[llvm-commits] PATCH: Teach LLVM to form minimal multiply DAGs

[Chandler Carruth]

Hello,

Richard and I hacked up a simple patch to form minimal multiply DAGs when
intermediate results are unused. The code should handle fully generic
formation of a minimal DAG of multiplies, sharing intermediate values
where-ever possible. Check out the test cases for some of the fun
simplifications it performs.

The only real problem is where to put it. Currently I've put it in
InstCombine, but not because that's really a good home for it (in fact, it
can't be the home, read on). The most "natural" home for this type of
transform is the Reassociate pass, but that has its own problems....

The reassociate pass does a really nice job of canonicalizing chains of
operations into left-linear trees that fully expose each intermediate
operation for CSE and combining with other operations. This is a really
nice property that seems a shame to loose. If we form the minimal multiply
DAG in reassociate  eagerly, we may miss chances to combine non-minimal
intermediate steps with other operations.

However, if we use a cleanup pass like instcombine to mop up one-use
left-linear trees like this patch does, we can easily get Reassociate and
InstCombine dueling over the canonical form, and into an inf-loop we go.

One possible fix is to have InstCombine form a left-linear tree after it
forms the minimal folding to try to keep Reassociate from undoing its
work... but this all feels a bit hackish. Especially the
deferred-processing hack required to ensure InstCombine performs this
optimization on the entire tree at once. Suggestions?
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