From cf42ae178deae475c4fdc2d927f91b4980ec8be5 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Fri, 5 Feb 2021 15:45:33 -0700
Subject: [PATCH] std.MultiArrayList: use `@memset` builtin for undefined

See comment for more details
---
 lib/std/multi_array_list.zig | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/std/multi_array_list.zig b/lib/std/multi_array_list.zig
index aaf105bb57..24269c2316 100644
--- a/lib/std/multi_array_list.zig
+++ b/lib/std/multi_array_list.zig
@@ -180,9 +180,15 @@ pub fn MultiArrayList(comptime S: type) type {
                 capacityInBytes(new_len),
                 .exact,
             ) catch {
+                const self_slice = self.slice();
                 inline for (fields) |field_info, i| {
                     const field = @intToEnum(Field, i);
-                    mem.set(field_info.field_type, self.slice().items(field)[new_len..], undefined);
+                    const dest_slice = self_slice.items(field)[new_len..];
+                    const byte_count = dest_slice.len * @sizeOf(field_info.field_type);
+                    // We use memset here for more efficient codegen in safety-checked,
+                    // valgrind-enabled builds. Otherwise the valgrind client request
+                    // will be repeated for every element.
+                    @memset(@ptrCast([*]u8, dest_slice.ptr), undefined, byte_count);
                 }
                 self.len = new_len;
                 return;