kernel mailz <kernelmailz@xxxxxxxxxxxxxx> writes:
> I've been fiddling my luck with gcc 4.3.2 inline assembly on powerpc
> There are a few queries
>
> 1. asm volatile or simply asm produce the same assembly code.
> Tried with a few examples but didnt find any difference by adding
> volatile with asm
>
> 2. Use of "memory" and clobbered registers.
>
> "memory" -
> a. announce to the compiler that the memory has been modified
> b. this instruction writes to some memory (other than a listed output)
> and GCC shouldn’t cache memory values in registers across this asm.
>
> I tried with stw and stwcx instruction, adding "memory" has no effect.
>
> Is there any example scenerio where gcc would generate different
> assembly by adding / removing "memory" ?
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An asm with no outputs is always considered to be volatile. To see the
affect of volatile, just try something like
asm ("# modify %0" : "=r" (i) : /* no inputs */ : /* no clobbers */);
Try it with and without optimization.
As the documentation says, the effect of adding a "memory" clobber is
that gcc does not cache values in registers across the asm. So the
effect will be shown in something like
int i = *p;
asm volatile ("# read %0" : : "r" (i));
return *p;
The memory clobber will only make a different when optimizing.
Ian
