[LLVMdev] Prevent instruction elimination

[Kenneth Uildriks]

On Mon, Oct 25, 2010 at 10:52 AM, John Criswell <criswell at illinois.edu> wrote:
> On 10/25/10 10:43 AM, Duncan Sands wrote:
>> Hi John,
>>
>>> As for instructions, I don't know of an instruction which does nothing,
>>> won't be removed by optimization, and yet does not inhibit
>>> optimization.  Perhaps a local alloca and a volatile load or store would
>>> do the trick?  Being volatile, the compiler won't remove it (or if it
>>> does, it's a bug, and you should file a bug report), and since it loads
>>> from a memory object not used for anything else, alias analysis should
>>> be able to see that it doesn't interefere with any other load/store.
>> LLVM certainly will remove volatile loads and stores to local variables
>> (at least in simple situations).  I suggest using an empty asm statement.
>
> Really?  Isn't that illegal?  The whole point of "volatile" is to tell
> the compiler that it should not remove a load/store.  Optimizing them
> away seems counter-intuitive and directly contradicts the documented
> behavior in the LLVM Language Reference manual (which states that the
> number of volatile loads/stores will not be changed).

If a local variable doesn't escape the function, no other thread can
touch it, and a volatile load from it is thus proven equivalent to a
regular load.


>
> -- John T.
>
>> Ciao,
>>
>> Duncan.
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