[LLVMdev] Prevent instruction elimination

[Devang Patel]

On Oct 25, 2010, at 12:09 PM, Xinfinity wrote:

> 
> 
> Devang Patel wrote:
>> 
>> 
>> What are you going to do if "optimized LLVM code" is hoisted above or
>> sinked below LLVM_dummy_inst by the optimizer ? It seems you are looking
>> for a way communicate some info for a block of instructions. If that is
>> the case then one solution is to extract interesting block of instructions
>> in a separate function and make sure that function is not inlined. If this
>> is not feasible than attaching your info with each instructions in
>> "optimized LLVM code" block is better than trying to find artificial
>> barriers to communicate some info.
>> -
>> Devang
>> 
>> 
> 
> Indeed, I want to use the metadata as barriers. They are meant to mark the
> beginning and the end of the compound statement of the pragma.
> 
> #pragma my_pragma{
>  code
> }
> 
> Therefore, I expect that the dummy-instructions will be part of different
> basic blocks, and I can determine the set of blocks in "code", inside the
> pragma region, from the control flow graph. So, if the instructions are
> reordered inside the same basic block, it will not pose any problems. Also,
> if both the beginning and the end of the barrier are part of the same block,
> it can be handled.
> 
> I modify clang in order to insert dummy instructions in the location of the
> pragma (in the beginning and the end of the pragma scope). I use a map
> (source_location, pragma) and I insert the dummy instruction when this
> location is reached in the code generator. It seems difficult to attach the
> metadata to the first and the last instruction emitted for the compound
> statement, as the "code" inside the pragma region can be anything.
> 
> Do you consider it would be a better idea to attach metadata to all
> instructions emitted between source_location marking the beginning and the
> source_location marking the end of the region?

Yes, that's how, now, we keep track of lexical scopes for debug info's use.
-
Devang