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Re: AArch64 patches for glibc master.

On Wed, 11 Dec 2013, Marcus Shawcroft wrote:

> Inspection of sysdeps/ieee754/dbl-64/Makefile suggests that we assume
> -ffp-contract=fast and target specific source files with
> -ffp-contract=off rather than taking the conservative approach of
> defaulting to -ffp-contract=off and calling out individual files with
> -ffp-contract=fast.
> Extending the existing mechanism to pass -ffp-contract=off for
> e_sqrt.c is trivial, but is this the right approach, I'm not sure how
> to go about proving that we don't have similar issues lurking latent
> elsewhere in ieee754/* which may surface when a future version of gcc
> figures out how to make more extensive use of fused multiply
> instructions.

It's quite possible that some architecture-specific libm files in fact 
rely on contracting taking place, if written for architectures known to 
have fused multiply-add.

I think you should arrange for -ffp-contract=off to be used for this 
particular file, and add an entry to the libm todo list on the wiki for 
reviewing C libm files to identify whether they use precision extension 
techniques that may rely on contracting not occurring; rely on 
contracting; or are indifferent to it.

Joseph S. Myers

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