[Bug middle-end/19154] miss-optimization of (x & pow2C) avr conditionals returning bool equivalent values

[schlie at comcast dot net]

------- Additional Comments From schlie at comcast dot net  2004-12-25 21:39 -------
Subject: Re:  miss-optimization of (x & pow2C) avr
 conditionals returning bool equivalent values

> I think the issue here is rtx_cost for avr sucks and does not give a good
> accurate cost matrix.

I considered that, and sure it doesn't help; but the form of the conditional
presented for code generation is actually different in otherwise identical
expressions, therefore implying that it's presumed that:

 if ((x >> C') & 1) 1 else 0;  => ((x >> C') & 1)

Is more efficient than:

 if ((x >> C') & 1) 1 else 0; => if (x & C) 1 else 0;

Which is target specific, and should not be presumed, as for targets which
support bit-test&branch:

 if (x & C) 1 else 0; => bit-test&branch x log2C; return 0; return 1;

Is significantly more efficient than a (multi-bit-shift & 1).

(therefore it doesn't seem proper for such return value dependant
assumptions to be made in a target neutral way)





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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19154