Re: [PATCH 2/2] powerpc: Switch to generic WARN_ON()/BUG_ON()

Olof Johansson
2007-10-12T02:35:36Z

On Fri, Oct 12, 2007 at 11:23:39AM +1000, Paul Mackerras wrote:
> Olof Johansson writes:
>
> > Not using the ppc-specific WARN_ON/BUG_ON constructs actually saves about
> > 4K text on a ppc64_defconfig. The main reason seems to be that prepping
> > the arguments to the conditional trap instructions is more work than
> > just doing a compare and branch.
>
> It might be more instructions but it takes fewer cycles, I would
> expect. Do you have the actual instruction sequences to compare?

Sure. Just looking at a couple of cases:

On range comparisons such as (a < CONST):
* without this patch, you end up with a comparison, then a cr-to-cr op +
mfcr + mask operation + tdnei.
* with the patch, you get a comparison and a conditional branch with
twi out of line.

On multiple comparisons like WARN_ON( a || b), it seems that there's an
added li to make the tdnei always hit for the first case. There it uses
a regular conditional branch, so no real difference in generated code
besides that.

Code examples. This was with a ppc64_defconfig, so CONFIG_POWER4_ONLY
wasn't enabled. It uses mfocrf when it is, which is alot cheaper. Still,
the rest of the code sequence is the same:

void irq_free_virt(unsigned int virq, unsigned int count)
{
unsigned long flags;
unsigned int i;

WARN_ON (virq < NUM_ISA_INTERRUPTS);
WARN_ON (count == 0 || (virq + count) > irq_virq_count);

spin_lock_irqsave(&irq_big_lock, flags);

Without the patch:

c00000000000b33c <.irq_free_virt>:
c00000000000b33c: 7c 08 02 a6 mflr r0
c00000000000b340: 2b 83 00 0f cmplwi cr7,r3,15
c00000000000b344: fb c1 ff f0 std r30,-16(r1)
c00000000000b348: fb e1 ff f8 std r31,-8(r1)
c00000000000b34c: 7c 9e 23 78 mr r30,r4
c00000000000b350: 7c 7f 1b 78 mr r31,r3
c00000000000b354: 4f dd e8 42 crnot 4*cr7+eq,4*cr7+gt
c00000000000b358: f8 01 00 10 std r0,16(r1)
c00000000000b35c: f8 21 ff 81 stdu r1,-128(r1)
c00000000000b360: 7c 00 00 26 mfcr r0
c00000000000b364: 54 00 ff fe rlwinm r0,r0,31,31,31
c00000000000b368: 0b 00 00 00 tdnei r0,0
c00000000000b36c: 2f a4 00 00 cmpdi cr7,r4,0
c00000000000b370: 38 00 00 01 li r0,1
c00000000000b374: 41 9e 00 1c beq cr7,c00000000000b390 <.irq_free_virt+0x54>
c00000000000b378: e9 22 80 d8 ld r9,-32552(r2)
c00000000000b37c: 7d 63 22 14 add r11,r3,r4
c00000000000b380: 80 09 00 00 lwz r0,0(r9)
c00000000000b384: 7f 8b 00 40 cmplw cr7,r11,r0
c00000000000b388: 7c 00 00 26 mfcr r0
c00000000000b38c: 54 00 f7 fe rlwinm r0,r0,30,31,31
c00000000000b390: 0b 00 00 00 tdnei r0,0
c00000000000b394: e8 62 80 f8 ld r3,-32520(r2)
c00000000000b398: 48 54 4c b1 bl c000000000550048 <._spin_lock_irqsave>
c00000000000b39c: 60 00 00 00 nop

With the patch:

c00000000000b0b0 <.irq_free_virt>:
c00000000000b0b0: 7c 08 02 a6 mflr r0
c00000000000b0b4: 2b 83 00 0f cmplwi cr7,r3,15
c00000000000b0b8: fb c1 ff f0 std r30,-16(r1)
c00000000000b0bc: fb e1 ff f8 std r31,-8(r1)
c00000000000b0c0: 7c 9e 23 78 mr r30,r4
c00000000000b0c4: 7c 7f 1b 78 mr r31,r3
c00000000000b0c8: f8 01 00 10 std r0,16(r1)
c00000000000b0cc: f8 21 ff 81 stdu r1,-128(r1)
c00000000000b0d0: 41 bd 00 08 bgt cr7,c00000000000b0d8 <.irq_free_virt+0x28>
c00000000000b0d4: 0f e0 00 00 twi 31,r0,0
c00000000000b0d8: 2f be 00 00 cmpdi cr7,r30,0
c00000000000b0dc: 41 9e 00 18 beq cr7,c00000000000b0f4 <.irq_free_virt+0x44>
c00000000000b0e0: e9 22 80 d8 ld r9,-32552(r2)
c00000000000b0e4: 7d 7f f2 14 add r11,r31,r30
c00000000000b0e8: 80 09 00 00 lwz r0,0(r9)
c00000000000b0ec: 7f 8b 00 40 cmplw cr7,r11,r0
c00000000000b0f0: 40 bd 00 08 ble cr7,c00000000000b0f8 <.irq_free_virt+0x48>
c00000000000b0f4: 0f e0 00 00 twi 31,r0,0
c00000000000b0f8: e8 62 80 f8 ld r3,-32520(r2)
c00000000000b0fc: 48 54 3f 75 bl c00000000054f070 <._spin_lock_irqsave>
c00000000000b100: 60 00 00 00 nop