llvm submittal 41050 created for powerpc64 C++ exception code generation: ld r2,40(r1) missing or skipped before bl __cxa_begin_catch code

[Mark Millard]

On 2019-Mar-12, at 22:08, Mark Millard <marklmi at yahoo.com> wrote:

> I have submitted:
> 
> https://bugs.llvm.org//show_bug.cgi?id=41050
> 
> for the clang 8 code generation problem of
> no code for setting r2 appropriately before
> the:
> 
> bl . . . <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3>
> 
> in unoptimized code ( no -O ). For the -O2 code:
> 
> ld r2,40(r1)
> 
> is present but is being skipped by the libunwind return
> to the code: it returns to the just-following bl
> instruction (like above) instead.
> 
> In both cases:
> 
> (gdb) x/32i  0x100007c0
>  0x100007c0 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3>:	std     r2,40(r1)
>  0x100007c4 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+4>:	ld      r12,-32608(r2)
>  0x100007c8 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+8>:	mtctr   r12
>  0x100007cc <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+12>:	ld      r11,-32592(r2)
>  0x100007d0 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+16>:	ld      r2,-32600(r2)
>  0x100007d4 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+20>:	bctr
>  0x100007d8 <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+24>:	.long 0x0
>  0x100007dc <00000018.plt_call.__cxa_begin_catch@@CXXABI_1.3+28>:	.long 0x0
> . . .
> 
> with an inappropriate r2 value leads to jumping to
> inappropriate places.
> 
> The example source code was:
> 
> #include <exception>
> 
> int main(void)
> {
>   try { throw std::exception(); }
>   catch (std::exception& e) {}
>   return 0;
> }
> 
> 
> 
> Note:
> 
> This is from investigations of head -r345044 using
> WITH_LLVM_LIBUNWIND= on powerpc64.
> 

The discussion on https://bugs.llvm.org//show_bug.cgi?id=41050
indicates that the ld r2,??? to restore the value appropriate to
the a.out code in my example should be happening via the library
holding libunwind's code instead of the ld executing in the
a.out code.

So: thus far it is viewed as a libunwind issue instead of a clang
one.


===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)