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

[Vladimir Kozlov]

On Wed, 4 May 2022 19:32:41 GMT, Vladimir Kozlov <kvn at openjdk.org> 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:
>>     Benchmark                      Mode  Cnt     Score     Error  Units
>>     FPComparison.equalDouble       avgt    5  2876.242 ±  58.875  ns/op
>>     FPComparison.equalFloat        avgt    5  3062.430 ±  31.371  ns/op
>>     FPComparison.isFiniteDouble    avgt    5   475.749 ±  19.027  ns/op
>>     FPComparison.isFiniteFloat     avgt    5   506.525 ±  14.417  ns/op
>>     FPComparison.isInfiniteDouble  avgt    5  1232.800 ±  31.677  ns/op
>>     FPComparison.isInfiniteFloat   avgt    5  1234.708 ±  70.239  ns/op
>>     FPComparison.isNanDouble       avgt    5  2255.847 ±   7.238  ns/op
>>     FPComparison.isNanFloat        avgt    5  2567.044 ±  36.078  ns/op
>> 
>>     After:
>>     Benchmark                      Mode  Cnt     Score     Error  Units
>>     FPComparison.equalDouble       avgt    5   594.636 ±   8.922  ns/op
>>     FPComparison.equalFloat        avgt    5   663.849 ±   3.656  ns/op
>>     FPComparison.isFiniteDouble    avgt    5   518.309 ± 107.352  ns/op
>>     FPComparison.isFiniteFloat     avgt    5   515.576 ±  14.669  ns/op
>>     FPComparison.isInfiniteDouble  avgt    5   621.185 ±  11.935  ns/op
>>     FPComparison.isInfiniteFloat   avgt    5   623.566 ±  15.206  ns/op
>>     FPComparison.isNanDouble       avgt    5   400.124 ±   0.762  ns/op
>>     FPComparison.isNanFloat        avgt    5   546.486 ±   1.509  ns/op
>> 
>> Thank you very much.
>
> src/hotspot/cpu/x86/x86_64.ad line 6998:
> 
>> 6996:   ins_encode %{
>> 6997:     __ cmovl(Assembler::parity, $dst$$Register, $src$$Register);
>> 6998:     __ cmovl(Assembler::notEqual, $dst$$Register, $src$$Register);
> 
> Should this be `equal`?

I see that you swapped `src, dst` in `match()` but `format` is sill incorrect and the code is confusing.

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