RFR: 8285973: x86_64: Improve fp comparison and cmove for eq/ne [v3]

[Quan Anh Mai]

On Sat, 21 May 2022 10:31:25 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

>> Hi,
>> 
>> This patch optimises the matching rules for floating-point comparison with respects to eq/ne on x86-64
>> 
>> 1, When the inputs of a comparison is the same (i.e `isNaN` patterns), `ZF` is always set, so we don't need `cmpOpUCF2` for the eq/ne cases, which improves the sequence of `If (CmpF x x) (Bool ne)` from
>> 
>>     ucomiss xmm0, xmm0
>>     jp      label
>>     jne     label
>> 
>> into
>> 
>>     ucomiss xmm0, xmm0
>>     jp      label
>> 
>> 2, The move rules for `cmpOpUCF2` is missing, which makes patterns such as `x == y ? 1 : 0` to fall back to `cmpOpU`, which have a really high cost of fixing the flags, such as
>> 
>>         xorl    ecx, ecx
>>         ucomiss xmm0, xmm1
>>         jnp     done
>>         pushf
>>         andq    [rsp], 0xffffff2b
>>         popf
>>     done:
>>         movl    eax, 1
>>         cmovel  eax, ecx
>> 
>> The patch changes this sequence into
>> 
>>     xorl    ecx, ecx
>>     ucomiss xmm0, xmm1
>>     movl    eax, 1
>>     cmovpl  eax, ecx
>>     cmovnel eax, ecx
>> 
>> 3, The patch also changes the pattern of `isInfinite` to be more optimised by using `Math.abs` to reduce 1 comparison and compares the result with `MAX_VALUE` since `>` is more optimised than `==` for floating-point types.
>> 
>> The benchmark results are as follow:
>> 
>>                                                      Before              After
>>     Benchmark                      Mode  Cnt     Score     Error    Score     Error   Unit   Ratio
>>     FPComparison.equalDouble       avgt    5  2876.242 ±  58.875  594.636 ±   8.922  ns/op    4.84
>>     FPComparison.equalFloat        avgt    5  3062.430 ±  31.371  663.849 ±   3.656  ns/op    4.61
>>     FPComparison.isFiniteDouble    avgt    5   475.749 ±  19.027  518.309 ± 107.352  ns/op    0.92
>>     FPComparison.isFiniteFloat     avgt    5   506.525 ±  14.417  515.576 ±  14.669  ns/op    0.98
>>     FPComparison.isInfiniteDouble  avgt    5  1232.800 ±  31.677  621.185 ±  11.935  ns/op    1.98
>>     FPComparison.isInfiniteFloat   avgt    5  1234.708 ±  70.239  623.566 ±  15.206  ns/op    1.98
>>     FPComparison.isNanDouble       avgt    5  2255.847 ±   7.238  400.124 ±   0.762  ns/op    5.64
>>     FPComparison.isNanFloat        avgt    5  2567.044 ±  36.078  546.486 ±   1.509  ns/op    4.70
>> 
>> Thank you very much.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comments

Thank you very much for your reviews and testing.

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

PR: https://git.openjdk.java.net/jdk/pull/8525