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Re: [PATCH] PR18002: Undo fold_single_bit_test in do_jump


> It is vital to get beyond the belief that tree-ssa form is related
> to the efficiency of the final machine code.  fold-const.c appears
> to convert expressions into more expensive forms, gimplification
> would appear to introduce large numbers of unnecessary variables and
> trivial assignments between them, spliting loop headers and critical
> blocks would appear to introduce large numbers of empty basic blocks
> and unconditional jumps.
>
> The truth is than none of the above should adversely affect a program's
> performance, and indeed the goal of applying these transformations is
> to improve the program globally, and not just tree-node by tree-node.

"Should" indeed, so this must be carefully monitored.  See PR tree-opt/18707 
for an example of a seamingly harmful form of canonicalization that ends up 
seriously hurting the performance of a loop.

-- 
Eric Botcazou


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