[llvm-dev] [cfe-dev] [test-suite] making polybench/symm succeed with "-Ofast" and "-ffp-contract=on"

[Joerg Sonnenberger via llvm-dev]

On Wed, Oct 12, 2016 at 02:35:16PM +0100, Renato Golin via cfe-dev wrote:
> Matching identical outputs to FP tests makes no sense because there's
> *always* an error bar.

That is plainly wrong and a very common misconception about floating
point. A very good example for something that is *required* to give the
very same result all the time is strtod. If a compiler change results in
different output, it is a bug. It is surprisingly difficult to ensure
that, but yes, there are floating point routines where absolute no
change must be added.

This doesn't mean that the rest of the proposal is wrong -- FMA
formation is after all valid inside expresions, so variance is possible.

Joerg