[webkit-dev] jit, unordered compare

[Zoltan Herczeg]

Hi,

unfortunately cmf was used by the old fpa (floating point accelerator),
which is replaced by vfp (vector floating point) for some time. Perhaps I
can try a "cmpvs reg0, reg0" instruction, which sets zero flag if one
argument is NaN (since reg0 is always equal to reg0). Otherwise it does
nothing (=nop).

Zoltan

> Hi Zoltan.
>
> I believe you're talking about this code for op_jfalse:
>
>>         zeroDouble(fpRegT0);
>>         emitLoadDouble(cond, fpRegT1);
>>         addJump(branchDouble(DoubleEqual, fpRegT0, fpRegT1), target);
>
> and this code for op_jtrue:
>
>>         zeroDouble(fpRegT0);
>>         emitLoadDouble(cond, fpRegT1);
>>         addJump(branchDouble(DoubleNotEqual, fpRegT0, fpRegT1),
>> target);
>
> The goal of this code is to perform a comparison that distinguishes
> { 0, NaN } from all other doubles. op_jfalse should branch in the case
> of 0 or NaN, and, inversely, op_jtrue should branch in all other cases.
>
> It sounds like the equal / zero condition does not indicate NaN on all
> platforms. Bummer.
>
> I'm not an expert in ARM (or other floating point processors, for that
> matter), so I'm not sure what the best abstraction is for this notion
> of "compares to zero as equal or unordered". Since IEEE double
> requires all comparisons to NaN to be implicitly false, I do expect
> that there is some efficient single branch that can do this on ARM.
>  From http://www.heyrick.co.uk/assembler/fpops.html#cmf, it looks like
> "the CMF instruction should be used to test for equality (ie when a
> BEQ or BNE is used afterwards) or to test for unorderedness (in the V
> flag)."
>
> Gavin, do you have any ideas here?
>
> Geoff
>
> On Oct 12, 2009, at 4:05 AM, Zoltan Herczeg wrote:
>
>> Hi,
>>
>> My stougle with USE_JSVALUE32_64 still continues on ARM. In
>> emit_op_jfalse, there is a comparison here (fpRegT0 contains 0.0):
>> addJump(branchDouble(DoubleEqual, fpRegT0, fpRegT1), target + 2);
>>
>> In x86, if either operand is NaN, the zero flag is set by definition
>> (ucomisd instruction). I have no idea why (NaN == anything is true?).
>> Unfortunately, ARM does not set the zero flag. What would be a
>> portable
>> way to improve this instruction? (and emit_op_jtrue) Perhaps this
>> affects
>> the possible thumb2 implementation with USE_JSVALUE32_64 as well.
>>
>> Zoltan
>>
>>
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>
>