Re: [PATCH][ARM] Fix RVCT interoperation issue

[Andrew Stubbs <ams at codesourcery dot com>]

On 09/03/11 16:12, Ramana Radhakrishnan wrote:
> On Fri, 2011-03-04 at 11:23 +0000, Andrew Stubbs wrote:
> > The attached patch, submitted on behalf of Dan Jacobowitz, fixes an
> > unwind bug when using RealView and libgcc.
> >
> > It's an old patch that has been in CodeSourcery and Linaro toolchains
> > for sometime, but somehow escaped being posted here.
>
> I would like to see some context around this patch before accepting or
> rejecting this, maybe a testcase (agreed that this is harder) or a
> description of the problem this patch is attempting to fix.

A testcase would be difficult. GCC won't produce anything that will 
exercise this code. We'd have to cook something up in hand-coded 
assembler, or maybe import something from RVCT - I don't know.

However, the logic can easily be compared with the ARM EHABI document, 
section 9.2:

http://infocenter.arm.com/help/topic/com.arm.doc.ihi0038a/IHI0038A_ehabi.pdf

The key sentences are:

 "If the high bit is set in the word containing N, then the type_info
  list is followed by a prel31 landing pad offset (with bit 31 clear)
  to be entered in the event that no type matches the thrown type. High
  bit clear in the N word signifies that implicitly the no match case
  should result in a call to __cxa_call_unexpected. When the high bit
  clear format is used, object producers must emit an R_ARM_NONE
  relocation to __cxa_call_unexpected to indicate the dependency to the
  linker."

(Thanks to Paul Brook for explaining this to me. :)

Andrew