Can compilers do this?

[bwburnsed@aol.com (BWBurnsed)]

I came across some very strange looking code that someone else wrote,
long, long ago, and (apparently) in a universe far, far away. But before I

make too many critical comments, I want to be sure I'm not missing
something.

Repeatedly in this code (in many files), there are places where a floating
point variable is tested to see if it is negative. However, the way it is
done is:

     if  X * abs(X)  <  0.0  then  ...

Is there (or was there ever) some pathological anomaly about floating
point 
implementations that would make a conversion (abs) and floating point
multiply 
preferable to testing a sign bit? Can, and will, optimizing compilers
recognize the
real test desired in such constructs, i.e. reduce it to a sign bit test?

Also, suppose Y and X are floating point variables, and M and B are 
CONSTANT floating point variables (not named numbers) initialized to
1.0  and  0.0  respectively. If one writes

     Y := M * X  +  B ;

can (and will) any compiler reduce this to

     Y := X ;

If not, could it do so if M and B were named numbers?

Thanks,
BwB