enable VM tracing in user code via {.define(nimVmTrace).} (#18244)

* enable VM tracing in user code via  `{.define(nimVmTrace).}`

* add vmutils.vmTrace

* add vmTrace

compiler/options.nim

@@ -330,6 +330,7 @@ type
     warnCounter*: int
     errorMax*: int
     maxLoopIterationsVM*: int ## VM: max iterations of all loops
+    isVmTrace*: bool
     configVars*: StringTableRef
     symbols*: StringTableRef ## We need to use a StringTableRef here as defined
                              ## symbols are always guaranteed to be style

compiler/vm.nim

@@ -548,9 +548,12 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
         "pc", $pc, "opcode", alignLeft($c.code[pc].opcode, 15),
         "ra", regDescr("ra", ra), "rb", regDescr("rb", instr.regB),
         "rc", regDescr("rc", instr.regC)]
-
+    if c.config.isVmTrace:
+      # unlike nimVMDebug, this doesn't require re-compiling nim and is controlled by user code
+      let info = c.debug[pc]
+      # other useful variables: c.loopIterations
+      echo "$# [$#] $#" % [c.config$info, $instr.opcode, c.config.sourceLine(info)]
     c.profiler.enter(c, tos)
-
     case instr.opcode
     of opcEof: return regs[ra]
     of opcRet:

compiler/vmops.nim

@@ -249,6 +249,9 @@ proc registerAdditionalOps*(c: PCtx) =
                   "isExported() requires a symbol. '" & $n & "' is of kind '" & $n.kind & "'", n.info)
     setResult(a, sfExported in n.sym.flags)
 
+  registerCallback c, "stdlib.vmutils.vmTrace", proc (a: VmArgs) =
+    c.config.isVmTrace = getBool(a, 0)
+
   proc hashVmImpl(a: VmArgs) =
     var res = hashes.hash(a.getString(0), a.getInt(1).int, a.getInt(2).int)
     if c.config.backend == backendJs: