Adding param virtual instruction to label stack entries before call pc (#43)

* Adding `param` virtual instruction to label stack entries before call pc

* add some comment explaining +1

* use spaces for indent

* correct comment

* power of 5 for a change and minor update

Author: ldemailly

Makefile

@@ -32,6 +32,8 @@ run: vm
 	./vm compile -loglevel debug programs/incr.asm
 	./vm run -loglevel debug programs/incr.vm
 	./vm compile -loglevel debug programs/loop.asm
+	./vm compile -loglevel debug programs/pow.asm
+	./vm run -loglevel debug programs/pow.vm
 	time ./vm run -profile-cpu cpu.pprof programs/loop.vm
 
 GEN:=cpu/instruction_string.go cpu/syscall_string.go

README.md

@@ -39,6 +39,7 @@ Assembler only:
 - `str8` for string (with the double or backtick quotes)
 - on a line preceding an instruction: _label_ + `:` label for the *R instruction (relative address calculation). _label_ starts with a letter.
 - `Var v1 v2 ...` virtual instruction that generates a `Push` instruction with the number of identifiers provided and defines labels for said variables starting at 0 (which will start with the value of the accumulator while the rest will start 0 initialized).
+- `Param p1 p2 ...` virtual instruction that generates stack labels for p1, p2 as offset from before the return PC (ie parameters pushed (via `Var` or `Push`) by the caller before calling `Call`)
 - `Return` virtual instruction that generates a `Ret n` where _n_ is such as a Var push is undone.
 
 ## Benchmarks

asm/asm.go

@@ -237,9 +237,9 @@ func compile(reader *bufio.Reader, writer *bufio.Writer) int {
 			if narg != 0 {
 				return log.FErrf("Expecting 0 arguments for return, got %d (%v)", narg, args)
 			}
-		case "var":
+		case "var", "param":
 			if narg == 0 {
-				return log.FErrf("Expecting at least 1 argument for var, got none")
+				return log.FErrf("Expecting at least 1 argument for %s, got none", instr)
 			}
 		case "incrr", "incrs", "sys", "syss", "storesb":
 			if narg != 2 {
@@ -281,6 +281,14 @@ func compile(reader *bufio.Reader, writer *bufio.Writer) int {
 				varmap[args[i]] = cpu.ImmediateData(i)
 			}
 			log.Debugf("Var -> Push %d and defined variables: %v", narg-1, varmap)
+		case "param":
+			// define more stack labels
+			start := returnN + 1 // +1 to skip over the return PC
+			for i := range narg {
+				varmap[args[i]] = cpu.ImmediateData(start + i)
+			}
+			log.Debugf("Param -> Defined parameters: %v", varmap)
+			continue
 		case "return":
 			data = false
 			op = op.SetOpcode(cpu.Ret)

programs/pow.asm

@@ -0,0 +1,23 @@
+; Demo of a subroutine that takes more than 1 input argument
+; (note that one of the arguments would probably be fine in accumulator but passing both
+; via the stack is more demonstrative)
+    loadi 5
+var base exp # accumulator (so: 2) pushed to stack slot 0, creates exp slot 1
+    loadi 3
+    stores exp
+    call pow
+    sys exit 0 ; note we leave the 2 variables in stack on exit which is fine for this demo
+
+pow:
+    ; inside the pow subroutine, s0 is the return address so s1 is the first argument (base) and s2 is the second argument (exp)
+    loadi 1  ; initialize result to 1
+  var result ; now we pushed one more so exp is at slot 3 and base at slot 2
+  param b e  ; b = base, e = exp, labels for their positions in the stack before PC
+  loop:
+    loads result
+    muls b
+    stores result
+    incrs -1 e
+    jnz loop
+    loads result
+    return