llvm dev - Feb 2013 - [LLVMdev] giving metadata a function parameter as an operand

Hi,

Is it possible to give an instruction metadata that accepts function argument as
operand? The 2 functions are in the same module so the metadata operand can be
resolved. Here is how I imagine it should look like:

      define i32 @f(i32 %v) {
        %v0 = call i32 @f1 (i32 %v, metadata !0) ; this ok
        ret i32 %v0
      }

      define i32 @f1 (i32 %v, metadata %md) { ; and this is ok too
        %t0 = add i32 %v, 1, !md !%md
      ;                           ^ here it says it expects an integer
        ret i32 %t0
      }

      !0 = metadata !{i32 4}

Thanks, Anat


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----- Original Message -----
> From: "Anat Shemer" <anat.shemer at intel.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Tuesday, February 12, 2013 6:33:41 AM
> Subject: [LLVMdev]  giving metadata a function parameter as an operand
> 
> 
> 
> Hi,
> 
> Is it possible to give an instruction metadata that accepts function
> argument as operand? The 2 functions are in the same module so the
> metadata operand can be resolved. Here is how I imagine it should
> look like:
> 
> define i32 @f(i32 %v) {
> %v0 = call i32 @f1 (i32 %v, metadata !0) ; this ok
> ret i32 %v0
> }
> 
> define i32 @f1 (i32 %v, metadata %md) { ; and this is ok too
> %t0 = add i32 %v, 1, !md !%md
> ; ^ here it says it expects an integer
> ret i32 %t0
> }
> 
> !0 = metadata !{i32 4}
Can you explain your motivation for wanting this?

 -Hal
> 
> Thanks, Anat
> 
> 
> 
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> 
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On Tue, Feb 12, 2013 at 4:33 AM, Shemer, Anat <anat.shemer at intel.com>
wrote:
> Hi,
>
> Is it possible to give an instruction metadata that accepts function
> argument as operand? The 2 functions are in the same module so the metadata
> operand can be resolved. Here is how I imagine it should look like:
>
> define i32 @f(i32 %v) {
>   %v0 = call i32 @f1 (i32 %v, metadata !0) ; this ok
>   ret i32 %v0
> }
>
> define i32 @f1 (i32 %v, metadata %md) { ; and this is ok too
>   %t0 = add i32 %v, 1, !md !%md
> ;                          ^ here it says it expects an integer
>   ret i32 %t0
> }
>
> !0 = metadata !{i32 4}
>From a cursory look at LLParser.cpp, this is not supported, since only
constant (= known at compile time) metadata nodes can be attached to
instructions. This makes sense, because metadata attached to
instructions are a complile-time property that should be available to
LLVM optimization passes and backends. What are the semantics of
having this value only known at runtime? Perhaps there's a better way
to achieve what you're trying to do.

Eli

Hi,

Thanks for looking into this.

The function f1 is inlined in the function f. Once this happens %md is a
constant.

I do this because I would like to inline f1 with different metadata values
depending on the call site. There is a finite number of possible metadata
values, practically 2 at the moment.

One alternative that I currently have in mind is to duplicate the function f1,
where the metadata is the only difference.

Another alternative has the advantage of duplicating only the instruction that
has the metadata attached but is more complex. (The idea is to transfer %md to
f1 as i32 and duplicate the instruction with the metadata in f1 in separate
basic blocks such that each copy has different metadata value. Additional
control flow will check the value of %md and jump to the right block
accordingly. This solution hopes that future passes will remove the redundant
blocks once f1 is inlined and %md becomes a constant.)

Both these options require code duplication with all the consequences of
maintainability.

And of course it's possible to write a pass that will handle this case.

But first I look for an existing solution.

Thanks, Anat 

-----Original Message-----
From: Eli Bendersky [mailto:eliben at google.com] 
Sent: Tuesday, February 12, 2013 18:24
To: Shemer, Anat
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] giving metadata a function parameter as an operand

On Tue, Feb 12, 2013 at 4:33 AM, Shemer, Anat <anat.shemer at intel.com>
wrote:
> Hi,
>
> Is it possible to give an instruction metadata that accepts function 
> argument as operand? The 2 functions are in the same module so the 
> metadata operand can be resolved. Here is how I imagine it should look
like:
>
> define i32 @f(i32 %v) {
>   %v0 = call i32 @f1 (i32 %v, metadata !0) ; this ok
>   ret i32 %v0
> }
>
> define i32 @f1 (i32 %v, metadata %md) { ; and this is ok too
>   %t0 = add i32 %v, 1, !md !%md
> ;                          ^ here it says it expects an integer
>   ret i32 %t0
> }
>
> !0 = metadata !{i32 4}
>From a cursory look at LLParser.cpp, this is not supported, since only
constant (= known at compile time) metadata nodes can be attached to
instructions. This makes sense, because metadata attached to instructions are a
complile-time property that should be available to LLVM optimization passes and
backends. What are the semantics of having this value only known at runtime?
Perhaps there's a better way to achieve what you're trying to do.
Eli
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Intel Israel (74) Limited

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.