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

[SUN Guoyun]

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

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

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

Changes: https://git.openjdk.org/jdk/pull/11685/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11685&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298813
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11685.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11685/head:pull/11685

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