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

[Mehdi Amini via llvm-dev]

> On Oct 25, 2016, at 9:09 AM, Chandler Carruth via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On Mon, Oct 24, 2016 at 10:48 PM Chris Lattner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
>> On Oct 24, 2016, at 1:07 PM, Peter Collingbourne <peter at pcc.me.uk <mailto:peter at pcc.me.uk>> 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
>> GlobalConstant
>> 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.)
> 
> Hi Peter,
> 
> I agree that it makes sense to introduce a new GlobalConstant IR node for this sort of thing.  That said, have you considered a design where GlobalConstant is still required to be a pointer type?  If you did this, you would end up with a simpler and less invasive design of:
> GlobalValue
> GlobalConstant
> GlobalIndirectSymbol
> GlobalAlias
> GlobalIFunc
> GlobalObject
> Function
> GlobalVariable
> I think that this would be better for (e.g.) the X86 backend anyway, since global objects can be assigned to specific addresses with linker maps, and thus have small addresses (and this is expressible with the range metadata).  This means that GlobalConstant and other GlobalValues should all be usable in the same places in principle.
> 
> If this works, it does seem better. But I can imagine it being hard to take the "load" of the global constant and turn it into a direct reference to a symbolic immediate operand to an instruction.

The linker “could" do it I think. For example ld64 is pattern matching to "optimize away” load from the GOT when possible, replacing then with nop and propagating the constant address.

It is still not perfect, as a register has to be used sometimes, and the nop will still be there taking space. But that may be enough for this use case?

— 
Mehdi


> 
> And it isn't clear that you can assign the "foo" in Peter's example an address even with a linker map -- it isn't a global object at all, it is a symbolic name for an immediate IIUC? (It's entirely possible I've misunderstood either what Peter needs or what you're suggesting, but I'd at least like to understand it! =D)
> 
> -Chandler
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