[llvm-dev] attribute of intrinsic function

[Philip Reames via llvm-dev]

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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