llvm dev - Mar 2012 - [LLVMdev] Remove subreg copies

Hi,

I'm facing a problem in my BE while trying to remove certain copies. 
Here is a code snippet which I would like to optimize

%vreg1<def> = READF32r; vRRegs:%vreg1
%vreg2<def> = COPY %vreg1:rsub_h; iRSubRegs:%vreg2 vRRegs:%vreg1
%vreg3<def> = COPY %vreg1:rsub_l; iRSubRegs:%vreg3 vRRegs:%vreg1

This code produces subreg-to-subreg copies but I would like to have 
direct uses of vreg1's subregisters instead.
I tried to add RAhints in order to form identity copies (vreg1 and 
vreg2/3 should be allocated to the same sub/super register) and let the 
RA removes them automatically but it didn't work on all the copies. Is 
there another method to get rid of these copies whenever possible ?

Ivan

On Mar 28, 2012, at 7:41 AM, Ivan Llopard <ivanllopard at gmail.com>
wrote:
> Hi,
> 
> I'm facing a problem in my BE while trying to remove certain copies. 
> Here is a code snippet which I would like to optimize
> 
> %vreg1<def> = READF32r; vRRegs:%vreg1
> %vreg2<def> = COPY %vreg1:rsub_h; iRSubRegs:%vreg2 vRRegs:%vreg1
> %vreg3<def> = COPY %vreg1:rsub_l; iRSubRegs:%vreg3 vRRegs:%vreg1
> 
> This code produces subreg-to-subreg copies but I would like to have 
> direct uses of vreg1's subregisters instead.
> I tried to add RAhints in order to form identity copies (vreg1 and 
> vreg2/3 should be allocated to the same sub/super register) and let the 
> RA removes them automatically but it didn't work on all the copies. Is 
> there another method to get rid of these copies whenever possible ?
I assume you are using svn trunk, otherwise I recommend you upgrade.

The output of -debug-only=regalloc should tell you why the register coalescer
isn't eliminating the copies.

/jakob

Hi Jakob,

Le 28/03/2012 18:51, Jakob Stoklund Olesen a écrit :
> On Mar 28, 2012, at 7:41 AM, Ivan Llopard<ivanllopard at gmail.com> 
wrote:
>
>> Hi,
>>
>> I'm facing a problem in my BE while trying to remove certain
copies.
>> Here is a code snippet which I would like to optimize
>>
>> %vreg1<def>  = READF32r; vRRegs:%vreg1
>> %vreg2<def>  = COPY %vreg1:rsub_h; iRSubRegs:%vreg2 vRRegs:%vreg1
>> %vreg3<def>  = COPY %vreg1:rsub_l; iRSubRegs:%vreg3 vRRegs:%vreg1
>>
>> This code produces subreg-to-subreg copies but I would like to have
>> direct uses of vreg1's subregisters instead.
>> I tried to add RAhints in order to form identity copies (vreg1 and
>> vreg2/3 should be allocated to the same sub/super register) and let the
>> RA removes them automatically but it didn't work on all the copies.
Is
>> there another method to get rid of these copies whenever possible ?
> I assume you are using svn trunk, otherwise I recommend you upgrade.
>
> The output of -debug-only=regalloc should tell you why the register
coalescer isn't eliminating the copies.
Thanks ! Looking into the coalescer, I realized I forgot to implement 
the hook getMatchingSuperRegClass(). Now, it works smoothly!

Regards,
Ivan
>
> /jakob
>