[libcxx-commits] [libcxx] [ASan][libc++] Hotfix: String annotations stack frame size (PR #76192)

[via libcxx-commits]

https://github.com/AdvenamTacet created https://github.com/llvm/llvm-project/pull/76192

This change affects optimizations during compilation and reduces size of stack frames.

It's also one of many changes suggested in https://github.com/llvm/llvm-project/pull/76165. As  https://github.com/llvm/llvm-project/pull/76082 was merged, I think that one should be as well.

>From 8e4fb9d0ae7123d85c567b410c265f989cfe75da Mon Sep 17 00:00:00 2001
From: Advenam Tacet <advenam.tacet at trailofbits.com>
Date: Thu, 21 Dec 2023 23:42:04 +0100
Subject: [PATCH] [ASan][libc++] Hotfix: String annotations stack frame size

This change affects optimizations during compilation and reduces size of stack frames.
---
 libcxx/include/string | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libcxx/include/string b/libcxx/include/string
index c676182fba8bac..f0af77c4d767ba 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -919,10 +919,16 @@ public:
 #  else
       _NOEXCEPT
 #  endif
+#ifndef _LIBCPP_HAS_NO_ASAN
       // Turning off ASan instrumentation for variable initialization with _LIBCPP_STRING_INTERNAL_MEMORY_ACCESS
       // does not work consistently during initialization of __r_, so we instead unpoison __str's memory manually first.
       // __str's memory needs to be unpoisoned only in the case where it's a short string.
       : __r_(((__str.__is_long() ? 0 : (__str.__annotate_delete(), 0)), std::move(__str.__r_))) {
+    // ASan TODO: ^ This line results in stack frame growth and isn't correctly optimized by the compiler.
+    //              It should be refactored to improve performance.
+#else
+      : __r_(std::move(__str.__r_)) {
+#endif
     __str.__r_.first() = __rep();
     __str.__annotate_new(0);
     if (!__is_long())