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Message-ID: <20200109220014.GX30412@brightrain.aerifal.cx> Date: Thu, 9 Jan 2020 17:00:14 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] math: move i386 sqrtf to C On Thu, Jan 09, 2020 at 10:00:06PM +0100, Szabolcs Nagy wrote: > > If we want to ensure correct rounding (important for sqrt[f]) even on > > broken compilers (some ppl use gcc 3.x, and pcc may be broken too?) > > perhaps we should just do the store from asm too? > > that would be a bit safer, but then correct compiler would > store twice i think. If you did something like: float y = expr_with_excess_precision; __asm__( "" : "+m"(y)); return y; then I think you'd get just one store and one load, as intended. It seems to work as intended here. Oddly though my local gcc (7.3) is gratuitously pushing/popping a single gpr to align stack to 8 (but not 16) despite being a leaf function. No idea why. > it's hard to get excited about this > issue: it only matters on m68k and i386 which are not the > main targets for new float code (and old code had to deal > with this and bigger brokenness already). Indeed, but context of present thread is getting rid of the i386 asm files so it's relevant here. > > Note that eval_as_float only helps if -ffloat-store is used, which is > > a nasty hack and also nonconforming, arguably worse than the behavior > > without it, so we should probably drop use of that as a fallback, and > > use fp_barrier[f] instead if needed. > > i think -ffloat-store almost always drops excess precision > including returns and assignments, so with that no > annotation is needed. but yes the way the annotation is > defined now is not useful against broken compilers or > non-standard excess precision setting, in glibc the > annotation is defined differently (with inline asm). I was thinking in the context of wanting to remove from configure the: || { test "$ARCH" = i386 && tryflag CFLAGS_C99FSE -ffloat-store ; } which is probably doing more harm than good. Do you know if there are things that'd break if we did that? I think eval_as_float should probably be defined as fp_barrierf to make it safe in your code, conditional on FLT_EVAL_METHOD>0 (and likewise >1 for eval_as_double). Rich
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