llvm dev - Sep 2015 - IR question re: constants

I'm curious.  Is there a specific technical reason that the high-level
IR doesn't have an instruction to simply set a register to a constant?
This limitation means we cannot hoist an "expensive" constant out of a
loop until we lower the IR to MachineInstrs.  Or more to the point, it
forces backends to undo any such hoisting done by frontends.

Or am I not understanding something?

                           -David

On 9/2/2015 1:16 PM, via llvm-dev wrote:
> I'm curious.  Is there a specific technical reason that the high-level
> IR doesn't have an instruction to simply set a register to a constant?
> This limitation means we cannot hoist an "expensive" constant out
of a
> loop until we lower the IR to MachineInstrs.  Or more to the point, it
> forces backends to undo any such hoisting done by frontends.
>
> Or am I not understanding something?
The LLVM IR doesn't have registers.  The computations are values and a 
constant represents itself.

-Krzysztof

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On 9/2/2015 1:24 PM, Krzysztof Parzyszek via llvm-dev
wrote:
>
> The LLVM IR doesn't have registers.  The computations are values and a
> constant represents itself.
On that note, you can hoist any constant yourself:

Define:
   %blah = llvm.my.own.intrinsic C
Then have:
	... = %blah
And lower the intrinsic to the value of its argument.

Alternatively, define
   %blah = select i1 true, C, 0.
The instruction selection happens on a basic-block basis, so if that 
"select" is in its own block, it will not be propagated into other 
blocks at that time.

There are limitations to each such approach though. In the above cases, 
the intrinsic won't be "understood" by the optimizer, but it can
be
considered expensive.  The select will be folded back into the constant 
if you do it too early, but this will allow further simplifications.

-Krzysztof

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Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation