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

[Kevin Buettner <kevinb@redhat.com>]

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 <main>:     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 <main>.
> 
> 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...