From 318b2cfc5ebeee5e27982bb5a76fa4c00d523772 Mon Sep 17 00:00:00 2001
From: heterodoxic <122719743+heterodoxic@users.noreply.github.com>
Date: Thu, 18 Apr 2024 21:58:01 +0200
Subject: [PATCH] =?UTF-8?q?allow=20having=20{.noinit.}=20on=20a=20complex?=
 =?UTF-8?q?=20type=20avoid=20memsets=20to=200=20for=20its=20=E2=80=A6=20(#?=
 =?UTF-8?q?23388)?=
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…instantiations (C/C++ backend)

AFAIK, #22802 expanded `noinit`'s utility by allowing the pragma to be
attached to types (thanks @jmgomez !).
I suggest broadening the scope a bit further: try to avoid `nimZeroMem`s
on a type level beyond imported C/C++ types[^1], saving us from
annotating the type instantiations with `noinit`.

If this change is deemed acceptable, I will also adjust the docs, of
course.

Adding tests for this change seems a bit problematic, as the effect of
this type annotation will be to work with uninitialized memory, which
*might* match 0 patterns.

[^1]: "complex value types" as already defined here:
https://github.com/nim-lang/Nim/blob/94c599687796f4ee3872c8aa866827b9ed33f52b/compiler/cgen.nim#L470-L471
---
 compiler/ccgtypes.nim | 3 +++
 compiler/cgen.nim     | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/compiler/ccgtypes.nim b/compiler/ccgtypes.nim
index 3590a17bcb..1366caab30 100644
--- a/compiler/ccgtypes.nim
+++ b/compiler/ccgtypes.nim
@@ -245,6 +245,9 @@ proc isImportedCppType(t: PType): bool =
 proc isOrHasImportedCppType(typ: PType): bool =
   searchTypeFor(typ.skipTypes({tyRef}), isImportedCppType)
 
+proc hasNoInit(t: PType): bool =
+  result = t.sym != nil and sfNoInit in t.sym.flags
+
 proc getTypeDescAux(m: BModule; origTyp: PType, check: var IntSet; kind: TypeDescKind): Rope
 
 proc isObjLackingTypeField(typ: PType): bool {.inline.} =
diff --git a/compiler/cgen.nim b/compiler/cgen.nim
index 043c014a21..e4ba416145 100644
--- a/compiler/cgen.nim
+++ b/compiler/cgen.nim
@@ -532,7 +532,7 @@ proc constructLoc(p: BProc, loc: var TLoc, isTemp = false) =
       linefmt(p, cpsStmts, "$1 = ($2)0;$n", [rdLoc(loc),
         getTypeDesc(p.module, typ, descKindFromSymKind mapTypeChooser(loc))])
   else:
-    if not isTemp or containsGarbageCollectedRef(loc.t):
+    if (not isTemp or containsGarbageCollectedRef(loc.t)) and not hasNoInit(loc.t):
       # don't use nimZeroMem for temporary values for performance if we can
       # avoid it:
       if not isOrHasImportedCppType(typ):