On 09/16/2014 11:49 AM, Peter Maydell wrote:
> On 16 September 2014 10:20, Tom Musta <tommu...@gmail.com> wrote:
>>
>> 1389 /* Compensate for very large offsets. */
>> 1390 if (add_off >= 0x8000) {
>> 1391 /* Most target env are smaller than 32k; none are larger
>> than 64k.
>> 1392 Simplify the logic here merely to offset by 0x7ff0,
>> giving us a
>> 1393 range just shy of 64k. Check this assumption. */
>> 1394 QEMU_BUILD_BUG_ON(offsetof(CPUArchState,
>> 1395 tlb_table[NB_MMU_MODES - 1][1])
>> 1396 > 0x7ff0 + 0x7fff);
>> 1397 tcg_out32(s, ADDI | TAI(TCG_REG_TMP1, base, 0x7ff0));
>> 1398 base = TCG_REG_TMP1;
>> 1399 cmp_off -= 0x7ff0;
>> 1400 add_off -= 0x7ff0;
>> 1401 }
>
> Is it possible to promote this BUILD_BUG_ON from "only on
> PPC hosts" to "on all builds" ? It's really checking a
> property of the target CPU's code, not a property of
> the TCG backend, and I bet a lot of our backends don't
> get built very often so we could easily miss breakage.
> I guess you'd need to define and check a worst-case value
> in a common header somewhere.
Meh. It is a property of the tcg backend, in that it is a property of the code
that immediately follows. And that's what makes the BUG_ON clear and obvious,
IMO.
For what it's worth, ppc as written has the smallest constraint of the current
backends, and I'm fairly confident that'll get built often-ish.
If you've got a rearrangement that puts the assert somewhere else, and keeps
the magic numbers understandable... I'll certainly have a look, but I don't see
how to retain the obviousness with a different placement.
r~
