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

[Martin Doerr]

On Wed, 28 Jan 2026 09:45:14 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> David Briemann has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   address review comments
>
> src/hotspot/cpu/ppc/c2_MacroAssembler_ppc.cpp line 684:
> 
>> 682:     xscmpeqdp(tmp, op1, op2);
>> 683:     xxsel(dst, first, second, tmp);
>> 684:     break;
> 
> This looks incorrect to me.
> 
> Also when I compare it with the disassembly of the C version[1]:
> 
> 
> double cmovf_eq(double op1, double op2, double src1, double src2) {
>     return op1 == op2 ? src1 : src2;
> }
> 
> cmovf_eq(double, double, double, double):
>         xscmpeqdp 1,2,1
>         xxsel 1,4,3,1
>         blr
> 
> 
> `cc` would be 0xA for `==` (looking at `operand cmpOp`[2]), right?
> 0xA is 0b1010. `exchange` would be 0 but I think `src1` and `src2` need to be exchanged.
> 
> Assume `op1` and `op2` are indeed equal in `op1 == op2 ? src1 : src2`. `tmp.dword[0]` will be set to 0xFFFF_FFFF_FFFF_FFFF by the `xscmpeqdp`. 
> 
> `xxsel` evaluates `(src1 & ~tmp) | (src2 & tmp)` so for the correct result `src1` and `src2` need to be swapped as also seen in the disassembly above.
> 
> [1] [disassembly of the C version](https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGEgOykrgAyeAyYAHI%2BAEaYxCAArKQADqgKhE4MHt6%2BASlpGQKh4VEssfFJdpgOmUIETMQE2T5%2BXIFVNQJ1DQTFkTFxibb1jc25bcM9faXliQCUtqhexMjsHOYAzGHI3lgA1CYbbgoE%2BIIAdAiH2CYaAIKb27uYB0cn%2BKiX17cP9%2BhL0Xoe2QLFQADcqAB9TAARwg/y8gJeqGSXFIewRSL2KLM6MxQIUKzRGIBBJWZjmB38Vnuezpe2ImAIywY2NRrwAIocOWyzK8AGJ7QnILh7EBC8mHGkPfxc%2B4/fEvEHgqHheGk5GovEa3naxFkkV6rHCilU6X0hlMlls0VgMDc3kCiUisXOsxSn4mWWev465UQyHATDq/Wa4mK3Uk0PO8M6k2Ur3m%2BmM5nEVko0XfDY8nFO4Wi8Umj3y73y33R/1Q2gEENYjNGoE4hsvfPNt0J6k/C0p60Z15uR2HQX511FjbSr0cjgLWicBK8PwcLSkVCcNzWaxCpYrF6bHikAiaacLADWIA2CXOAA4AJwANhvGivd7MN/8t4SgVnHEkC6PK84XgFBADQDyPBY4FgJA0BYZI6DichKBguD6HiZJkmQO9JHoYACGILwGBPPg6AIOJgIgaJ/2iMIGgAT04fdqOYYhaIAeWibRqkPbheBgthBFYhhaHopdeCwaIvGANwxFoYCeNILAWEMYBxFEhS8EZGowUwOTl0wVRqi8UiGN4MJSO/ZdaDwaJiDojwsH/PC8BYEzSG04hojSTAOUwJSjCsoxwL4AxgAUAA1PBM
 AAd1Y5JGFc/hBBEMR2CkGRBEUFR1DU3Q0QMQLTEsax9Gs4DIAWFFHAEOSAFpWI2PYapYZBki8blUiiuIuGXcE4mIPAsDKiAFg6Kq/AgVwxlaIIGHQaYBniNFUnSMapr0ZbCgYeaykGNFRtqEYmk8Fo9H2rpDu22Y9sOtbrqmMJ%2Bh2xaRu3VYJBnOc/zU1cOD2Dq4iwoFgGQZA9jwgiTz2CBcEIEgDjMDYuDmXhuK0OZT3PDRzn8B9/ASK8NA2O8r38fw8s4X9SEXHrANsECwNEiCYEQEAlgIVqCEQiBkPg4gIlYNZVGfGqsL2YHQYgcHCORoJ8CIfr0D0RLhFEcQ0uVzK1H/XLSCi2zkhMj6OHnKn/x%2B1ijI57EqD%2B1BOuIQGXnFsH8MIqGPFg3n4cRmXUePUgzw2DZziD0Ow/D/QKa%2BmmOCA%2Bm/fRyOODMaPeB%2BlHwIWdz0mcSQgA)
> 
> [2] [`operand cmpOp`](https://github.com/openjdk/jdk/blob/4ae4ffd5a3114aa2a3832818ee30dc38d9aa2b72/src/hotspot/...

Please note that C2 generates code which is equivalent to your C version:

xscmpeqdp vs0,vs1,vs2
xxsel   vs1,vs4,vs3,vs0

It's correct because the condition is already inverted by C2 (cc == 2). But, the code is very confusing. I agree with that it should be improved.

Btw. what the Spec suggests also seems to use the wrong order:

xscmpeqdp can be used to implement the C/C++/Java conditional operation, RESULT = (x=y) ? a:b.
xscmpeqdp fEQ,fX,fY
xxsel fRESULT,fA,fB,fEQ

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/29281#discussion_r2737516069