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Re: [patch] Fold x + x to x * 2


> From: Paul Schlie
> 
>> Zdenek Dvorak wrote:
>> 
>> during investigation of PR 17549 I noted that we do not fold x + x to x
>> * 2.  We also do not fold x * c + x to x * (c + 1).  (For non-floats.
>> For floats we do both, under some assumptions.)  In the case of PR
>> 17549, the ivopts end up producing expressions of type x+x+...+x with as
>> much as 8 operands (the expressions are then somehow folded in RTL
>> optimizers, I think, so we do not end up producing these sums and it
>> does not affect size of the final code, but still this behavior is quite
>> weird).  This patch implements these transformations.
> 
> How is perceived that (2 * x) is preferable to (x + x), as in general
> it's most likely not?
> 
> even (x + x + x + x) would guess should likely only be target neutrally folded
> to (t = (x + x), (t + t)), not (4 * x); where then based on the target
> machine, likely to (x << 2) or (x * 4), but not before.
> 
> Do I misunderstand?

Or is it presumed that multiplication is idealized GCC canonical form
where possible, and all machine descriptions need to define more optimal
conversions to alternate forms were preferable? (is this documented?)




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