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Date: Tue, 14 Jan 2020 13:58:35 -0500
From: Rich Felker <>
Subject: Re: Q: dealing with missing removal of excess precision

On Tue, Jan 14, 2020 at 07:50:58PM +0100, Szabolcs Nagy wrote:
> * Alexander Monakov <> [2020-01-14 21:17:55 +0300]:
> > What to do with functions that return values with excess precision from asm?
> > 
> > Many of the remaining x87-based asm implementations do not remove excess
> > precision on return. This looks like a bug, but matches Glibc behavior.
> > Moving such functions to C would either introduce removal of excess precision,
> > or result in code that lies to the compiler by pretending that inline asm keeps
> > values within precision of float/double, where in reality it might not.
> the intention is to remove excess precision so it's good that's happening.

Agreed. It may be a slight performance regression but it's the right
thing to do anyway.

> otoh it would be nice if there was a way to tell the compiler not to
> remove it (e.g. in case the asm already took care of it) even in c99
> standard mode.

Perhaps this happens if the output constraint is tied to a float
rather than a long double?


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