(gmp.info.gz) Assembler Functional Units

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 Functional Units
 ----------------
 
 When choosing an approach for an assembler loop, consideration is given
 to what operations can execute simultaneously and what throughput can
 thereby be achieved.  In some cases an algorithm can be tweaked to
 accommodate available resources.
 
    Loop control will generally require a counter and pointer updates,
 costing as much as 5 instructions, plus any delays a branch introduces.
 CPU addressing modes might reduce pointer updates, perhaps by allowing
 just one updating pointer and others expressed as offsets from it, or
 on CISC chips with all addressing done with the loop counter as a
 scaled index.
 
    The final loop control cost can be amortised by processing several
 limbs in each iteration ( Assembler Loop Unrolling).  This at
 least ensures loop control isn't a big fraction the work done.
 
    Memory throughput is always a limit.  If perhaps only one load or
 one store can be done per cycle then 3 cycles/limb will the top speed
 for "binary" operations like `mpn_add_n', and any code achieving that
 is optimal.
 
    Integer resources can be freed up by having the loop counter in a
 float register, or by pressing the float units into use for some
 multiplying, perhaps doing every second limb on the float side (
 Assembler Floating Point).
 
    Float resources can be freed up by doing carry propagation on the
 integer side, or even by doing integer to float conversions in integers
 using bit twiddling.
 

Info Catalog

(gmp.info.gz) Assembler Cache Handling

(gmp.info.gz) Assembler Coding

(gmp.info.gz) Assembler Floating Point