[llvm-dev] attribute of intrinsic function

[Xiangyang Guo via llvm-dev]

Thanks for your reply, Philip.

You are right, when I use LLVM-3.8, the 'argmemonly' shows up. Previously,
I use LLVM-3.7.

I think idempotent is what I want. Can you tell me how to add idempotent
attribute to the function? Thanks.

Regards,

Xiangyang

2016-03-24 14:30 GMT-07:00 Philip Reames <listmail at philipreames.com>:

>
>
> On 03/24/2016 12:45 PM, Xiangyang Guo via llvm-dev wrote:
>
> Hi,
>
> When I define an intrinsic function with memory write permission, my
> assumption is that we can either attach [IntrReadWriteArgMem] or [] to the
> intrinsic function. Based on the comment of the source code ,
> "IntrReadWriteArgMem - This intrinsic reads and writes only from memory
> that one of its arguments points to, but may access an unspecified amount."
> "If no property is set, the worst case is assumed (it may read and write
> any memory it can get access to and it may have other side effects)".
>
> But when I define the intrinsic function with different attributes, there
> is no difference when I dump the IR. For example, two intrinsic functions
> are
>
> def int_foo5 : Intrinsic<[], [llvm_ptr_ty], [IntrReadWriteArgMem]>;
> def int_foo6 : Intrinsic<[], [llvm_ptr_ty], []>;
>
> Then I write a very simple module, in each module the intrinsic function
> is called twice as shown below:
>
> /******the intrinsic function foo5 is used******/
> ; Function Attrs: nounwind
> define void @_Z3fooPi(i8* %a) #0 {
>   call void @llvm.foo5(i8* %a)
>   call void @llvm.foo5(i8* %a)
>   ret void
> }
> ; Function Attrs: nounwind
> declare void @llvm.foo5(i8*) #0
> attributes #0 = { nounwind }
>
> This is strange.  I would expect this to introduce the argmemonly
> attribute.  Can you confirm that you're on tip-of-tree and not some
> previous LLVM release version?
>
>
>
> /****** the intrinsic function foo6 is used******/
> ; Function Attrs: nounwind
> define void @_Z3fooPi(i8* %a) #0 {
>   call void @llvm.foo6(i8* %a)
>   call void @llvm.foo6(i8* %a)
>   ret void
> }
> ; Function Attrs: nounwind
> declare void @llvm.foo6(i8*) #0
> attributes #0 = { nounwind }
>
> And my when I apply -O3 optimization level for these two bitcode, I hope
> the second call can be eliminated. But from the result, it doesn't.
>
> Removing the previous call would be an incorrect optimization.
> Specifically, the intrinsic "foo5" could be an increment of the passed
> memory location.  Removing one of the two increments would be incorrect.
>
> I suspect you actually know something stronger about the intrinsic.  Is it
> possible readonly?  Or write only?  Or idempotent?  Or something else?
>
> For intrinsic functio foo5, I don't understand it. I mean, when I apply
> [IntrReadWriteArgMem] to this intrinsic function, I hope the LLVM can know
> the second one is redundant because nothing is changed. Can you tell why
> LLVM cannot optimize it in this case?
>
> Any input is appreciable.
>
> Regards,
>
> Xiangyang
>
>
>
>
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