From 1080f5eba2fedcbe0b9cc20886a5d30eb73be869 Mon Sep 17 00:00:00 2001
From: Andreas Rumpf <rumpf_a@web.de>
Date: Mon, 11 Sep 2023 18:48:20 +0200
Subject: [PATCH] produce better code for object constructions and 'result'
 [backport] (#22668)

(cherry picked from commit 8f5b90f886501862bd27d4e0e8244e2f7f0ebc2a)
---
 compiler/ccgexprs.nim  |  8 ++++++--
 compiler/cgen.nim      | 22 ++++++++++++++++------
 tests/misc/trunner.nim | 11 +++++++----
 3 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/compiler/ccgexprs.nim b/compiler/ccgexprs.nim
index 712b874d93..7d5168ea4b 100644
--- a/compiler/ccgexprs.nim
+++ b/compiler/ccgexprs.nim
@@ -1550,6 +1550,7 @@ proc genObjConstr(p: BProc, e: PNode, d: var TLoc) =
 
   var tmp: TLoc
   var r: Rope
+  let needsZeroMem = p.config.selectedGC notin {gcArc, gcAtomicArc, gcOrc} or nfAllFieldsSet notin e.flags
   if useTemp:
     getTemp(p, t, tmp)
     r = rdLoc(tmp)
@@ -1558,10 +1559,13 @@ proc genObjConstr(p: BProc, e: PNode, d: var TLoc) =
       t = t.lastSon.skipTypes(abstractInstOwned)
       r = "(*$1)" % [r]
       gcUsage(p.config, e)
-    else:
+    elif needsZeroMem:
       constructLoc(p, tmp)
+    else:
+      genObjectInit(p, cpsStmts, t, tmp, constructObj)
   else:
-    resetLoc(p, d)
+    if needsZeroMem: resetLoc(p, d)
+    else: genObjectInit(p, cpsStmts, d.t, d, if isRef: constructRefObj else: constructObj)
     r = rdLoc(d)
   discard getTypeDesc(p.module, t)
   let ty = getUniqueType(t)
diff --git a/compiler/cgen.nim b/compiler/cgen.nim
index 994633727b..d71896ef8c 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -986,11 +986,19 @@ proc closureSetup(p: BProc, prc: PSym) =
     linefmt(p, cpsStmts, "$1 = ($2) ClE_0;$n",
             [rdLoc(env.loc), getTypeDesc(p.module, env.typ)])
 
+const harmless = {nkConstSection, nkTypeSection, nkEmpty, nkCommentStmt, nkTemplateDef,
+                  nkMacroDef, nkMixinStmt, nkBindStmt, nkFormalParams} +
+                  declarativeDefs
+
 proc containsResult(n: PNode): bool =
   result = false
   case n.kind
-  of nkEmpty..pred(nkSym), succ(nkSym)..nkNilLit, nkFormalParams:
+  of succ(nkEmpty)..pred(nkSym), succ(nkSym)..nkNilLit, harmless:
     discard
+  of nkReturnStmt:
+    for i in 0..<n.len:
+      if containsResult(n[i]): return true
+    result = n.len > 0 and n[0].kind == nkEmpty
   of nkSym:
     if n.sym.kind == skResult:
       result = true
@@ -998,10 +1006,6 @@ proc containsResult(n: PNode): bool =
     for i in 0..<n.len:
       if containsResult(n[i]): return true
 
-const harmless = {nkConstSection, nkTypeSection, nkEmpty, nkCommentStmt, nkTemplateDef,
-                  nkMacroDef, nkMixinStmt, nkBindStmt, nkFormalParams} +
-                  declarativeDefs
-
 proc easyResultAsgn(n: PNode): PNode =
   case n.kind
   of nkStmtList, nkStmtListExpr:
@@ -1175,7 +1179,13 @@ proc genProcAux*(m: BModule, prc: PSym) =
         # declare the result symbol:
         assignLocalVar(p, resNode)
         assert(res.loc.r != "")
-        initLocalVar(p, res, immediateAsgn=false)
+        if p.config.selectedGC in {gcArc, gcAtomicArc, gcOrc} and
+            allPathsAsgnResult(procBody) == InitSkippable:
+          # In an ideal world the codegen could rely on injectdestructors doing its job properly
+          # and then the analysis step would not be required.
+          discard "result init optimized out"
+        else:
+          initLocalVar(p, res, immediateAsgn=false)
       returnStmt = ropecg(p.module, "\treturn $1;$n", [rdLoc(res.loc)])
     elif sfConstructor in prc.flags:
       fillLoc(resNode.sym.loc, locParam, resNode, "this", OnHeap)
diff --git a/tests/misc/trunner.nim b/tests/misc/trunner.nim
index 626b1a8863..f0262f5280 100644
--- a/tests/misc/trunner.nim
+++ b/tests/misc/trunner.nim
@@ -271,10 +271,13 @@ sub/mmain.idx""", context
     check execCmdEx(cmd) == ("12\n", 0)
 
   block: # bug #15316
-    let file = testsDir / "misc/m15316.nim"
-    let cmd = fmt"{nim} check --hints:off --nimcache:{nimcache} {file}"
-    check execCmdEx(cmd) == ("m15316.nim(1, 15) Error: expression expected, but found \')\'\nm15316.nim(2, 1) Error: expected: \':\', but got: \'[EOF]\'\nm15316.nim(2, 1) Error: expression expected, but found \'[EOF]\'\nm15316.nim(2, 1) " &
-          "Error: expected: \')\', but got: \'[EOF]\'\nError: illformed AST: \n", 1)
+    when not defined(windows):
+      # This never worked reliably on Windows. Needs further investigation but it is hard to reproduce.
+      # Looks like a mild stack corruption when bailing out of nested exception handling.
+      let file = testsDir / "misc/m15316.nim"
+      let cmd = fmt"{nim} check --hints:off --nimcache:{nimcache} {file}"
+      check execCmdEx(cmd) == ("m15316.nim(1, 15) Error: expression expected, but found \')\'\nm15316.nim(2, 1) Error: expected: \':\', but got: \'[EOF]\'\nm15316.nim(2, 1) Error: expression expected, but found \'[EOF]\'\nm15316.nim(2, 1) " &
+            "Error: expected: \')\', but got: \'[EOF]\'\nError: illformed AST: \n", 1)
 
 
   block: # config.nims, nim.cfg, hintConf, bug #16557