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Re: Unnecessary PRE optimization


On 12/23/2009 03:05 PM, Joern Rennecke wrote:

So if this is only useful for a limited set of targets, why isn't it controlled by an option or a target hook so that it is only turned on on the targets where it is deemed to make sense overall?

Well, this optimization is basically the opposite of loop-invariant motion, so there is some merit in not doing it: you could also tweak loop-invariant motion to not hoist unnecessarily (and increase register pressure unnecessarily) instead of relying on fwprop undoing it. Indeed in trunk loop-invariant motion is not hoisting cheap addresses anymore. Bingfeng is seeing the problem because in his case PRE is doing the hoisting rather than LIM.


Paolo


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