llvm dev - Dec 2012 - [LLVMdev] target specific ways to extend ConstantExpr

The particular case we are looking at is converting a pointer from one address
space to another address space. There is one operand and one output, both are
the same pointer type, except for their address space. The pointers are of the
same size. The operation is a bit-changing operation.

We are using intrinsic functions for the instructions. We need a solution for
the ConstantExpr.

Instead of adding one target specific expression, it would be better to have a
more generic scheme in LLVM.


From: Chris Lattner [mailto:clattner at apple.com]
Sent: Monday, December 17, 2012 12:48 PM
To: Yuan Lin
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] target specific ways to extend ConstantExpr


On Dec 17, 2012, at 11:26 AM, Yuan Lin <yulin at nvidia.com<mailto:yulin
at nvidia.com>> wrote:


I am looking for a way to allow ConstantExpr to express target specific
operations, which will be used in global initializers.

The recommended way to extend LLVM IR is using intrinsic functions. But this
does not work for ConstantExpr, which the global initializer uses.

Should we make ConstantExpr to support some kind of side-effect free intrinsic
function calls? If we want to work on such a patch, what are the things we
should look out for?

http://llvm.org/bugs/show_bug.cgi?id=10368 seems also express this desire.


That PR is still accurate as to the direction I think ConstantExprs should go. 
What specifically does this target-specific constant need to look like?

-Chris


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On Dec 17, 2012, at 4:23 PM, Yuan Lin <yulin at nvidia.com> wrote:
> The particular case we are looking at is converting a pointer from one
address space to another address space. There is one operand and one output,
both are the same pointer type, except for their address space. The pointers are
of the same size. The operation is a bit-changing operation.
>  We are using intrinsic functions for the instructions. We need a solution
for the ConstantExpr.
Sounds like a bitcast to me.  Is there a reason the bitcast constexpr isn't
working for you?
> Instead of adding one target specific expression, it would be better to
have a more generic scheme in LLVM.
I agree!  However, fixing this is also a lot of work.

-Chris

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We cannot use bitcast because bitcast is a value preserving operation, but the
address space conversion operation we have is a bit-changing operation.

Where will the bulk of work be? Can we make a ConstantExpr which has a new
Opcode (or reuse call) and a list of operands? The first operand is a string.

From: Chris Lattner [mailto:clattner at apple.com]
Sent: Monday, December 17, 2012 4:39 PM
To: Yuan Lin
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] target specific ways to extend ConstantExpr


On Dec 17, 2012, at 4:23 PM, Yuan Lin <yulin at nvidia.com<mailto:yulin at
nvidia.com>> wrote:


The particular case we are looking at is converting a pointer from one address
space to another address space. There is one operand and one output, both are
the same pointer type, except for their address space. The pointers are of the
same size. The operation is a bit-changing operation.
 We are using intrinsic functions for the instructions. We need a solution for
the ConstantExpr.

Sounds like a bitcast to me.  Is there a reason the bitcast constexpr isn't
working for you?


Instead of adding one target specific expression, it would be better to have a
more generic scheme in LLVM.

I agree!  However, fixing this is also a lot of work.

-Chris


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