RFR: 8375536: PPC64: Implement special MachNodes for floating point CMove [v2]

[Richard Reingruber]

On Thu, 29 Jan 2026 11:55:03 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> Thanks, I was looking at 3.1B which doesn't have the programming note.
>> 
>> Thanks also for the example. The trick to get the cmove is that the condition mustn't be true or false all the time.
>> 
>> We should also have OptoAssembly that shows the condition and the registers.
>> For testing I've added
>> 
>>   format %{ "cmovD_cmpD  $dst, $op1 $cop $op2, $src1, $src2\n\t" %}
>> 
>> My testmethod
>> 
>>     static double dontinline_cmovf(double op1, double op2, double src1, double src2) {
>>         return op1 == op2 ? src1 : src2;
>>     }
>> 
>> produces this OptoAssembly
>> 
>> ============================= C2-compiled nmethod ==============================
>> #r090 F0:F0   : parm 0: double <- note: register names are incorrect
>> #r088 F0:F0   : parm 2: double
>> #r086 F0:F0   : parm 4: double
>> #r084 F0:F0   : parm 6: double
>> #r283 R1+76: old out preserve
>> #r282 R1+72: old out preserve
>> // ...
>> 028     cmovD_cmpD  F1, F1 ne F2, F3, F4
>> 
>> testing...
>> 
>>         res = dontinline_cmovf(1d, 2d, 3d, 4d);
>>         System.out.println(op1 + " == " + op2 + " ? " + src1 + " : " + src2);
>>         System.out.println("-> " + res);
>> 
>> 1.0 == 2.0 ? 3.0 : 4.0
>> -> 4.0
>> 
>> Very weired: the condition `==` is flipped to `ne` but `src1` and `src2` are not swapped since 4.0 is in `F4` which is `src2` according to the OptoAssembly above.
>> 
>> I would have expected
>> 
>> 
>> 028     cmovD_cmpD  F1, F1 ne F2, F4, F3
>> 
>> Is there an explanation?
>
> The condition is adjusted for `CMove` with match rules like `match(Set dst (CMoveD (Binary cop cr) (Binary dst src)));`. If the condition is `false`, the value `dst` is kept. If the condition is `true`, the value `src` is used. So, the comment for PPC64 should actually say "dst = (op1 cmp(cc) op2) ? src2 : src1;".

Are you saying the condition is only adjusted if `dst` and `src1` are identical? Probably not...

I guess this originates from L44, L45. below The left tree is associated with false and the right with true.

```c++
  38	  CMoveNode( Node *bol, Node *left, Node *right, const Type *t ) : TypeNode(t,4)
  39	  {
  40	    init_class_id(Class_CMove);
  41	    // all inputs are nullified in Node::Node(int)
  42	    // init_req(Control,nullptr);
  43	    init_req(Condition,bol);
  44	    init_req(IfFalse,left);
  45	    init_req(IfTrue,right);
  46	  }


So we cannot think of a CMoveNode as a ternary operator.

What about adapting the match rules swapping src1 and src2 and implemeting `op1 cond op2 ? src1 : src2`?
This would be less confusing then implementing `op1 cond op2 ? src2 : src1`

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PR Review Comment: https://git.openjdk.org/jdk/pull/29281#discussion_r2741383030