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

Anton Blanchard
2010-05-17T04:38:03Z

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>
---

Index: linux-cpumask/include/linux/types.h
===================================================================
--- linux-cpumask.orig/include/linux/types.h 2010-04-08 19:46:02.916696580 +1000
+++ linux-cpumask/include/linux/types.h 2010-05-17 13:21:59.135964013 +1000
@@ -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