[jcranmer]

In practice, all of the C compilers will (or can be made to) conform to IEEE 754, although #pragma STDC FENV_ACCESS ON support is very spotty (and consequently non-default rounding-mode support), although Clang/LLVM has been working on this for the past several months.

(That reminds me, I need to harangue the llvm-libc folks to add that pragma to their fpenv code for correctness sake).

[shakna]

The clang docs explicitly say that they don't support Annex F at all. Is this story finally going to change? So that you have at least partial support?

[titzer]

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.

[jcranmer]

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.

[zaitanz]

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.