[PD-dev] d_math.c "break[s] strict-aliasing rules"

[Hans-Christoph Steiner]

Is there any particular reason why it was done this way?  Does any  
object if  it was fixed?

It seems that -fstrict-aliasing is basic to compiler optimization  
since gcc -O2 enables it.  There has been a lot of work recently in  
making gcc produce faster code. It would be very nice if we could  
take advantage of that.  I think -fstrict-aliasing would be a  
necessary part of that.

.hc


On Apr 18, 2007, at 4:55 PM, Miller Puckette wrote:

> I think there are several places where "strict-aliasing" isn't
> followed.  I always compile with -fno-strict-aliasing to prevent
> problems.
>
> cheers
> Miller
>
> On Wed, Apr 18, 2007 at 11:19:20AM -0400, Hans-Christoph Steiner  
> wrote:
>>
>> So it seems that this bug in d_math.c is triggered by turning on the
>> Apple-recommended optimization flags:
>>
>> http://sourceforge.net/tracker/index.php?
>> func=detail&aid=1692649&group_id=55736&atid=478070
>>
>> I did notice that there are these warnings in the source.  IIRC,
>> optimization generally requires strict aliasing, so it seems that
>> these warnings are probably related to the above bug:
>>
>> d_math.c: In function 'init_rsqrt':
>> d_math.c:79: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules
>> d_math.c: In function 'q8_rsqrt':
>> d_math.c:93: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules
>> d_math.c: In function 'q8_sqrt':
>> d_math.c:101: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules
>>
>> Here are the lines in question:
>>
>> 79:    *(long *)(&f) = l;
>> 93:    long l = *(long *)(&f);
>> 101:   long l = *(long *)(&f);
>>
>> Can anyone speak to what's this for and what it can be replaced with
>> so as to follow "strict-aliasing rules".  Maybe we could use
>> something like this instead (from http://www.cs.tut.fi/~jkorpela/
>> round.html ):
>>
>> #define round(x) ((x) < LONG_MIN-0.5 || (x) > LONG_MAX+0.5 ?\
>> error() : ((x)>=0?(long)((x)+0.5):(long)((x)-0.5))
>>
>> This requires that you have #include <limits.h> and that you have an
>> error handling routine called error which is a function of type long.
>>
>>
>> .hc
>>
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