Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition

Heiko Carstens
2010-05-17T08:57:43Z

On Mon, May 17, 2010 at 02:34:57PM +1000, Anton Blanchard wrote:
>
> When looking at a performance problem on PowerPC, I noticed some awful code
> generation:
>
> c00000000051fc98: 3b 60 00 01 li r27,1
> ...
> c00000000051fca0: 3b 80 00 00 li r28,0
> ...
> c00000000051fcdc: 93 61 00 70 stw r27,112(r1)
> c00000000051fce0: 93 81 00 74 stw r28,116(r1)
> c00000000051fce4: 81 21 00 70 lwz r9,112(r1)
> c00000000051fce8: 80 01 00 74 lwz r0,116(r1)
> c00000000051fcec: 7d 29 07 b4 extsw r9,r9
> c00000000051fcf0: 7c 00 07 b4 extsw r0,r0
>
> c00000000051fcf4: 7c 20 04 ac lwsync
> c00000000051fcf8: 7d 60 f8 28 lwarx r11,0,r31
> c00000000051fcfc: 7c 0b 48 00 cmpw r11,r9
> c00000000051fd00: 40 c2 00 10 bne- c00000000051fd10
> c00000000051fd04: 7c 00 f9 2d stwcx. r0,0,r31
> c00000000051fd08: 40 c2 ff f0 bne+ c00000000051fcf8
> c00000000051fd0c: 4c 00 01 2c isync
>
> We create two constants, write them out to the stack, read them straight back
> in and sign extend them. What a mess.
>
> It turns out this bad code is a result of us defining atomic_t as a
> volatile int.
>
> We removed the volatile attribute from the powerpc atomic_t definition years
> ago, but commit ea435467500612636f8f4fb639ff6e76b2496e4b (atomic_t: unify all
> arch definitions) added it back in.

With your patches we can also revert 39475179d40996b4efa662e3825735a84d2526d1
"[S390] Improve code generated by atomic operations." which was created for
the very same reason.

Thanks!