llvm dev - Oct 2010 - [LLVMdev] How to assign a constant to a register?

Other than:
%x = add i32 5,0 ; suppose I want to assign 5 to %x

is there any other way? Something like x86's mov instruction

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hi,

On Mon, Oct 18, 2010 at 12:39 PM, leledumbo <leledumbo_cool at
yahoo.co.id> wrote:
>
> Other than:
> %x = add i32 5,0 ; suppose I want to assign 5 to %x
you do not need to do this at the level of LLVM IR, or do you mind to
tell us the reason for this?

best regards
ether
>
> is there any other way? Something like x86's mov instruction
>
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> View this message in context:
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> Sent from the LLVM - Dev mailing list archive at Nabble.com.
>
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Hi,
> Other than:
> %x = add i32 5,0 ; suppose I want to assign 5 to %x
>
> is there any other way? Something like x86's mov instruction
let me say that in general doing this is pointless.  Due to SSA form, if %x is
set to 5 you can't set it to something else later.  Thus everywhere that you
use %x you might as well just directly use 5 there instead.  A common situation
is that you have a register %x, and due to performing optimizations you discover
that in fact %x will always have the value 5.  You can use RAUW (aka the
replaceAllUsesWith method) to replace %x with 5 everywhere.

Ciao,

Duncan.

> let me say that in general doing this is pointless.  Due to SSA form,
> if %x is
> set to 5 you can't set it to something else later.  Thus everywhere
> that you
> use %x you might as well just directly use 5 there instead.
But the cost of doing that might be greater than the costs
of keeping it in a register.  Suppose the code was ORing a value
with 5 and the target only had OR-register and not OR-immediate.
You would expect local code generation to handle OR-immediate by
locally materialising the constant into a spare register.
But if the usage was in a loop it would be better (at the cost of
register pressure) to materialise 5 into a register outside of the
loop and use the register repeatedly in the loop.

Al

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