Structure-return bug on powerpc32

[Andreas Tobler]

Wim Lewis wrote:
> I've been looking into some libffi test failures on netbsd/ppc. To make 
> a long story short, under some circumstances (returning a small 
> aggregate that's not a multiple of 4 bytes), ffi_call() will write past 
> the end of the buffer passed to rvalue.
> 
> In the cls_6byte.c test, this manifests itself as the first field in 
> g_dbl getting mysteriously set to zero after the call to ffi_call() in 
> main, since g_dbl is allocated after res_dbl with no padding. (This 
> surprised me a little, but the debugger confirms that &g_dbl is six 
> bytes higher than &res_dbl.) As a result, the *next* test in that file 
> will fail, since g_dbl no longer has the expected value.
> 
> The actual clobbering takes place at this point in src/powerpc/sysv.S:
> 
>  > L(smst_8byte):
>  >         stw     %r3,0(%r30)
>  >         stw     %r4,4(%r30)
>  >         b       L(done_return_value)
> 
> where r30 contains 'rvalue'. This ends up writing 8 bytes even for a 
> 6-byte return struct.
> 
> I'm not familiar enough with ppc assembly to suggest a patch; I'm not 
> really sure what the code immediately before the smst_8byte label is 
> doing. One idea is to do a read-modify-write and use insrwi/rlwinm to 
> modify only the portion of the word containing the returned struct. But 
> the struct may be aligned at the end of a memory region, so maybe the 
> only correct implementation for oddly-sized aggregates is a sequence of 
> rotates and stswi or stbu instructions.
> 
> 
> regards
>    Wim Lewis / wiml@hhhh.org

Which ABI does netbsd/ppc use? The original SYSV or the same as Linux? 
If it is the original SYSV you might try to build like FreeBSD does.

Andreas