RFR: 8291336: Add ideal rule to convert floating point multiply by 2 into addition [v2]

[SuperCoder79]

On Wed, 27 Jul 2022 15:54:01 GMT, Quan Anh Mai <duke at openjdk.org> wrote:

>> SuperCoder79 has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Apply changes from code review and improve benchmark
>
> src/hotspot/share/opto/mulnode.cpp line 439:
> 
>> 437: // Check to see if we are multiplying by a constant 2 and convert to add, then try the regular MulNode::Ideal
>> 438: Node *MulFNode::Ideal(PhaseGVN *phase, bool can_reshape) {
>> 439:   const TypeF *t1 = in(1)->bottom_type()->isa_float_constant();
> 
> `phase->type(Node*)` refers to the type inferred by the GVN in this phase, while `Node::bottom_type()` refers to the loosest type the node can have. For example, the bottom type of an `AddINode` is always `TypeInt::INT` (every `int` value possible) while the GVN can ensure a stricter type if it knows both the inputs are integers between 0 and 10. In this case, you obtain the correct type nonetheless because `ConNode` extends `TypeNode`, a family of nodes which has their bottom types updated by the GVN. In general, in idealisation, it is more efficient to use `phase->type(Node*)`.

Thank you for this perspective! I have updated the code to use `phase->type()`.

> src/hotspot/share/opto/mulnode.cpp line 442:
> 
>> 440:   const TypeF *t2 = in(2)->bottom_type()->isa_float_constant();
>> 441: 
>> 442:   // x * 2 -> x + x
> 
> Since constants are always pushed to the right of the expression, you don't need to try both permutations of the pattern.

Done, thanks!

> test/hotspot/jtreg/compiler/c2/irTests/TestMulBy2.java line 37:
> 
>> 35:  * @run driver compiler.c2.irTests.TestMulBy2
>> 36:  */
>> 37: public class TestMulBy2 {
> 
> Please use a more general name such as `MulFNodeIdealizationTests`

Done

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PR: https://git.openjdk.org/jdk/pull/9642