[LLVMdev] GlobalValues appear in their own use lists?

[Chris Lattner]

On Mar 10, 2014, at 11:59 AM, Sean Callanan <scallanan at apple.com> wrote:

> In the following IR module:
> –
> define i8 @foo() #0 {
> entry:
>   %call0 = call i8 @bar()
>   ret i8 %call0
> }
> 
> declare i8 @bar() #1
> –
> @bar() gets marked as its own user in top-of-tree LLVM.  I patched the Verifier to check it (but didn’t commit the patch):
> –
> Index: lib/IR/Verifier.cpp
> ===================================================================
> --- lib/IR/Verifier.cpp	(revision 203468)
> +++ lib/IR/Verifier.cpp	(working copy)
> @@ -360,6 +360,11 @@
>            "Global is external, but doesn't have external or weak linkage!",
>            &GV);
>  
> +  for (Value::const_use_iterator UI = GV.use_begin(), UE = GV.use_end();
> +       UI != UE; ++UI) {
> +    Assert1(*UI != &GV, "Global values cannot be their own uses!", &GV);
> +  }
> +
>    Assert1(!GV.hasAppendingLinkage() || isa<GlobalVariable>(GV),
>            "Only global variables can have appending linkage!", &GV);
> –
> Is it ever reasonable for a global value to be its own use?

No, this is never possible.  The only globals that have uses are GlobalVariables and Aliases, and both of them require that the operand have a different type than the global itself (one level of pointer is removed).  Since LLVM 2.0, it isn’t possible to have a "pointer to itself” type anymore.

>  If not, can I commit this patch?

Sure, but note that it won’t apply to mainline.  use iterators got reworked to be C++’11ified and got renamed in the process.

> This really causes problems for LLDB in various parts of our logic, including infinite loops when following use chains, but a major issue is that we try to strip certain global values out of the module, and if they’re their own uses then that doesn’t work because we can never eliminate the last use.

Are you sure that they are direct uses like this?  If *is* possible to have global variables that use themselves through constant expressions.  A C example would be something like:

extern void *G;
void *G = &G;

which compiles to:

@G = global i8* bitcast (i8** @G to i8*), align 8

-Chris
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