[PATCH][PR debug/60655] Power/GCC: Reject cross-section symbol subtraction

[Alan Modra]

On Tue, Sep 02, 2014 at 09:47:15AM -0400, David Edelsohn wrote:
> Alan,
> 
> Any feedback?

It is just papering over the real bug(s), of course, so I'd be
inclined to say this doesn't belong on trunk.

If you take a look at the assembly that is failing, you find gcc is
trying to output a location expression for a parameter of mp_compare.
After replacing the 335-.LANCHOR0 with 0:

objdump -sj.debug_loc
 0180                   000000f8 00000104
 0190 000f7a00 03000000 00220300 00000022
 01a0 9f

readelf -r
00000195  00000401 R_PPC_ADDR32           00000000   .bss + 0

readelf -wo
    00000188 000000f8 00000104 (DW_OP_breg10 (r10): 0; DW_OP_addr: 0; DW_OP_plus; DW_OP_addr: 0; DW_OP_plus; DW_OP_stack_value)

Putting this all together, and knowing that .LANCHOR0 == .bss+0, the
expresssion is really:

    00000188 000000f8 00000104 (DW_OP_breg10 (r10): 0; DW_OP_addr: .LANCHOR0; DW_OP_plus; DW_OP_addr: 335-.LANCHOR0; DW_OP_plus; DW_OP_stack_value)

ie. ((r10) + 0) + (.LANCHOR0) + (335-.LANCHOR0)

Clearly this could be simplified to (r10) + 335, so it might be that
var-tracking is being confused by section anchors.  However, it is
more than just a missing simplification because the location
expression doesn't make any sense at all..


In fact if you look at the corresponding location expression for
-fno-section-anchors code you get something quite screwy too.

0000018f 00000108 00000114 (DW_OP_breg10 (r10): 0; DW_OP_addr: 144; DW_OP_plus; DW_OP_stack_value

which is (r10) + &modulus.

If I have the interpretation of all the debug info correct, what this
is saying is that in the epilogue of upton_modmult, the value for the
"r2" parameter of mp_compare is found by adding the address of
"modulus" to the contents of r10.  The correct answer is simply that
the "r2" parameter of mp_compare is in r10.


> On Tue, Sep 2, 2014 at 6:28 AM, Maciej W. Rozycki
> <macro@codesourcery.com> wrote:
> > Hi,
> >
> >  Similarly to ARM, where this issue was seen originally, and likely many
> > other targets, the Power ABI does not appear to have a relocation defined
> > to support taking a difference of two symbols in different sections each.
> > This is seen as a failure in gcc.c-torture/compile/pr60655-2.c:

-- 
Alan Modra
Australia Development Lab, IBM