llvm dev - Dec 2008 - [LLVMdev] optimization whith call of Intrinsics

Dear LLVMer ;

i'm trying to use LLVM for a specific target, using different memory banks.
I have written the frontend to generate a valid IR and want to use the 
existing passes (as defined in tool opt) to optimize the code.

The target has specific instructions, so following advices given in 
documentation, I created an intrinsic function, with attribute 
[IntrReadArgMem].

My test case is very simple :  I repeat twice a set of instructions and 
would like that the optimizer delete the second one;
the set is
{  INSTR 1 :  store of data in bank 1;
    INSTR 2 :  call of intrinsic (read only)
    INSTR 3 :  store of data in bank 2; }

When calling the intrinsic function, the optimizer doesn't work well.
(If I delete INSTR2 and INSTR3, optimization is good)
If I use only one memory bank, the problem is the same. It seems that 
the call of intrinsics is not well supported by existing optimization 
passes.
Does anybody have an idea about this problem ? ( just below, the IR 
generated by LLVM after optimization)
Thanks.

Julien



; ModuleID = 'TheModule'
@MemSys = global i16 0        ; <i16*> [#uses=2]
@MemSysSigA = global i16 3 addrspace(2)         ; <i16 addrspace(2)*> 
[#uses=2]
@reg_ai_beg = global i16 405 addrspace(1)         ; <i16 addrspace(1)*> 
[#uses=2]

define void @OFUNC(i16* %N0, i16* %N1) nounwind {
entry:
    store i16 1, i16 addrspace(1)* @reg_ai_beg
    %outLLVMInt = tail call i16 @llvm.octo.su.opa.rd(i16* @MemSys)       
 ; <i16> [#uses=1]
    store i16 %outLLVMInt, i16 addrspace(2)* @MemSysSigA
    store i16 1, i16 addrspace(1)* @reg_ai_beg
    %outLLVMInt5 = tail call i16 @llvm.octo.su.opa.rd(i16* @MemSys)   
     ; <i16> [#uses=1]
    store i16 %outLLVMInt5, i16 addrspace(2)* @MemSysSigA
    ret void
}

declare i16 @llvm.octo.su.opa.rd(i16*) nounwind readonly











--

On Thu, Dec 4, 2008 at 7:38 AM, Julien Schmitt <jschmitt at dibcom.fr>
wrote:
> Dear LLVMer ;
>
> i'm trying to use LLVM for a specific target, using different memory
banks.
> I have written the frontend to generate a valid IR and want to use the
> existing passes (as defined in tool opt) to optimize the code.
>
> The target has specific instructions, so following advices given in
> documentation, I created an intrinsic function, with attribute
> [IntrReadArgMem].
>
> My test case is very simple :  I repeat twice a set of instructions and
> would like that the optimizer delete the second one;
> the set is
> {  INSTR 1 :  store of data in bank 1;
>    INSTR 2 :  call of intrinsic (read only)
>    INSTR 3 :  store of data in bank 2; }
>
> When calling the intrinsic function, the optimizer doesn't work well.
> (If I delete INSTR2 and INSTR3, optimization is good)
> If I use only one memory bank, the problem is the same. It seems that
> the call of intrinsics is not well supported by existing optimization
> passes.
> Does anybody have an idea about this problem ? ( just below, the IR
> generated by LLVM after optimization)
> Thanks.
>
> Julien
>
>
>
> ; ModuleID = 'TheModule'
> @MemSys = global i16 0        ; <i16*> [#uses=2]
> @MemSysSigA = global i16 3 addrspace(2)         ; <i16 addrspace(2)*>
> [#uses=2]
> @reg_ai_beg = global i16 405 addrspace(1)         ; <i16
addrspace(1)*>
> [#uses=2]
>
> define void @OFUNC(i16* %N0, i16* %N1) nounwind {
> entry:
>    store i16 1, i16 addrspace(1)* @reg_ai_beg
>    %outLLVMInt = tail call i16 @llvm.octo.su.opa.rd(i16* @MemSys)
>  ; <i16> [#uses=1]
>    store i16 %outLLVMInt, i16 addrspace(2)* @MemSysSigA
>    store i16 1, i16 addrspace(1)* @reg_ai_beg
>    %outLLVMInt5 = tail call i16 @llvm.octo.su.opa.rd(i16* @MemSys)
>     ; <i16> [#uses=1]
>    store i16 %outLLVMInt5, i16 addrspace(2)* @MemSysSigA
>    ret void
> }
>
> declare i16 @llvm.octo.su.opa.rd(i16*) nounwind readonly
Dead store elimination is that pass that could do the optimizations in
question.  The second store to @reg_ai_beg should be eliminated; I'm
not really surprised that it doesn't implement this particular case,
though.  For the stores to @MemSysSigA, LLVM doesn't know that
llvm.octo.su.opa.rd doesn't read @MemSysSigA, so neither store can be
eliminated.

You should be able to solve both these problems by writing a special
alias analysis pass that knows about the behavior of
llvm.octo.su.opa.rd.  See http://llvm.org/docs/AliasAnalysis.html for
details.

-Eli

Thanks Eli.
I will try your solution, but then, what is the difference between 
[IntrReadArgMem] and
[IntrReadMem] when specifying an intrinsic ? It seems that both options 
specify that a function is 'readonly'
(maybe the difference is not well supported for then moment ?)

Julien

> Dead store elimination is that pass that could do the optimizations in
> question.  The second store to @reg_ai_beg should be eliminated; I'm
> not really surprised that it doesn't implement this particular case,
> though.  For the stores to @MemSysSigA, LLVM doesn't know that
> llvm.octo.su.opa.rd doesn't read @MemSysSigA, so neither store can be
> eliminated.
>
> You should be able to solve both these problems by writing a special
> alias analysis pass that knows about the behavior of
> llvm.octo.su.opa.rd.  See http://llvm.org/docs/AliasAnalysis.html for
> details.
>
> -Eli
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> CONFIDENTIAL NOTICE: The contents of this message, including any
attachments, are confidential and are intended solely for the use of the person
or entity to whom the message was addressed. If you are not the intended
recipient of this message, please be advised that any dissemination,
distribution, or use of the contents of this message is strictly prohibited. If
you received this message in error, please notify the sender. Please also
permanently delete all copies of the original message and any attached
documentation. Thank you.
>   












--