[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Friedman, Eli via llvm-dev]

On 10/24/2016 1:07 PM, Peter Collingbourne via llvm-dev wrote:
> On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk 
> <mailto:peter at pcc.me.uk>> wrote:
>
>     The specific change I have in mind is to allow !range metadata on
>     GlobalObjects. This would
>     be similar to existing !range metadata, but it would apply to the
>     "address" of the attached GlobalObject, rather than any value
>     loaded from it. Its presence on a GlobalObject would also imply
>     that the address of the GlobalObject is "fixed" at link time.
>
> Going back to IR-level representation: here is an alternative 
> representation based on a suggestion from Eli.
>
> Introduce a new type of GlobalValue called GlobalConstant. 
> GlobalConstant would fit into the GlobalValue hierarchy like this:
>
>   * GlobalValue
>       o GlobalConstant
>       o GlobalPointer
>           + GlobalIndirectSymbol
>               # GlobalAlias
>               # GlobalIFunc
>           + GlobalObject
>               # Function
>               # GlobalVariable
>
> GlobalValue would no longer be assumed to be of pointer type. The 
> getType() overload that takes a PointerType, as well as getValueType() 
> would be moved down to GlobalPointer. (A nice side benefit of this is 
> that it would help flush out cases where we are unnecessarily 
> depending on global pointee types.)
>
> A GlobalConstant can either be a definition or a declaration. A 
> definition would look like this:
>
> @foo = globalconst i32 42

This is equivalent to writing "foo = 42" in assembly?

> while a declaration would look like this:
>
> @foo = external globalconst i32
>
> GlobalConstant could also hold a linkage and visibility. Looking at 
> the other attributes that a GlobalValue can hold, many of them do not 
> seem appropriate for GlobalConstant and could potentially be moved to 
> GlobalPointer.
>
> Thoughts?
>

How do you plan to use this?  The concept makes sense, but I've never 
actually seen anyone use symbols this way.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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