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

Nicholas Piggin
2021-08-13T06:08:22Z

Excerpts from Christophe Leroy's message of April 14, 2021 2:38 am:
> powerpc BUG_ON() and WARN_ON() are based on using twnei instruction.
>
> For catching simple conditions like a variable having value 0, this
> is efficient because it does the test and the trap at the same time.
> But most conditions used with BUG_ON or WARN_ON are more complex and
> forces GCC to format the condition into a 0 or 1 value in a register.
> This will usually require 2 to 3 instructions.
>
> The most efficient solution would be to use __builtin_trap() because
> GCC is able to optimise the use of the different trap instructions
> based on the requested condition, but this is complex if not
> impossible for the following reasons:
> - __builtin_trap() is a non-recoverable instruction, so it can't be
> used for WARN_ON
> - Knowing which line of code generated the trap would require the
> analysis of DWARF information. This is not a feature we have today.
>
> As mentioned in commit 8d4fbcfbe0a4 ("Fix WARN_ON() on bitfield ops")
> the way WARN_ON() is implemented is suboptimal. That commit also
> mentions an issue with 'long long' condition. It fixed it for
> WARN_ON() but the same problem still exists today with BUG_ON() on
> PPC32. It will be fixed by using the generic implementation.
>
> By using the generic implementation, gcc will naturally generate a
> branch to the unconditional trap generated by BUG().
>
> As modern powerpc implement zero-cycle branch,
> that's even more efficient.
>
> And for the functions using WARN_ON() and its return, the test
> on return from WARN_ON() is now also used for the WARN_ON() itself.
>
> On PPC64 we don't want it because we want to be able to use CFAR
> register to track how we entered the code that trapped. The CFAR
> register would be clobbered by the branch.
>
> A simple test function:
>
> unsigned long test9w(unsigned long a, unsigned long b)
> {
> if (WARN_ON(!b))
> return 0;
> return a / b;
> }
>
> Before the patch:
>
> 0000046c <test9w>:
> 46c: 7c 89 00 34 cntlzw r9,r4
> 470: 55 29 d9 7e rlwinm r9,r9,27,5,31
> 474: 0f 09 00 00 twnei r9,0
> 478: 2c 04 00 00 cmpwi r4,0
> 47c: 41 82 00 0c beq 488 <test9w+0x1c>
> 480: 7c 63 23 96 divwu r3,r3,r4
> 484: 4e 80 00 20 blr
>
> 488: 38 60 00 00 li r3,0
> 48c: 4e 80 00 20 blr
>
> After the patch:
>
> 00000468 <test9w>:
> 468: 2c 04 00 00 cmpwi r4,0
> 46c: 41 82 00 0c beq 478 <test9w+0x10>
> 470: 7c 63 23 96 divwu r3,r3,r4
> 474: 4e 80 00 20 blr
>
> 478: 0f e0 00 00 twui r0,0
> 47c: 38 60 00 00 li r3,0
> 480: 4e 80 00 20 blr

That's clearly better because we have a branch anyway.

>
> So we see before the patch we need 3 instructions on the likely path
> to handle the WARN_ON(). With the patch the trap goes on the unlikely
> path.
>
> See below the difference at the entry of system_call_exception where
> we have several BUG_ON(), allthough less impressing.
>
> With the patch:
>
> 00000000 <system_call_exception>:
> 0: 81 6a 00 84 lwz r11,132(r10)
> 4: 90 6a 00 88 stw r3,136(r10)
> 8: 71 60 00 02 andi. r0,r11,2
> c: 41 82 00 70 beq 7c <system_call_exception+0x7c>
> 10: 71 60 40 00 andi. r0,r11,16384
> 14: 41 82 00 6c beq 80 <system_call_exception+0x80>
> 18: 71 6b 80 00 andi. r11,r11,32768
> 1c: 41 82 00 68 beq 84 <system_call_exception+0x84>
> 20: 94 21 ff e0 stwu r1,-32(r1)
> 24: 93 e1 00 1c stw r31,28(r1)
> 28: 7d 8c 42 e6 mftb r12
> ...
> 7c: 0f e0 00 00 twui r0,0
> 80: 0f e0 00 00 twui r0,0
> 84: 0f e0 00 00 twui r0,0
>
> Without the patch:
>
> 00000000 <system_call_exception>:
> 0: 94 21 ff e0 stwu r1,-32(r1)
> 4: 93 e1 00 1c stw r31,28(r1)
> 8: 90 6a 00 88 stw r3,136(r10)
> c: 81 6a 00 84 lwz r11,132(r10)
> 10: 69 60 00 02 xori r0,r11,2
> 14: 54 00 ff fe rlwinm r0,r0,31,31,31
> 18: 0f 00 00 00 twnei r0,0
> 1c: 69 60 40 00 xori r0,r11,16384
> 20: 54 00 97 fe rlwinm r0,r0,18,31,31
> 24: 0f 00 00 00 twnei r0,0
> 28: 69 6b 80 00 xori r11,r11,32768
> 2c: 55 6b 8f fe rlwinm r11,r11,17,31,31
> 30: 0f 0b 00 00 twnei r11,0
> 34: 7d 8c 42 e6 mftb r12

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).

On typical ppc32 CPUs, maybe it's a more obvious win. As you say there
is the CFAR issue as well which makes it a problem for 64s. It would
have been nice if it could use the same code though.

Maybe one day gcc's __builtin_trap() will become smart enough around
conditional statements that it it generates better code and tries to
avoid branches.

Thanks,
Nick