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

Oliver Stannard via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 26 08:50:24 PST 2019

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

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