[R-SIG-Mac] .Machine differences between Intel and M1

[Simon Urbanek]

Matt,

yes, arm64 does not support long doubles. In C the long double type is 64-bit there so has the same precision as doubles (this is allowed by the standard).

Cheers,
Simon


> On Dec 22, 2021, at 9:21 AM, Matthew Heun via R-SIG-Mac <r-sig-mac using r-project.org> wrote:
> 
> All:
> 
> I'm seeing some test failures on a new M1 Pro machine that I do not see on my Intel machine.  I'm investigating whether the test failures are caused by machine precision differences.  On my M1 Pro machine, differences of large numbers are greater than a specified tolerance.  (On my Intel machine, differences between the supposed same numbers are within tolerance.)
> 
> The output of .Machine shows some differences.  I have 2 questions below, each identified by "****".
> 
> (1) For sizeof.longdouble, I see the following:
> 
> Intel machine:
> 
>> $sizeof.longdouble
>> [1] 16
> 
> M1 Pro machine:
> 
>> $sizeof.longdouble
>> [1] 8
> 
> 
> 
> ?.Machine says:
> 
> sizeof.longdouble	
> the number of bytes in a C long double type. Will be zero if there is no such type (or its use was disabled when R was built), otherwise possibly12 (most 32-bit builds) or 16 (most 64-bit builds).
> 
> The M1 Pro uses a 64-bit architecture.  So this result is surprising to me.
> 
> Furthermore, 
> 
>>> capabilities("long.double")
>> long.double 
>>       TRUE 
> 
> So somebody thinks that long doubles are supported.
> 
> **** Is the difference in sizeof.longdouble between the Intel and M1 architectures expected?
> 
> 
> (2) Also, my M1 Pro machine is missing additional fields (that the Intel machine reports):
> 
>> $longdouble.eps
>> [1] 1.084202e-19
>> 
>> $longdouble.neg.eps
>> [1] 5.421011e-20
>> 
>> $longdouble.digits
>> [1] 64
>> 
>> $longdouble.rounding
>> [1] 5
>> 
>> $longdouble.guard
>> [1] 0
>> 
>> $longdouble.ulp.digits
>> [1] -63
>> 
>> $longdouble.neg.ulp.digits
>> [1] -64
>> 
>> $longdouble.exponent
>> [1] 15
>> 
>> $longdouble.min.exp
>> [1] -16382
>> 
>> $longdouble.max.exp
>> [1] 16384
> 
> **** Is there a reason why the above entries are missing from the output of .Machine on the M1 Pro machine?
> 
> 
> 
> My R installation is:  4.1.2 Patched (2021/12/16, r81394)
> 
> 
> 
> Any help will be appreciated.
> 
> 
> 
> Cheers,
> 
> Matt
> 
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