From: user5857@newsgrouper.org.invalid   
      
   Stephen Fuld  posted:   
      
   > On 12/28/2025 8:22 AM, John Savard wrote:   
   > > On Mon, 22 Dec 2025 20:00:06 +0000, Waldek Hebisch wrote:   
   > >> John Savard  wrote:   
   > >>> On Thu, 18 Dec 2025 22:25:08 +0000, Anton Ertl wrote:   
   > >>>   
   > >>>> It is certainly possible to decode potential instructions at every   
   > >>>> starting position in parallel, and later select the ones that actually   
   > >>>> correspond to the end of the previous instruction,   
   > >>>   
   > >>> Oh, yes, I had always realized that, but dismissed it as far too   
   > >>> wasteful.   
   > >>   
   > >> Well, Mitch claims average 35 bits per instructions, that means about   
   > >> 90% utilization of decoders, so not bad.   
   > >   
   > > His minimum instruction size is 32 bits, but I was going for 16 bits.   
   > >   
   > >> Also, consider that alternative to variable length instructions is to   
   > >> use longer instructions or more of them.   
   > >   
   > > What I did instead was use variable-length instructions, but add a prefix   
   > > at the beginning of any 256-bit block of instructions that contained them   
   > > which directly showed where each instruction began.   
   > >   
   > > My intent was to avoid the disadvantages you identify for fixed-length   
   > > instructions, but avoid the disadvantage of variable-length instructions   
   > > too.   
   >   
   > I understand your goal, however . . .   
   >   
   > How many bits do you "waste" on the prefix?   
      
   For code like:: (undoctored compiler output)   
      
   .LBB1_34:   
   	mov	r1,#0   
   	br	.LBB1_35   
   .LBB1_39:   
   	mov	r1,#28   
   	exit	r16,r0,0,32   
   .LBB1_36:   
   	mov	r1,#0   
   	exit	r16,r0,0,32   
   .LBB1_37:   
   	call	processRestart   
   	beq0	r1,.LBB1_38   
   .LBB1_35:   
   	exit	r16,r0,0,32   
   .LBB1_38:   
   	lduh	r1,[ip,gRestartsLeft]   
   	br	.LBB1_2   
   .Lfunc_end1:   
      
   You are going to eat a lot of headers for 2 instruction BBs.   
      
   Also:: Can you return to the middle of a block ??   
          Can you make multiple CALLs from a single block ??   
          Can you fit an entire loop in a single block ??   
          Can you fit a loop with calls in a single block ??   
          Does each unique label of a switch(i) require its own block ??   
      
   > Since I think any branch must target the beginning of a block, and in   
   > general, a routine will not end on a block boundary, there will be   
   > "wasted" bits at the end of the last block before a "label".  Have you   
   > determined for a "typical" program, how many bits are wasted due to this?   
   >   
   > The point I am making is that you will "cancel" at least some of the   
   > savings of 16 bit instructions.  You should take this into account   
   > before committing to your plan.   
      
   I suspect the block-boundaries will consume more space than the 16-bit   
   instructions can possibly save.   
   >   
   >   
      
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