Re: "Mini" tags to reduce the number of op codes

On 4/3/2024 2:30 PM, MitchAlsup1 wrote:

BGB-Alt wrote:
 

On 4/3/2024 11:43 AM, Stephen Fuld wrote:

There has been discussion here about the benefits of reducing the number of op codes.  One reason not mentioned before is if you have fixed length instructions, you may want to leave as many codes as possible available for future use.  Of course, if you are doing a 16-bit instruction design, where instruction bits are especially tight, you may save enough op-codes to save a bit, perhaps allowing a larger register specifier field, or to allow more instructions in the smaller subset.
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It is in this spirit that I had an idea, partially inspired by Mill’s use of tags in registers, but not memory.  I worked through this idea using the My 6600 as an example “substrate” for two reasons.  First, it

                66000

Sorry.  Typo.

has several features that are “friendly” to the idea.  Second, I know Mitch cares about keeping the number of op codes low.
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Please bear in mind that this is just the germ of an idea.  It is certainly not fully worked out.  I present it here to stimulate discussions, and because it has been fun to think about.
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The idea is to add 32 bits to the processor state, one per register (though probably not physically part of the register file) as a tag.  If set, the bit indicates that the corresponding register contains a floating-point value.  Clear indicates not floating point (integer, address, etc.).  There would be two additional instructions, load single floating and load double floating, which work the same as the other 32- and 64-bit loads, but in addition to loading the value, set the tag bit for the destination register.  Non-floating-point loads would clear the tag bit.  As I show below, I don’t think you need any special "store tag" instructions.

 What do you do when you want a FP bit pattern interpreted as an integer, or vice versa.

As I said below, if you need that, you can use an otherwise :"useless" instruction, such as ORing a register with itself the modify the tag bits.

 

When executing arithmetic instructions, if the tag bits of both sources of an instruction are the same, do the appropriate operation (floating or integer), and set the tag bit of the result register appropriately.
If the tag bits of the two sources are different, I see several possibilities.
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1.    Generate an exception.
2.    Use the sense of source 1 for the arithmetic operation, but perform the appropriate conversion on the second operand first, potentially saving an instruction

 Conversions to/from FP often require a rounding mode. How do you specify that?

Good point.

 

3.    Always do the operation in floating point and convert the integer operand prior to the operation.  (Or, if you prefer, change floating point to integer in the above description.)
4.    Same as 2 or 3 above, but don’t do the conversions.
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I suspect this is the least useful choice.  I am not sure which is the best option.
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Given that, use the same op code for the floating-point and fixed versions of the same operations.  So we can save eight op codes, the four arithmetic operations, max, min, abs and compare.  So far, a net savings of six opcodes.
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But we can go further.  There are some opcodes that only make sense for FP operands, e.g. the transcendental instructions.  And there are some operations that probably only make sense for non-FP operands, e.g. POP, FF1, probably shifts.  Given the tag bit, these could share the same op-code.  There may be several more of these.

 Hands waving:: "Danger Will Robinson, Danger" more waving of hands.

Agreed.

I think this all works fine for a single compilation unit, as the compiler certainly knows the type of the data.  But what happens with separate compilations?  The called function probably doesn’t know the

 The compiler will certainly have a function prototype. In any event, if FP
and Integers share a register file the lack of prototype is much less stress-
full to the compiler/linking system.
 

tag value for callee saved registers.  Fortunately, the My 66000 architecture comes to the rescue here.  You would modify the Enter and Exit instructions to save/restore the tag bits of the registers they are saving or restoring in the same data structure it uses for the registers (yes, it adds 32 bits to that structure – minimal cost).  The same mechanism works for interrupts that take control away from a running process.

 Yes, but we do just fine without the tag and without the stuff mentioned above. Neither ENTER nor EXIT care about the 64-bit pattern in the register.

I think you need it for callee saved registers to insure the tag is set correctly for the calling program upon return to it.

 

I don’t think you need to set or clear the tag bits without doing anything else, but if you do, I think you could “repurpose” some other instructions to do this, without requiring another op-code.   For example, Oring a register with itself could be used to set the tag bit and Oring a register with zero could clear it.  These should be pretty rare.

 

That is as far as I got.  I think you could net save perhaps 8-12 op codes, which is about 10% of the existing op codes - not bad.  Is it worth it?

 No.
 

           To me, a major question is the effect on performance.  What

is the cost of having to decode the source registers and reading their respective tag bits before knowing which FU to use?

 The problem is you have put decode dependent on dynamic pipeline information.
I suggest you don't want to do that. Consider a change from int to FP instruction
as a predicated instruction, so the pipeline cannot DECODE the instruction at
hand until the predicate resolves. Yech.

Good point.
--
  - Stephen Fuld
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