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 8d4fbcfbe0 ("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
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
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b286e07fb771a664b631cd07a40b09c06f26e64b.1618331881.git.christophe.leroy@csgroup.eu
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