llvm dev - Sep 2014 - [LLVMdev] Converting back to SSA form

Hi,

I'm wondering how I can convert "register allocated" code back to
SSA form.
I realized from MachineRegisterInfo.h that a function leaves SSA form only
once and cannot be taken back to it.
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On 9/23/14, 10:52 AM, Rahman Lavaee wrote:
> Hi,
>
> I'm wondering how I can convert "register allocated" code
back to SSA
> form. I realized from MachineRegisterInfo.h that a function leaves SSA 
> form only once and cannot be taken back to it.
Are you wanting to put MachineInstr's into SSA form?  If you want to do 
that, you'll need to implement the SSA construction algorithm yourself 
so that it works on MachineInstrs.  Also, such a pass will need to work 
with virtual registers as you'll need an unlimited supply of temporary 
variables.  In essence, you'll be undoing register allocation.

Can you describe why you want to put MachineInstrs into SSA form?  I 
suspect that putting MachineIntrs back into SSA form is not what you 
want to do.

Regards,

John Criswell

Thanks John,

The reason I want to do this is that register allocator works only on SSA
form, and if you instrument regallocated code with non-regallocated machine
instructions, then you cannot regallocate the result.

A workaround is to assign physical registers while doing the
instrumentation, which I don't think is as easy as the above.

On Tue, Sep 23, 2014 at 11:01 AM, John Criswell <jtcriswel at gmail.com>
wrote:
> On 9/23/14, 10:52 AM, Rahman Lavaee wrote:
>
>> Hi,
>>
>> I'm wondering how I can convert "register allocated" code
back to SSA
>> form. I realized from MachineRegisterInfo.h that a function leaves SSA
form
>> only once and cannot be taken back to it.
>>
>
> Are you wanting to put MachineInstr's into SSA form?  If you want to do
> that, you'll need to implement the SSA construction algorithm yourself
so
> that it works on MachineInstrs.  Also, such a pass will need to work with
> virtual registers as you'll need an unlimited supply of temporary
> variables.  In essence, you'll be undoing register allocation.
>
> Can you describe why you want to put MachineInstrs into SSA form?  I
> suspect that putting MachineIntrs back into SSA form is not what you want
> to do.
>
> Regards,
>
> John Criswell
>
>
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