[12] RFR(M): 8210215: C2 should optimize trichotomy calculations

[Vladimir Kozlov]

Okay. My point is that such optimization should be handled separately and not part of 
optimize_trichotomy(). You can have multi-inputs Region (not just 2 or 3) with the same If nodes case.

Thanks,
Vladimir

On 10/2/18 12:20 AM, Tobias Hartmann wrote:
> Hi Vladimir,
> 
> On 01.10.2018 20:00, Vladimir Kozlov wrote:
>> We already have this optimization (for no Phi nodes) in RegionNode::Ideal() [1]:
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/2f1698b6db15/src/hotspot/share/opto/cfgnode.cpp#l432
>>
>> I don't think we need to duplicate it here - just return false. Did you found that it does not work
>> in your case?
> 
> That optimization only works if the region has no phi users but for shape 2, the region *has* a phi
> user. We can only remove the if because we know that the phi is merging the same value 'x':
> 
>        If
>       /  \
>    Proj  Proj
>      \    /  [...]
>       \  /   /
>       Region1
>        |
>        |  x x  y
>         \ | | /
>           Phi
> 
> For example, without this, TestTrichotomyExpressions::testEqual7 is not optimized. Below is the
> ideal graph for this test, node 41 is the corresponding region and 44 is the useless if because the
> only phi user 71 returns the same value for inputs 1 and 2.
> 
> Thanks,
> Tobias
> 
> 
> Ideal graph for TestTrichotomyExpressions::testEqual7:
> 
>   0	Root	===  0  65  [[ 0  1  3  40  49  51 ]] inner
>   11	Parm	===  3  [[ 33 ]] Parm1: int !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   10	Parm	===  3  [[ 33 ]] Parm0: int !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   33	CmpI	=== _  10  11  [[ 34  43 ]]  !jvms: TestTrichotomyExpressions::compare7 @ bci:2
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   34	Bool	=== _  33  [[ 35 ]] [ne] !jvms: TestTrichotomyExpressions::compare7 @ bci:2
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   5	Parm	===  3  [[ 35 ]] Control !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   43	Bool	=== _  33  [[ 44 ]] [lt] !jvms: TestTrichotomyExpressions::compare7 @ bci:11
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   36	IfTrue	===  35  [[ 44 ]] #1 !jvms: TestTrichotomyExpressions::compare7 @ bci:2
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   35	If	===  5  34  [[ 36  37 ]] P=0.666667, C=98484.000000 !jvms:
> TestTrichotomyExpressions::compare7 @ bci:2 TestTrichotomyExpressions::testEqual7 @ bci:2
>   44	If	===  36  43  [[ 45  46 ]] P=0.500000, C=65656.000000 !jvms:
> TestTrichotomyExpressions::compare7 @ bci:11 TestTrichotomyExpressions::testEqual7 @ bci:2
>   51	ConI	===  0  [[ 71 ]]  #int:1
>   40	ConI	===  0  [[ 71  71 ]]  #int:0
>   3	Start	===  3  0  [[ 3  5  6  7  8  9  10  11 ]]  #{0:control, 1:abIO, 2:memory, 3:rawptr:BotPTR,
> 4:return_address, 5:int, 6:int}
>   37	IfFalse	===  35  [[ 41 ]] #0 !jvms: TestTrichotomyExpressions::compare7 @ bci:2
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   45	IfTrue	===  44  [[ 41 ]] #1 !jvms: TestTrichotomyExpressions::compare7 @ bci:11
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   46	IfFalse	===  44  [[ 41 ]] #0 !jvms: TestTrichotomyExpressions::compare7 @ bci:11
> TestTrichotomyExpressions::testEqual7 @ bci:2
>   71	Phi	===  41  40  40  51  [[ 65 ]]  #bool !orig=[63] !jvms: TestTrichotomyExpressions::testEqual7
> @ bci:13
>   9	Parm	===  3  [[ 65 ]] ReturnAdr !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   8	Parm	===  3  [[ 65 ]] FramePtr !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   7	Parm	===  3  [[ 65 ]] Memory  Memory: @BotPTR *+bot, idx=Bot; !jvms:
> TestTrichotomyExpressions::testEqual7 @ bci:-1
>   6	Parm	===  3  [[ 65 ]] I_O !jvms: TestTrichotomyExpressions::testEqual7 @ bci:-1
>   41	Region	===  41  46  45  37  [[ 41  65  71 ]]  !jvms: TestTrichotomyExpressions::compare7 @
> bci:19 TestTrichotomyExpressions::testEqual7 @ bci:2
>   65	Return	===  41  6  7  8  9 returns 71  [[ 0 ]]
>