src/dst register for andI_rReg_imm255 and andI_rReg_imm65535

Christian Thalinger Christian.Thalinger at Sun.COM
Tue Sep 22 01:56:07 PDT 2009

Christian Thalinger wrote:
> Tom Rodriguez wrote:
>> On Sep 21, 2009, at 12:06 PM, Christian Thalinger wrote:
>>> Vladimir Kozlov wrote:
>>>> Christian,
>>>> Can you tell more about the problem you see?
>>>> The instructions you pointing use dst register only:
>>>> "movzbl  $dst, $dst\t# int & 0xFF"
>>> Right.  Well, it's not a problem, I just wonder why it does not use  
>>> the
>>> src register:
>>> "movzbl  $dst, $src\t# int & 0xFF"
>>> but instead we have to emit a spill instruction to move src to dst.
>> That will occur if there are other users of src.  If it's the last use  
>> then we shouldn't emit the spill.
> I was looking at some assembly output while testing something else and I
> noticed that it's not the case.  Maybe LoopUnrollLimit=1 changes the way
> code is emitted, but in this small loop there is no other use of src and
> we have a spill:
> 042   B2: #	B2 B3 <- B1 B2 	Loop: B2-B2 inner stride: not constant
> Freq: 1e+06
> 042   	movl    R11, RBP	# spill
> 045   	movzbl  R11, R11	# int & 0xFF
> 049   	addl    RBX, R11	# int
> 04c   	incl    RBP	# int
> 04e   	cmpl    RBP, #2147483647
> 054   	jl,s   B2	# loop end  P=1.000000 C=809121.000000

Seems I'm a bit confused today... of course there is another use of src.
 But what is the reason we need a spill here?  src doesn't get changed
in the movzbl instruction.

-- Christian

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