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Re: Activate -mrecip with -ffast-math?
On Jun 18, 2007, at 2:27 PM, Bradley Lucier wrote:
But even if sqrt is corrected for 0.0 * inf, there would still be
a lot of problems with the combinations of NR-enhanced rsqrt and
rcp. Consider for example:
1.0/sqrt(a/b) alias rsqrt(a/b)
Having a=0, b != 0, the result is inf.
As already stated, -ffast-math turns on -ffinite-math-only, which
allows the compiler to assume that a result of inf cannot happen,
so gcc is allowed to ignore this possiblity. Producing NaN instead
of inf seems to be allowed.
Let me restate this.
If -ffinite-math-only is specified, then producing NaN instead of inf
should be allowed.
If -fno-finite-math-only is specified, then the generated code should
"do the right thing" if an argument or result is inf or NaN.
In any case, I would consider it an error if the argument is finite,
the result is supposed to be finite, and inf or NaN is produced.
Brad