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Date: Fri, 20 Mar 2020 14:12:50 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Q: dealing with missing removal of excess precision

On Sun, Feb 23, 2020 at 07:14:08PM +0300, Alexander Monakov wrote:
> On Sat, 22 Feb 2020, Rich Felker wrote:
> 
> > I'm not well acquainted with SSE, and only so-so with x87, so pretty
> > much I'm reading them for higher-level issues with tooling
> > compatibility (like the concerns I already raised and looked up and
> > seem to have resolved about x87 constraints and non-GCC compilers) and
> > logic, then planning to apply and test them. I think being aware of
> > non-obvious mistake modes that have already been found would be a lot
> > more useful than staring at things, especially if the bugs you've
> > found are in subtleties of the insn behavior or constraint behavior.
> 
> Okay, thanks. I found two issues:
> 
> 1. i386 lrint* functions mistakenly used fistpll instead of fistpl (I
> posted the fix for x32 asm after noticing my own mistake).
> 
> 2. Some functions bind a 32-bit lvalue as output for fnstsw %ax, which
> as the operand says writes only 16 bits. They should be changed to either
> use a 16-bit lvalue, or a zero-initialized 32-bit lvalue with "+a" constraint.
> 
> Plus, not bugs, but still worth mentioning:
> 
> 3. The new remquol in C could alternatively be implemented by using fxam
> to extract sign bits instead of loading them from stack slots. The current
> approach makes sense given the ABI, but an implementation aiming for better
> code after inlining could choose to use fxam instead of forcing a spill.
> 
> 4. I did not manage to find a copy of Figueroa's "When is double rounding
> innocuous", but I could cite e.g. "Innocuous Double Rounding of Basic
> Arithmetic Operations" by Pierre Roux instead (in i386/sqrtf.c).
> 
> If you like you can fetch a Git tree with my patches from 
> 
>     https://git.sr.ht/~amonakov/musl
> 
> (issue 1 is already corrected in that repo)

Hi. I'm trying to catch up on this and other patches after being sick
and not getting much done for a while. Is there anything else I should
be aware of before going forward with these?

One minor cosmetic change I'd like to make to commit names if you
don't object is changing "x86-family" to "x87-family" for the commits
that are "i386 + long double functions on x86_64" since I found it
confusing when there's one commit for "x86 family" then a separate one
for x86_64 versions of the same function. Let me know if you think of
any alternative that might be more clear than "x87"

Rich

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