llvm dev - Jun 2008 - [LLVMdev] Transforming ConstantExprs to Instructions

Hi Chris,
> > [ Snip replacing constantexprs with instructions ] 
> Ok, this is not possible in general though, global variable initializers 
> have to be constants, not instructions.
Yeah, so if not all uses can be replaced, my pass will just have to skip the
variable.
> Is it possible to design the pass to work with both?  The general approach 
> is to make stuff handle "User"s instead of Instructions.  It is
much more
> compile time efficient to just handle the two forms rather than converting 
> them back and forth.
With both I assume you mean both GEP instrs and GEP constantexprs? If so then
yes, I am intending to make it work with both, and that's exactly why I need
to convert the constantexprs to instructions (Since, unless I'm very much
mistaken, a gepconstantexpr won't work with an alloca, so I can't leave
them
intact).

Gr.

Matthijs
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On Jun 18, 2008, at 1:36 AM, Matthijs Kooijman wrote:
>> Is it possible to design the pass to work with both?  The general  
>> approach
>> is to make stuff handle "User"s instead of Instructions.  It
is
>> much more
>> compile time efficient to just handle the two forms rather than  
>> converting
>> them back and forth.
> With both I assume you mean both GEP instrs and GEP constantexprs?  
> If so then
> yes, I am intending to make it work with both, and that's exactly  
> why I need
> to convert the constantexprs to instructions (Since, unless I'm very  
> much
> mistaken, a gepconstantexpr won't work with an alloca, so I can't  
> leave them
> intact).
What do you mean?  A constantexpr gep can be used as the size of an  
alloca.

-Chris

On Wed, Jun 18, 2008 at 10:19 PM, Chris Lattner <sabre at nondot.org>
wrote:
> What do you mean?  A constantexpr gep can be used as the size of an
> alloca.
A constantexpr as an argument to an alloca is legal, but a
constantexpr can't use an alloca.

Suppose you have a global struct, and you load the third member.  The
IL for this is something like "load i32* getelementptr ( %struct @x,
i32 0, i32 2)".  Now you want to replace the global @x with a local
alloca %x (assuming all the appropriate safety checks are done).
"load i32* getelementptr (%struct %x, i32 0, i32 2)" is illegal; it
has to be split into "%loadaddr = getelementptr %struct %x, i32 0, i32
2" and "load i32* %loadaddr".

-Eli

Hi Chris,
> What do you mean?  A constantexpr gep can be used as the size of an  
> alloca.
I meant the other way around. Ie,

@A = internal global { i32, i32 }
%B = load getelementptr { i32, i32 }* @A, i32 0, i32 0

would trivially translate to

%A = alloca { i32, i32 }
%B = load getelementptr { i32, i32 }* %A, i32 0, i32 0

which is not legal AFAICS (since %A is not a Constant).

Gr.

Matthijs
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