RFR: 8264020: Optimize double negation elimination

[Tobias Hartmann]

On Sun, 28 Mar 2021 01:36:56 GMT, Eric Liu <github.com+10482586+theRealELiu at openjdk.org> wrote:

> For the double negation '-(-x)', c2 could reduce the redundant 'sub'
> through SubNode::Ideal. But every time when 'sub' is removed, there
> would generate two useless nodes SubNode(x, 0)[1] and AddNode(x, -0)[2],
> which would be removed by later phases but should have optimized better
> to 'x' itself without generating those new stuffs.
> 
> This patch is a small fix for SubNode's Ideal, by leaving this special
> case to 'Identity' which handled double negation already[3].
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L238
> [2] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L181
> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L55

Otherwise, looks good to me.

src/hotspot/share/opto/subnode.cpp line 239:

> 237: 
> 238:   // Convert "0 - (x-y)" into "y-x", leave the double negation "-(-y)" to SubNode::Identity().
> 239:   if(t1 == TypeInt::ZERO && op2 == Op_SubI && phase->type(in2->in(1)) != TypeInt::ZERO)

Since you are modifying that code, please also add parentheses around the body, update the code style and add a whitespace between `if` and the condition.

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Marked as reviewed by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3231