Date: Sat, 21 Mar 2020 21:19:58 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Q: dealing with missing removal of excess precision On Fri, Mar 20, 2020 at 02:12:50PM -0400, Rich Felker wrote: > 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" With the fixups mentioned (included in attached patches), i386 and x86_64 are passing libc-test and seem fine. OK if I merge them (rebasing the fixups in as fixups)? With s/x86/x87/ naming? Rich View attachment "0001-fabsf-i386-fixup.patch" of type "text/plain" (553 bytes) View attachment "0002-sqrt-i386-fixup.patch" of type "text/plain" (748 bytes) View attachment "0003-sqrt-i386-fixup-2.patch" of type "text/plain" (651 bytes)
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