fixes status-im/nimbus-eth2#1549 (nim-lang#16146)

* fixes status-im/nimbus-eth2#1549 [backport:1.4]

* test fixup

compiler/ccgcalls.nim

@@ -249,13 +249,15 @@ proc openArrayLoc(p: BProc, formalType: PType, n: PNode): Rope =
     else: internalError(p.config, "openArrayLoc: " & typeToString(a.t))
 
 proc withTmpIfNeeded(p: BProc, a: TLoc, needsTmp: bool): TLoc =
-  if needsTmp and a.lode.typ != nil:
-    var tmp: TLoc
-    getTemp(p, a.lode.typ, tmp, needsInit=false)
-    genAssignment(p, tmp, a, {})
-    tmp
+  # Bug https://github.com/status-im/nimbus-eth2/issues/1549
+  # Aliasing is preferred over stack overflows.
+  # Also don't regress for non ARC-builds, too risky.
+  if needsTmp and a.lode.typ != nil and p.config.selectedGC in {gcArc, gcOrc} and
+      getSize(p.config, a.lode.typ) < 1024:
+    getTemp(p, a.lode.typ, result, needsInit=false)
+    genAssignment(p, result, a, {})
   else:
-    a
+    result = a
 
 proc genArgStringToCString(p: BProc, n: PNode, needsTmp: bool): Rope {.inline.} =
   var a: TLoc

compiler/cgen.nim

@@ -471,6 +471,14 @@ proc getTemp(p: BProc, t: PType, result: var TLoc; needsInit=false) =
   result.storage = OnStack
   result.flags = {}
   constructLoc(p, result, not needsInit)
+  when false:
+    # XXX Introduce a compiler switch in order to detect these easily.
+    if getSize(p.config, t) > 1024 * 1024:
+      if p.prc != nil:
+        echo "ENORMOUS TEMPORARY! ", p.config $ p.prc.info
+      else:
+        echo "ENORMOUS TEMPORARY! ", p.config $ p.lastLineInfo
+      writeStackTrace()
 
 proc getTempCpp(p: BProc, t: PType, result: var TLoc; value: Rope) =
   inc(p.labels)

tests/ccgbugs/targ_lefttoright.nim

@@ -16,6 +16,7 @@ discard """
 2,2
 1,2
 '''
+  cmd: "nim c --gc:orc $file"
 """
 
 template test =