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cmd/compile: add reassociate pass to find better optimization opportunities within commutative expressions #60015

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@ryan-berger ryan-berger commented May 5, 2023 •
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[WIP] - separating out ILP work and reassociation work

reassociation should help (a + (b + (a + (b + (a + (b + a))))) get optimized to (4a + 4b),

ilp should help (a + (b + (c + d))) turn into (a + b) + (c + d)

ryan-berger added 2 commits May 5, 2023 16:31
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cmd/compile: add reassociate ssa pass to rebalance commutative operat…
c69b20d 
…ions

Currently the compiler groups expressions with commutative operations
such as a + b + c + d as so: (a + (b + (c + d))) which is suboptimal for
CPU instruction pipelining.

This pass balances commutative expressions as shown above to (a + b) + (c + d)
to optimally pipeline them. It also attempts to reassociate constants to as far
right of the commutative expression as possible for better constant folding
opportunities.

Below is a benchmark from crypto/md5 on an MacBook Pro M2:

          trunk          reassociate
Hash1K-8  433.7Mi ± 0%   499.4Mi ± 4%  +15.17% (p=0.000 n=10)
Hash8K-8  454.3Mi ± 1%   524.9Mi ± 1%  +15.53% (p=0.000 n=10)
....
geomean   284.4Mi        327.5Mi       +15.15%

Other CPU architectures tried showed very little change (+/-1%) on
this particular benchmark but tight mathematical code stands to gain
greatly from this optimization

Fixes golang#49331
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Patch Set 3:

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Message from Keith Randall:

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Message from Ryan Berger:

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Message from Ryan Berger:

Patch Set 6:

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Message from Keith Randall:

Patch Set 6: Hold+1

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@ryan-berger ryan-berger changed the title cmd/compile: add reassociate pass to rebalance commutative operations cmd/compile: add reassociate pass to find better optimization opportunities within commutative expressions May 18, 2023
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