Re: [PATCH 3/4] softfloat: fix floatx80 pseudo-denormal comparisons

["Alex Bennée" <alex.bennee@linaro.org>]

Joseph Myers <joseph@codesourcery.com> writes:

> On Fri, 1 May 2020, Alex Bennée wrote:
>
>> OK - so these only turn up in i386?
>
> Patch 1, silencing sNaN, is about generic semantics of IEEE floating-point 
> conversions (which are implemented correctly in various other cases in 
> QEMU), and would be equally applicable to m68k (I believe, without having 
> m68k hardware to test).
>
> Patches 2 and 3 are i386-specific (just like everything in the existing 
> softfloat code relating to floatx80 subnormals), because m68k interprets 
> biased exponent zero differently.
>
> Patch 4 would apply equally to m68k, because all that matters there is 
> that a certain representation is a small nonzero value, not exactly what 
> value it is.
>
> None of these apply to any other architectures supported by QEMU.
>
>> We have two tests currently (float_convs and float_madds) which
>> currently exercise the various combinations of limits and NaN types
>> using some common float_helpers.c support. Maybe extend it for have a
>> table of the various ext80 types and write a i386 only test case to
>> exercise the functions you fixed?
>
> It seems to me that appropriate tests would be entirely i386-specific (in 
> tests/tcg/i386?).

Yes.

> How are such tests supposed to signal success or 
> failure, since all the tests currently there seem to exit with status 0 
> unconditionally?

Non-zero exit. The float_convs and madds tests always pass but the
second phase is a diff with a reference output which may fails.
Whichever is easier for your test case.

> I do have a test I'm using to check these fixes (in C code for convenience 
> of implementation, with only a little inline asm), but it's not suitable 
> for inclusion as-is, since it includes many tests that currently fail 
> (e.g. for exceptions generated, since the i386 floating-point support in 
> QEMU currently discards exceptions from the softfloat code; one of the 
> things I intend to fix but haven't yet).  It also doesn't yet cover all 
> the problems I think I've found so far in the floating-point support in 
> the i386 port (at least ten such bugs beyond the ones fixed in the present 
> patch series).  And it might well depend on details of compiler code 
> generation to test some of the bugs effectively.

OK - we certainly want to include tests for fixed functionality as we
add it. It's something we are trying to get better at since the big
re-write a few years ago.

-- 
Alex Bennée