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.
Micah
From: [email protected] [mailto:[email protected]] On Behalf Of James
Molloy
Sent: Saturday, September 29, 2012 5:30 AM
To: Owen Anderson
Cc: Villmow, Micah; [email protected]; [email protected];
[email protected]
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.
Cheers,
James
On 29 September 2012 03:16, Owen Anderson
<[email protected]<mailto:[email protected]>> wrote:
On Sep 28, 2012, at 9:45 AM, James Molloy
<[email protected]<mailto:[email protected]>> 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.
--Owen
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