diff options
| author | Anton Blanchard <anton@samba.org> | 2010-05-17 14:34:57 +1000 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2010-05-17 07:57:27 -0700 |
| commit | 81880d603d00c645e0890d0a44d50711c503b72b (patch) | |
| tree | 69d4bb390d8d2595017192b51feb08e2fed3c26a | |
| parent | f3d46f9d3194e0329216002a8724d4c0957abc79 (diff) | |
atomic_t: Remove volatile from atomic_t definition
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.
To dig up an old quote from Linus:
> The fact is, volatile on data structures is a bug. It's a wart in the C
> language. It shouldn't be used.
>
> Volatile accesses in *code* can be ok, and if we have "atomic_read()"
> expand to a "*(volatile int *)&(x)->value", then I'd be ok with that.
>
> But marking data structures volatile just makes the compiler screw up
> totally, and makes code for initialization sequences etc much worse.
And screw up it does :)
With the volatile removed, we see much more reasonable code generation:
c00000000051f5b8: 3b 60 00 01 li r27,1
...
c00000000051f5c0: 3b 80 00 00 li r28,0
...
c00000000051fc7c: 7c 20 04 ac lwsync
c00000000051fc80: 7c 00 f8 28 lwarx r0,0,r31
c00000000051fc84: 7c 00 d8 00 cmpw r0,r27
c00000000051fc88: 40 c2 00 10 bne- c00000000051fc98
c00000000051fc8c: 7f 80 f9 2d stwcx. r28,0,r31
c00000000051fc90: 40 c2 ff f0 bne+ c00000000051fc80
c00000000051fc94: 4c 00 01 2c isync
Six instructions less.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
| -rw-r--r-- | include/linux/types.h | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/include/linux/types.h b/include/linux/types.h index c42724f8c802..23d237a075e2 100644 --- a/include/linux/types.h +++ b/include/linux/types.h @@ -188,12 +188,12 @@ typedef u32 phys_addr_t; typedef phys_addr_t resource_size_t; typedef struct { - volatile int counter; + int counter; } atomic_t; #ifdef CONFIG_64BIT typedef struct { - volatile long counter; + long counter; } atomic64_t; #endif |