[14] RFR (XS): 8234392: C2: Extend Matcher::match_rule_supported_vector() with element type information

[Vladimir Ivanov]

> Yes, fully agree. Updated version:
>    http://cr.openjdk.java.net/~vlivanov/8234392/webrev.01/
> 
> Got rid of ret_value and enhanced the comments, but also fixed 
> long-standing bug you noticed: AbsVF should have the same additional 
> checks as NegVF (since 512bit vandps is also introduced in AVX512DQ).

Additionally, got rid of ret_value in Matcher::match_rule_supported() 
and moved Op_RoundDoubleModeV there since it doesn't depend on vector 
length:

+    case Op_RoundDoubleModeV:
+      if (VM_Version::supports_avx() == false) {
+        return false; // 128bit vroundpd is not available
+      } break;

Best regards,
Vladimir Ivanov

> On 11.12.2019 02:31, John Rose wrote:
>> Actually I have one more comment about the new classification logic:
>>
>> The “ret_value” idiom is terrible.
>>
>> I see a function which is complex, with something like this at the top:
>>
>>    if (… simple size check …) {
>>      ret_value = false;   // size not supported
>>    } …
>>
>> I want to read something more decisive like this:
>>
>>    if (… simple size check …) {
>>      return false;   // size not supported
>>    } …
>>
>> The “ret_value” thingy adds only noise, and no clarity.
>>
>> It is more than an annoyance, hence my comment here.
>> The problem is that if I want to understand the quick check
>> above, I have to scroll down *past all the other checks* to see
>> if some joker does “ret_value = true” before the “return ret_value”,
>> subverting my understanding of the code.  Basically, the “ret_value”
>> nonsense makes the code impossible to break into understandable
>> parts.
>>
>> — Grumpy John
>>
>> On Dec 10, 2019, at 12:34 PM, John Rose <john.r.rose at oracle.com 
>> <mailto:john.r.rose at oracle.com>> wrote:
>>>
>>>> It would severely penalize Xeon Phis (which lack BW, DQ, and VL 
>>>> extensions), but maybe there'll be a moment when Skylake 
>>>> (F+CD+BW+DQ+VL) can be chosen as the baseline.
>>>
>>> Yeah, I saw that coming after I visited the trusty intrinsics guide
>>> https://software.intel.com/sites/landingpage/IntrinsicsGuide/
>>>>
>>>> Anyway, I got your idea and it makes perfect sense to me to collect 
>>>> such ideas.
>>>
>>