[PATCH] D104853: [X86] Add description of FXAM instruction

[Serge Pavlov via Phabricator via llvm-commits]

sepavloff added a comment.

In D104853#2838797 <https://reviews.llvm.org/D104853#2838797>, @craig.topper wrote:

> FXAM appears to be two uops where FTST is one on modern Intel CPUs based on Agner Fog's data. Agner's data for some AMD CPUs shows ~20 cycles of latency.

Could tuning scheduling for this instruction be subsequent work?

> What do you intend to use this for?

This function is a convenient basis for implementation of `isnan` and other classification functions for fp80. Now these functions are implemented using FUCOM* instructions. They however raise exception if argument is signaling NaN, such behavior is not suitable for the classification functions.


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https://reviews.llvm.org/D104853