RFR: 8352635: Improve inferencing of Float16 operations with constant inputs [v5]

[Jatin Bhateja]

On Wed, 4 Jun 2025 06:17:23 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Extending tests and review resolutions
>
> src/hotspot/share/opto/convertnode.cpp line 294:
> 
>> 292:       // Conditions under which floating point constant can be considered for a pattern match.
>> 293:       // 1. Constant must lie within Float16 value range, this will ensure that
>> 294:       // we don't unintentially round off float constant to enforce a pattern match.
> 
> What do you mean by `enforce a pattern match`?
> 
> Are you just trying to say that we have to be careful with the pattern matching here, and we cannot just round off the float constant? Do you have an example where that rounding would lead to issues?

import jdk.incubator.vector.*;
public class verify_rounding {
   public static void check() {
       for (int i = 0; i < 65550; i++) {
           short post_rounding = Float.floatToFloat16(Float.float16ToFloat(Float.floatToFloat16((float)i)) * 2049.0f);
           short pre_rounding = Float16.float16ToRawShortBits(Float16.multiply(Float16.valueOf((float)i), Float16.valueOf((float)2049.0f)));
           if (pre_rounding != post_rounding) {
              System.out.println("Mismatch at val = " + (float)i);
              System.out.println("post_rounding val = " + post_rounding);
              System.out.println("pre_rounding val = " + pre_rounding);
              break;
           }
       }
   }

   public static void main(String [] args) {
      check();
   }
}


CPROMPT>java --add-modules=jdk.incubator.vector -cp . verify_rounding
WARNING: Using incubator modules: jdk.incubator.vector
Mismatch at val = 3.0
post_rounding val = 28161
pre_rounding val = 28160

Since we intend to infer Float16 IR using patten match,  hence it may be incorrect to transform post_rounting pattern to pre_rounding.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24179#discussion_r2126295150