Optimize storing into uninit locations for arrays and seqs. (#24619)

Author: planetis-m

lib/system.nim

@@ -2965,14 +2965,16 @@ when notJSnotNims and not defined(nimSeqsV2):
       assert y == "abcgh"
     discard
 
-proc arrayWith*[T](y: T, size: static int): array[size, T] {.raises: [].} =
+proc arrayWith*[T](y: T, size: static int): array[size, T] {.noinit, nodestroy, raises: [].} =
   ## Creates a new array filled with `y`.
-  result = zeroDefault(array[size, T])
   for i in 0..size-1:
-    result[i] = y
+    when (NimMajor, NimMinor, NimPatch) >= (2, 3, 1):
+      result[i] = `=dup`(y)
+    else:
+      wasMoved(result[i])
+      `=copy`(result[i], y)
 
-proc arrayWithDefault*[T](size: static int): array[size, T] {.raises: [].} =
+proc arrayWithDefault*[T](size: static int): array[size, T] {.noinit, nodestroy, raises: [].} =
   ## Creates a new array filled with `default(T)`.
-  result = zeroDefault(array[size, T])
   for i in 0..size-1:
     result[i] = default(T)

lib/system/seqs_v2.nim

@@ -144,8 +144,11 @@ proc grow*[T](x: var seq[T]; newLen: Natural; value: T) {.nodestroy.} =
     xu.p = cast[typeof(xu.p)](prepareSeqAddUninit(oldLen, xu.p, newLen - oldLen, sizeof(T), alignof(T)))
   xu.len = newLen
   for i in oldLen .. newLen-1:
-    wasMoved(xu.p.data[i])
-    `=copy`(xu.p.data[i], value)
+    when (NimMajor, NimMinor, NimPatch) >= (2, 3, 1):
+      xu.p.data[i] = `=dup`(value)
+    else:
+      wasMoved(xu.p.data[i])
+      `=copy`(xu.p.data[i], value)
 
 proc add*[T](x: var seq[T]; y: sink T) {.magic: "AppendSeqElem", noSideEffect, nodestroy.} =
   ## Generic proc for adding a data item `y` to a container `x`.