in mind for this:
> > * Users reading the disassembly of compiled code. It will be
> quicker/easier to do this if the disassembly shows which value is in each
> register and stack slot, rather than the user having to reverse-engineer
> this by hand.
> > * Compiler developers, who can use it to understand the debug info
> emitted by the compiler, and spot missing or incorrect debug info. In fact,
> I've already spotted one LLVM bug while writing this patch: in the function
> `baz` in M2, the debug info claims that variable `a` is in `r0` between PC
> addresses 0x14 and 0x8, which isn't true.
> >
> > My questions for the LLVM community are:
> > * Is this an acceptable change for llvm-objdump, or is this adding too
> much complexity to be worth it?
> > * The patch currently uses unicode box-drawing characters, is this OK?
> If not, what would people rather see? A plain ASCII version of this, or
> some completely different format?
> > * The patch displays DWARF expressions in an ad-hoc syntax, which is a
> mix of C and ARM assembly (square brackets for memory access). Is there an
> existing syntax which would be better for this? I think it's important that
> the common cases like "load 4 bytes from memory at SP+4" are displayed
> concisely.
> >
> > Oliver
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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