From: sfuld@alumni.cmu.edu.invalid   
      
   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.   
      
      
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