🚀 go-pugleaf

|> :	    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??

The Intel386(tm) architecture has SETcc (set on condition)
instructions that put 0/1 into a register depending on the condition
codes. So you would have:
    
    	cmp 	ebx,ecx 	! assuming absdx in ebx, dmax in ecx
    	setle	al  	    	! put 0/1 into al 

Set-conditional is rather common - I believe it is in the MIPS R2000
architecture (and I am sure I will be corrected if wrong), and the AMD
29K architecture.  Minor differences according to whether 0/1 or 0/-1
is stored.
    	


|> 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??

The Cydra-5 had a conditional assignment instruction (like the phi
function in single assignment code). Semantics: dest := IF icr[N] THEN
val1 ELSE val2.  Cydra also conditionalized nearly all instructions,
as does the Acorn Risk machine, although I do not know if the ARM has
a conditional select.

Note that with a bit of masking conditional select can be acheived on
the 88K, as on most other machines.



|> 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.

I have seen some of the better x86 compilers compile this without a
branch.
--

Andy Glew, glew@ichips.intel.com
Intel Corp., M/S JF1-19, 5200 NE Elam Young Parkway, 
Hillsboro, Oregon 97124-6497

This is a private posting; it does not indicate opinions or positions
of Intel Corp.