[PATCH] Reassociate in favor of grouping previously paired operands

[Daniel Berlin]

For reference, can you explain the difference between this and the
last patch I posted that does the same thing?


On Mon, Jun 8, 2015 at 4:04 PM, Xuetian Weng <xweng at google.com> wrote:
> Hi dberlin, meheff, majnemer, jingyue,
>
> This patch enhances Reassociate to group operands that are paired
> previously. With this patch, Reassociate can expose in general more
> opportunities for later redundancy elimination. For example,
>
>   foo(a + c);
>   foo(a + (b + c));
>
> Previously,
>
>   t1 = a + c;
>   foo(t);
>   t2 = b + c;
>   t3 = a + t2;
>   foo(t3); // using 5 instructions
>
> With this patch, since a, c are paired together in t1, later instruction
> will try to pair a, c together so it can be optimized to
>
>   t1 = a + c;
>   foo(t1);
>   t2 = t1 + b;
>   foo(t2); // using 4 instructions
>
> We implement this idea leveraging a data structure called
> LastOperandPairsMap that stores the most recently paired operands. It is
> indexed by a Value and its corresponding Opcode in this pair.
>
> For example, LastOperandPairsMap[x][op] = <y, I> means there is an instruction I
> in form of (op x, y) or or (op y, x) in the current block.
>
> Each time when function ReassocaiteExpression sorts the operands of an
> expression, it searches the map for any pair of leaves that have been
> reassociated recently, and moves such pair (if any) to the end of the
> operand list so that they are guaranteed to be grouped together by function
> RewriteExprTree later.
>
> Note that this optimization overlaps quite a bit with NaryReassociate.
> However, since NaryReassociate requires ScalarEvolution and is a
> iterative algorithm, NaryReassociate is much more expensive than the
> algorithm introduced in this patch and thus cannot be run as pervasively
> as Reassociate. This optimization allows Reassociate to cheaply handle
> many cases NaryReassociate is designed for.
>
> http://reviews.llvm.org/D10327
>
> Files:
>   lib/Transforms/Scalar/Reassociate.cpp
>   test/Transforms/Reassociate/pair.ll
>
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