[llvm-dev] attribute of intrinsic function

[Xiangyang Guo via llvm-dev]

Hi, Mehdi,

I use the following code to generate the IR:

*  std::vector<Type *> arg_type;*
*  std::vector<Value *> margs;*
*  margs.push_back(ptr_a);*
*  callee = Intrinsic::getDeclaration(mod, Intrinsic::foo5, arg_type);*
*  Value *dest_value = Builder.CreateCall(callee, margs);*
*  Value *dest_value1 = Builder.CreateCall(callee, margs);*

And even if I add the 'argmemonly' manually, it does not help. The problems
come from two places:
(1) when I use the following code to add the attribute, the attribute is
not there in the bit code. Maybe it's because the intrinsic function can
only have one attribute?

* AttributeSet func__Z3fooPi_PAL;*
* {*
*  SmallVector<AttributeSet, 4> Attrs;*
*  AttributeSet PAS;*
*   {*
*    AttrBuilder B;*
*    B.addAttribute(Attribute::NoUnwind);*
*    B.addAttribute(Attribute::ArgMemOnly);*
*    PAS = AttributeSet::get(mod->getContext(), ~0U, B);*
*   }*
*  Attrs.push_back(PAS);*
*  func__Z3fooPi_PAL = AttributeSet::get(mod->getContext(), Attrs);*
* }*


*// this is the new bitcode*
; Function Attrs: nounwind argmemonly
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*) #1
attributes #0 = { nounwind argmemonly }
attributes #1 = { nounwind }

(2) when I change the .ll file to add argmemonly'' manually, it does not
work either.

Ane idea?

Thanks,

Xiangyang


2016-03-24 13:12 GMT-07:00 Mehdi Amini <mehdi.amini at apple.com>:

>
> > On Mar 24, 2016, at 12:45 PM, Xiangyang Guo via llvm-dev <
> llvm-dev at lists.llvm.org> 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 }
>
> I'd expect foo4 to have the "argmemonly" attribute.
> How did you generate this IR?
> If you round-trip to bitcode, what happens? I expect the attribute to be
> added.
>
> --
> Mehdi
>
>
>
> >
> >
> > /****** 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. 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
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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