[LLVMdev] [pocl-devel] [cfe-dev] SPIR provisional specification is now available in the Khronos website

Villmow, Micah Micah.Villmow at amd.com
Mon Oct 1 09:36:12 PDT 2012

Maybe it would be easier to provide a bitcode example of this problem.

After thinking about this more, I'm not sure if this is applicable to SPIR itself. For you to have a constant GEP expression, you have to know the pointer size in order to correctly generate the expression. Since the pointer size itself is not known, I don't yet see how you can generate a constant expression that is valid SPIR.


From: mankeyrabbit at gmail.com [mailto:mankeyrabbit at gmail.com] On Behalf Of James Molloy
Sent: Saturday, September 29, 2012 5:30 AM
To: Owen Anderson
Cc: Villmow, Micah; llvmdev at cs.uiuc.edu; pocl-devel at lists.sourceforge.net; cfe-dev at cs.uiuc.edu
Subject: Re: [LLVMdev] [pocl-devel] [cfe-dev] SPIR provisional specification is now available in the Khronos website

Yes, it would.

But I was concerned Micah was just going to write it off as an implementation detail, so I felt that I should offer a "less correct but less work" option for him to consider.


On 29 September 2012 03:16, Owen Anderson <resistor at mac.com<mailto:resistor at mac.com>> wrote:

On Sep 28, 2012, at 9:45 AM, James Molloy <james at jamesmolloy.co.uk<mailto:james at jamesmolloy.co.uk>> wrote:

You can easily simplify this problem with a restriction in SPIR: disallow ConstantExpr casts - no ptrtoint constant expression. Because GlobalVariables have pointer type, if you disallow converting their type to non-pointer type in a constantexpr, the number of constantexpr subclasses you have to deal with is drastically reduced (to essentially just BitCast and GEP).

Wouldn't an easier solution just be not to represent them as constants in the first place?  For instance, you could have a built-in function to get the address of local N, where N is taken as a parameter.  You can call the builtins at the beginning of the kernel, and then proceed to use them as you wish without having to worry about reversing a constant folding later.  Plus, if a given vendor's backend wants the address to get constant folded, it's easy to do replaceAllUsesWith of the call with a global, and run an appropriate constant folding pass.


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