Re: [PATCH v2 1/2] powerpc/bug: Remove specific powerpc BUG_ON() and WARN_ON() on PPC32

Nicholas Piggin
2021-08-26T15:04:45Z

Excerpts from Segher Boessenkool's message of August 27, 2021 12:37 am:
> On Thu, Aug 26, 2021 at 11:57:52PM +1000, Nicholas Piggin wrote:
>> Excerpts from Segher Boessenkool's message of August 26, 2021 10:49 pm:
>> > On Thu, Aug 26, 2021 at 01:26:14PM +1000, Nicholas Piggin wrote:
>> >> Excerpts from Segher Boessenkool's message of August 19, 2021 1:06 am:
>> >> > On Fri, Aug 13, 2021 at 04:08:13PM +1000, Nicholas Piggin wrote:
>> >> >> This one possibly the branches end up in predictors, whereas conditional
>> >> >> trap is always just speculated not to hit. Branches may also have a
>> >> >> throughput limit on execution whereas trap could be more (1 per cycle
>> >> >> vs 4 per cycle on POWER9).
>> >> >
>> >> > I thought only *taken* branches are just one per cycle?
>> >>
>> >> Taken branches are fetched by the front end at one per cycle (assuming
>> >> they hit the BTAC), but all branches have to be executed by BR at one
>> >> per cycle
>> >
>> > This is not true. (Simple) predicted not-taken conditional branches are
>> > just folded out, never hit the issue queues. And they are fetched as
>> > many together as fit in a fetch group, can complete without limits as
>> > well.
>>
>> No, they are all dispatched and issue to the BRU for execution. It's
>> trivial to construct a test of a lot of not taken branches in a row
>> and time a loop of it to see it executes at 1 cycle per branch.
>
> (s/dispatched/issued/)

?

>
> Huh, this was true on p8 already. Sorry for my confusion. In my
> defence, this doesn't matter for performance on "real code".
>
>> > Correctly predicted simple conditional branches just get their prediction
>> > validated (and that is not done in the execution units). Incorrectly
>> > predicted branches the same, but those cause a redirect and refetch.
>>
>> How could it validate prediction without issuing? It wouldn't know when
>> sources are ready.
>
> In the backend. But that is just how it worked on older cores :-/

Okay. I don't know about older cores than POWER9. Backend would normally
include execution though. Only other place you could do it if you don't
issue/exec would be after it goes back in order, like completion. But
that would be horrible for mispredict penalty.

>> >> The first problem seems like the show stopper though. AFAIKS it would
>> >> need a special builtin support that does something to create the table
>> >> entry, or a guarantee that we could put an inline asm right after the
>> >> builtin as a recognized pattern and that would give us the instruction
>> >> following the trap.
>> >
>> > I'm not quite sure what this means. Can't you always just put a
>> >
>> > bla: asm("");
>> >
>> > in there, and use the address of "bla"?
>>
>> Not AFAIKS. Put it where?
>
> After wherever you want to know the address after. You will have to
> make sure they stay together somehow.

I still don't follow.

> It is much easier to get the address of something, not the address after
> it. If you know it is just one insn anyway, that isn't hard to handle
> either (even if prefixed insns exist).
>
>> > If not, you need to say a lot
>> > more about what you actually want to do :-/
>>
>> We need to put the address (or relative offset) of the trap instruction
>> into an entry in a different section. Basically this:
>>
>> __builtin_trap();
>> asm ("1: \n\t"
>> " .section __bug_table,\"aw\" \n\t"
>> "2: .4byte 1b - 2b - 4 \n\t"
>> " .previous");
>>
>> Where the 1: label must follow immediately after the trap instruction,
>> and where the asm must be emitted even for the case of no-return traps
>> (but anything following the asm could be omitted).
>
> The address *after* the trap insn? That is much much harder than the
> address *of* the trap insn.

No the address of the trap instruction, hence the -4. I have the label
after because that is the semantics I have from inline asm.

If you could give a built in that put a label at the address of the trap
instruction that could be used later by inline asm then that could work
too:

__builtin_labeled_trap("1:");
asm (" .section __bug_table,\"aw\" \n\t"
"2: .4byte 1b - 2b \n\t"
" .previous");