That is disappointing. OK. How about this, then?
Peter
llc[58]$ cat /tmp/x.c
#define DIRTY_FLAG 0x4
volatile int state;
#define _ATOMIC_BIC(_dst,_src) do { \
__asm__ __volatile__ ("bic\t%0, %1" \
: "=m" (_dst) \
: "g" (_src)); \
} while (0)
#define _ATOMIC_BIS(_dst,_src) do { \
__asm__ __volatile__ ("bis\t%0, %1" \
: "=m" (_dst) \
: "g" (_src)); \
} while (0)
void doit (int x) {
if (x) {
_ATOMIC_BIS(state, (state >> 1) & DIRTY_FLAG);
} else {
_ATOMIC_BIC(state, (state << 1) & ~DIRTY_FLAG);
}
}
llc[59]$ /msp430/install/lts-20120406-20120502/bin/msp430-gcc -Os -S /tmp/x.c
llc[60]$ cat x.s
.file "x.c"
.cpu 430
.mpy none
.text
.p2align 1,0
.global doit
.type doit,@function
/***********************
* Function `doit'
***********************/
doit:
cmp #0, r15
jeq .L2
mov &state, r15
rra r15
and #4, r15
#APP
; 17 "/tmp/x.c" 1
bis &state, r15
; 0 "" 2
#NOAPP
ret
.L2:
mov &state, r15
rla r15
and #llo(-5), r15
#APP
; 19 "/tmp/x.c" 1
bic &state, r15
; 0 "" 2
#NOAPP
ret
.Lfe1:
.size doit,.Lfe1-doit
;; End of function
.comm state,2,2
llc[61]$
On Sun, Sep 2, 2012 at 3:21 PM, William Swanson <swanson...@gmail.com> wrote:
> On Sun, Sep 2, 2012 at 1:02 PM, Peter Bigot <big...@acm.org> wrote:
>> 1) The solution mentioned before and made explicit below, which is not
>> optimal but does appear to meet your stated requirements for
>> atomicity;
>
> Right, I have been investigating that solution, and it's not 100%
> reliable. Depending on what the branch structure looks like, the
> compiler can sometimes insert a jump which destroys the peephole
> optimization. For example:
>
> if (blah)
> flags |= some_bit;
> else
> flags &= ~some_bit;
>
> This sometimes compiles to:
>
> TST.B blah
> MOV.B &flags, R14
> JZ else
> BIS.B #0x0008, R14
> JMP skip
> else:
> AND.B #0x00f7, R14
> skip:
> MOV.B R14, &flags
>
> I can't just "set it and forget it" with option 1; I have to verify
> that the assembly matches my intent, and re-verify it each time I
> change something. Option 3 would be more work than option 2, but would
> solve this problem once and for all for everyone.
>
> -William
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