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0 pointstitzer5y ago0 comments

No, they consistently perform optimizations that violate IEEE 754 rules, such as reassociation and commutation of expressions. These are difficult to observe (unless you are in the habit of observing -0 vs 0), but yes, you can observe them. They are very precisely specified by IEEE 754 and C/C++ compilers perform illegal optimizations.

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jcranmer5y ago

They do those optimizations when you enable fast-math flags. Some compilers (e.g., icc) enable those by default. When fast-math is not enabled, then the IEEE 754-violating optimizations are disabled.

Source: I've been working on such optimizations this past week. And that means establishing precisely which flags I need to have enabled in order for optimization to kick in.

zaitanz5y ago

This is very true. Once upon a time -o3 would enable fast-math. We're very careful to ensure we use test models to verify the outputs of release binaries. Enabling fast-math as you have said optimises the equations in a way that produces different results.

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0 pointstitzer5y ago0 comments

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jcranmer5y ago

They do those optimizations when you enable fast-math flags. Some compilers (e.g., icc) enable those by default. When fast-math is not enabled, then the IEEE 754-violating optimizations are disabled.

Source: I've been working on such optimizations this past week. And that means establishing precisely which flags I need to have enabled in order for optimization to kick in.

zaitanz5y ago

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