Re: [PATCH 0/8] Non-contiguous address range support

Kevin Buettner
2018-07-11T15:27:45Z

Ping.

On Mon, 25 Jun 2018 23:32:39 -0700
Kevin Buettner <kevinb@redhat.com> wrote:

> This eight part patch sequence adds (further) support for
> non-contiguous address ranges to GDB.
>
> This sequence of patches was motivated by GCC bug 84550:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84550
>
> There is a test case posted to that bug along with some analysis of
> the underlying problem.
>
> There is also a GDB bug for the same issue; it's 23021, but at the
> moment, there is little there aside from a link to the GCC bug
> mentioned above. But here's a link anyway:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=23021
>
> A quick synopsis of the problem is as follows...
>
> Recent versions of gcc can generate code in which a function is split
> into at least two non-contiguous address ranges. As I understand it,
> the idea here is to separate code which gcc does not expect to execute
> in normal operation from the rest of the code. Doing this may result
> in better cache locality for the normal case. The generated code for
> the example in GCC bug 84550 separated a call to abort() from the rest
> of the code comprising the function.
>
> In the course of my investigation, I identified at least four
> problems:
>
> 1) Stepping into a function from a function which occupies non-contiguous
> address ranges does not always work. It was not uncommon to see the
> program run to completion when attempting to do a step.
>
> 2) Setting a breakpoint on a function with non-contiguous address ranges
> causes a breakpoint to be placed on more than one location. When a
> breakpoint is set on the "cold" address range, this is almost certainly
> incorrect. The breakpoint should instead be set only on code near the
> entry point(s).
>
> 3) The disassemble command did not work correctly. E.g. here is what I
> found during my analysis of 84550:
>
> (gdb) x/i 'main.cold.0'
> 0x4010e0 <main()>: mov %rax,%rdi
> (gdb) x/i main
> 0x4011a0 : push %r12
> (gdb) disassemble main
> Dump of assembler code for function main():
> 0x00000000004010e0 <+0>: mov %rax,%rdi
> ...
> [No addresses starting at 0x4011a0 are shown]
>
> 4) Display of addresses associated with the non-contiguous function are
> confusing. E.g. in the above example, note that GDB thinks that
> the address associated with main.cold.0 is <main()>, but that there's
> also a minsym called main which is displayed as .
>
> There are probably several other problems which are related those
> identified above.
>
> I discovered that the stepping problem could be "fixed" by disabling
> the find_pc_partial_function cache. This cache keeps track of the
> most recent result (of calling find_pc_partial_function). If
> find_pc_partial_function is called with an address which falls within
> the cache range, then that's considered to be a cache hit and the most
> recent result is returned. Obviously, this won't work correctly for
> functions which occupy non-contiguous (disjoint) address ranges where
> other functions might be placed in the gap.
>
> So one of the problems that needed to be solved was to make the
> caching code work correctly. It is interesting to note that stepping
> _did_ work when the cache was disabled. This is/was due to GDB
> already having some (albeit incomplete) support for non-contiguous
> addresses in the form of blockvector address maps. Code responsible
> for mapping addresses to blocks (which form the lower levels of
> find_pc_partial_function) handle this case correctly.
>
> To solve the problem of incorrect disassembly, we need to be able
> to iterate over all of the ranges associated with a block.
>
> Finally, we need to distinguish between the entry pc and the lowest
> address in a block. I discovered that this lack of distinction was
> the cause of the remainder of the bugs including some which seemed to
> be introduced by fixing the problems noted above. Once this
> distinction is made, it will be straightforward to add full support for
> DW_AT_entry_pc. I considered adding this support as part of this
> patch series, but decided to wait until the community weighs in on my
> work thus far...