provide experimental op handler switch -d:lowmem:1 for low memory C compiler

why:
  on 32bit windows 7, there seems to be a 64k memory ceiling for the gcc
  compiler which was exceeded on some test platform.

details:
  compiling VM2 for low memory C compiler can be triggered with
  "make ENABLE_VM2LOWMEM". this comes with a ~24% longer execution time
  of the test suite against old VM and optimised VM2.

Makefile

@@ -80,10 +80,15 @@ ifneq ($(ENABLE_EVMC), 0)
 NIM_PARAMS := $(NIM_PARAMS) -d:evmc_enabled
 endif
 
+# disabled by default, enable with ENABLE_VM2SLOW=1
+ifneq ($(if $(ENABLE_VM2LOWMEM),$(ENABLE_VM2LOWMEM),0),0)
+NIM_PARAMS := $(NIM_PARAMS) -d:vm2_enabled -d:lowmem:1
+else
 # disabled by default, enable with ENABLE_VM2=1
 ifneq ($(if $(ENABLE_VM2),$(ENABLE_VM2),0),0)
 NIM_PARAMS := $(NIM_PARAMS) -d:vm2_enabled
 endif
+endif
 
 deps: | deps-common nat-libs nimbus.nims
 ifneq ($(USE_LIBBACKTRACE), 0)

README.md

@@ -190,8 +190,29 @@ make nimbus
 
 build/nimbus
 ```
+### <a name="make-xvars"></a>Experimental make variables
 
-### Development tips
+Apart from standard make flags (see link in the next [chapter](#devel-tips)),
+the following make variables can be set to control which version of a virtual
+engine is compiled. The variables are listed with decreasing priority (in
+case of doubt, the lower prioritised variable is ignored when the higher on is
+available.)
+
+ * ENABLE_EVMC=1<br>
+   Enable mostly EVMC compliant wrapper around the native nim VM
+
+ * ENABLE_VM2LOWMEM=1<br>
+   Enable new re-factored version of the native nim VM. This version is not
+   optimised and coded in a way so that low memory compilers can handle it
+   (observed on 32 bit windows 7.)
+
+ * ENABLE_VM2=1<br>
+   Enable new re-factored version of the native nim VM.
+
+For these variables, using &lt;variable&gt;=0 is ignored and &lt;variable&gt;=2
+has the same effect as &lt;variable&gt;=1 (ditto for other numbers.)
+
+### <a name="devel-tips"></a>Development tips
 
 Interesting Make variables and targets are documented in the [nimbus-build-system](https://github.com/status-im/nimbus-build-system) repo.
 

nimbus/vm2/interpreter_dispatch.nim

@@ -9,6 +9,10 @@
 # according to those terms.
 
 const
+  # help with low memory when compiling
+  lowmem {.intdefine.}: int = 0
+  lowMemoryCompileTime {.used.} = lowmem > 0
+
   # debugging flag
   kludge {.intdefine.}: int = 0
   breakCircularDependency {.used.} = kludge > 0
@@ -99,7 +103,7 @@ proc toCaseStmt(forkArg, opArg, k: NimNode): NimNode =
       of Create, Create2, Call, CallCode, DelegateCall, StaticCall:
         quote do:
           `forkCaseSubExpr`
-          if not c.continuation.isNil:
+          if not `k`.cpt.continuation.isNil:
             break
       of Stop, Return, Revert, SelfDestruct:
         quote do:
@@ -117,9 +121,29 @@ proc toCaseStmt(forkArg, opArg, k: NimNode): NimNode =
     echo ">>> ", result.repr
 
 
-macro genDispatchMatrix(fork: Fork; op: Op; k: Vm2Ctx): untyped =
+macro genDispatchMatrix(fork: Fork; op: Op; k: Vm2Ctx): untyped {.used.} =
   result = fork.toCaseStmt(op, k)
 
+
+template genLowMemDispatcher(fork: Fork; op: Op; k: Vm2Ctx) {.used.} =
+  if op == Stop:
+    handleStopDirective(k)
+    break
+
+  if BaseGasCosts[op].kind == GckFixed:
+    handleFixedGasCostsDirective(fork, op, k)
+  else:
+    handleOtherDirective(fork, op, k)
+
+  case c.instr
+  of Create, Create2, Call, CallCode, DelegateCall, StaticCall:
+    if not k.cpt.continuation.isNil:
+      break
+  of Return, Revert, SelfDestruct:
+    break
+  else:
+    discard
+
 # ------------------------------------------------------------------------------
 # Public functions
 # ------------------------------------------------------------------------------
@@ -144,7 +168,7 @@ proc selectVM*(c: Computation, fork: Fork) {.gcsafe.} =
     #   # we could use manual jump table instead
     #   # TODO lots of macro magic here to unravel, with chronicles...
     #   # `c`.logger.log($`c`.stack & "\n\n", fgGreen)
-
+    when not lowMemoryCompileTime:
       when defined(release):
         #
         # FIXME: OS case list below needs to be adjusted
@@ -154,7 +178,7 @@ proc selectVM*(c: Computation, fork: Fork) {.gcsafe.} =
             {.warning: "*** Win64/VM2 handler switch => computedGoto".}
             {.computedGoto, optimization: speed.}
           else:
-          # computedGoto is not compiling on github/ci (out of memory) -- jordan
+            # computedGoto not compiling on github/ci (out of memory) -- jordan
             {.warning: "*** Win32/VM2 handler switch => optimisation disabled".}
             # {.computedGoto, optimization: speed.}
 
@@ -179,6 +203,11 @@ proc selectVM*(c: Computation, fork: Fork) {.gcsafe.} =
 
       genDispatchMatrix(fork, c.instr, desc)
 
+    else:
+      {.warning: "*** low memory compiler mode => program will be slow".}
+
+      genLowMemDispatcher(fork, c.instr, desc)
+
 # ------------------------------------------------------------------------------
 # End
 # ------------------------------------------------------------------------------