🚀 go-pugleaf

In article <1991Jun14.134338.4673@linus.mitre.org>, bs@frieda.mitre.org (Robert D. Silverman) writes:

|> :	    ibig  = absdx .le. dmax
|> 	    ^^^^^^^^^^^^^^^^^^^^^^^^
|> 
|> I would be very interested in seeing the assembler code that gets
|> emitted for this line of Fortran. How can this statement get executed
|> WITHOUT a branch??
|>  
|> I also have never seen a machine that has conditional assignment
|> implemented as an instruction! (As indicated above). What machine is
|> this? Could you show us the assembler syntax??
|> 
|> I ** believe ** that the above conditional assignment will get compiled
|> into a test and branch. I don't know of any machine that provides
|> instructions to do otherwise.
|> 
Motorola 88000, for one.  The trick is that the compare instruction puts
the condition code bit vector into a general register instead of a dedicated
condition code register, so that the bitfield instructions can be used
on the condition codes.  For example:

	cmp	scratch,absdx,dmax	; Compare; condition codes to scratch
	extu	ibig,scratch,1<le>	; Extract the le bit into ibig,
					; right-adjusted.

The condition code bit vector is also redundant, in the sense that
if any bit is set, there is another bit, in this case greater-than, which
is known to be clear, and vice versa.  In many cases it turns out the
redundancy is useful.  It's certainly near-as-no-never-mind free.

The ibig = absdx .le. dmax example just barely scratches the surface.  For
example:
	if( a condition b )
		i = i + 1;

can be rendered as:

	cmp	rscratch,x,y
	extu	rscratch,rscratch,1<condition>
	addu	i,i,rscratch

and lots more.  I'll leave it to the compiler folks to post the most aggressively
clever example.

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