From: ThatWouldBeTelling@thevillage.com   
      
   Stephen Fuld wrote:   
   > On 9/4/2025 12:06 PM, EricP wrote:   
   >> Stephen Fuld wrote:   
   >>> On 9/4/2025 10:19 AM, EricP wrote:   
   >>>> BGB wrote:   
   >>>>> On 9/3/2025 9:42 PM, EricP wrote:   
   >>>>>> MitchAlsup wrote:   
   >>>>>>>   
   >>>>>>> However, I also found that STs need an immediate and a   
   >>>>>>> displacement, so,   
   >>>>>>> Major == 0b'001001 and minor == 0b'011xxx has 4 ST instructions with   
   >>>>>>> potential displacement (from D12ds above) and the immediate has   
   >>>>>>> the size of the ST. This provides for::   
   >>>>>>>          std    #4607182418800017408,[r3,r2<<3,96]   
   >>>>>>   
   >>>>>> Compare and Branch can also use two immediates as it   
   >>>>>> has reg-reg or reg-imm compares plus displacement.   
   >>>>>> And has high enough frequency to be worth considering.   
   >>>>>>   
   >>>>>   
   >>>>> Can be done, yes.   
   >>>>>   High enough frequency/etc, is where the possible debate lies.   
   >>>>>   
   >>>>>   
   >>>>> Checking stats, it can effect roughly 1.9% of the instructions.   
   >>>>> Or, around 11% of branches; most of the rest being unconditional or   
   >>>>> comparing against 0 (which can use the Zero Register). Only a   
   >>>>> relative minority being compares against non-zero constants.   
   >>>>   
   >>>> The only instruction usage stats I have are from those VAX papers:   
   >>>> A Case Study of VAX-11 Instruction Set Usage For Compiler Execution,   
   >>>> 1982   
   >>>>   
   >>>> That shows about 12% instructions are conditional branch and 9% CMP.   
   >>>> That says to me that almost all Bcc are paired with a CMP,   
   >>>> and very few use the flags set as a side effect of ALU ops.   
   >>>   
   >>> OK, but does this tell you how many of the CMPs are to a value of   
   >>> zero? I expect these to be a significant enough percentage to skew   
   >>> your analysis.   
   >>   
   >> Looking at   
   >> Measurement and Analysis of Instruction Use in VAX 780, 1982   
   >>   
   >> VAX had a TST instruction which was the same as CMP src,#0.   
   >> TST has < 2% usage while CMP 10-12%.   
   >   
   > Thanks.  That's interesting.  So perhaps ~15% of all compares are to   
   > zero.  I would have expected higher.   
      
   Oh no I didn't mean that. I meant that a compiler that wanted to   
   to compare with zero would use a TST instruction not a CMP.   
   That could be used any branch GT, GE, LE, LT, EQ, NE.   
   And that is < 2%   
      
   It would use CMP to compare with a number other than 0, which was 10-12%.   
      
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