RFR: JDK-8298813: [C2] Converting double to float cause a loss of precision and resulting crypto.aes scores fluctuate

[Tobias Hartmann]

On Thu, 15 Dec 2022 02:52:51 GMT, SUN Guoyun <duke at openjdk.org> wrote:

> Hi all,
> For C2, convert double to float cause a loss of precision,
> 
> <pre><code class="cpp">
> ./chaitin.cpp:221
> _high_frequency_lrg = MIN2(double(OPTO_LRG_HIGH_FREQ), _cfg.get_outer_loop_frequency());
> </code></pre>
> 
> Here, _high_frequency_lrg type is float, so maybe has a loss of precision. when it be used:
> 
> <pre><code class="cpp">
> ./coalesce.cpp:379
> if( lrg._maxfreq >= _phc.high_frequency_lrg() ) {
>    ...
> }
> </code></pre>
> Here, lrg._maxfreq type is double, so _high_frequency_lrg will be convert double again. But now, due to the loss of precision of _high_frequency_lrg, the conditions here may be true or false.
> 
> There are two cases that I tested for SPECjvm2008 crypto.aes.
> case 1:
> <pre><code class="shell">
> //chaitin.cpp:221
> // fcvt.s.d $f0,$f0 #double->float
> d = 16.994714324523816
> f = 16.9947147
> 
> //coalesce.cpp:379
> // fcvt.d.s $f0,$f0 #float->double
> // fcmp.sle.d $fcc2,$f0,$f1
> (gdb) i r fa0
> fa0 {f = 0x0, d = 0x10} {f = -1.08420217e-19, d = 16.994714736938477}
> (gdb) i r fa1
> fa1 {f = 0x0, d = 0x10} {f = -7.68722312e-24, d = 16.994714324523816}
> </code></pre>
> 
> case2:
> <pre><code class="shell">
> //chaitin.cpp:221
> // fcvt.s.d $f0,$f0
> d = 16.996332681816536
> f = 16.9963322
> 
> //coalesce.cpp
> // fcvt.d.s $f0,$f0
> // fcmp.sle.d $fcc2,$f0,$f1
> (gdb) i r fa0
> fa0 {f = 0x0, d = 0x10} {f = -1.08420217e-19, d = 16.996332168579102}
> (gdb) i r fa1
> fa1 {f = 0x0, d = 0x10} {f = -1.73570044e-14, d = 16.996332681816536}
> </code></pre>
> 
> The above two cases result in different block generation（case2 can insert new SpillCopyNodes), and resulting score on cryto.aes is fluctuate.
> 
> This is a patch to fix this problem. Please help review it.
> 
> Thanks,
> Sun Guoyun

This looks reasonable to me but I'm not an expert in that area. I'll run some performance testing for sanity and report back once it passed.

src/hotspot/share/opto/chaitin.hpp line 481:

> 479:   Block **_blks;                // Array of blocks sorted by frequency for coalescing
> 480: 
> 481:   double _high_frequency_lrg;    // Frequency at which LRG will be spilled for debug info

Suggestion:

  double _high_frequency_lrg;   // Frequency at which LRG will be spilled for debug info

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

PR: https://git.openjdk.org/jdk/pull/11685