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Re: [PATCH] Small optimization for emit_case_bit_tests (PR tree-optimization/63464)

On Fri, Oct 10, 2014 at 12:55:21PM +0200, Richard Biener wrote:
> On Fri, 10 Oct 2014, Jakub Jelinek wrote:
> > This patch adds a small optimization to emit_case_bit_tests,
> > instead of emitting (for high, low, mask all constants)
> > (x - low) <= (high - low) && ((1 << (x - low)) & mask)
> > if high is smaller than BITS_PER_WORD and low > 0 we can emit
> > x <= high && ((1 << x) & (mask << low))
> > and avoid subtraction.  Do this only if mask << low isn't more costly than
> > mask.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> But isn't this a general optimization?

The && always has to mean separate basic blocks, as otherwise it is
undefined behavior.  I'd think this optimization would be too specialized
for a general optimization, and unsure in which pass it would be desirable.

> Also testing for RTX costs this early sounds bogus.

Well, the bit test optimization is already decided based on other rtx costs.


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