floats and doubles on a VAX 750 with an FPA

[buck at NRL-CSS.ARPA]

From:  Joe Buck <buck at NRL-CSS.ARPA>

	Recently I ran some tests to calculate the cost of the phrase
	"all floating arithmetic in C is carried out in double-precision..."
	(K&R pp 184 section 6.2 -- to quote chapter and verse).  It turned
	out to be really expensive.  For instance, the code generated by
	the C compiler to evaluate "a *= b"
	
		cvtfd	-12(fp),r0
		muld2	r0,-20(fp)
		cvtdf	-20(fp),-12(fp)
	
	turned out to be four times as expensive as the equivalent
	floating point operation
	
		mulf2	-20(fp),-12(fp)
	
If memory is not a problem, you could declare some or all of the variables
double and get rid of the cvt's, before going to a lot of trouble to convert
the assembler output, or to modify the code. Since I don't see any reason
why a muld2 should cost more than twice a mulf2, this should double the
speed.

I understand that the new ANSI standard will allow single precision
computation, but it's not clear whether support for this would be
required or optional.

-Joe