[compiler-rt] [NFC][sanitizer] Update `DTLS_on_tls_get_addr` failure comment (PR #109546)

[Vitaly Buka via llvm-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/109546

Also return `nullptr` on failure, as is all caller
handler `nullptr` and `{tls_beg, 0}` in the same
manner.


>From 5ad995d1b10c52d2c2d2dc6b4e20dbade53c0abd Mon Sep 17 00:00:00 2001
From: Vitaly Buka <vitalybuka at google.com>
Date: Sat, 21 Sep 2024 13:01:44 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.4
---
 .../lib/sanitizer_common/sanitizer_tls_get_addr.cpp  | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
index 6d5fad3e7b4a4b..e08bcb435ca841 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
@@ -158,10 +158,18 @@ DTLS::DTV *DTLS_on_tls_get_addr(void *arg_void, void *res,
     return dtv;
   }
   VReport(2, "__tls_get_addr: Can't guess glibc version\n");
-  // This may happen inside the DTOR of main thread, so just ignore it.
+  // This may happen inside the DTOR a thread, or async signal handlers before
+  // thread initialization, so just ignore it.
+  //
+  // If the unknown block is dynamic TLS, unlikely we will be able to recognize
+  // it in future, mark it as done with '{tls_beg, 0}'.
+  //
+  // If the block is static TLS, possible reason of failed detection is nullptr
+  // in `static_tls_begin`. Regardless of reasons, the future handling of static
+  // TLS is still '{tls_beg, 0}'.
   dtv->beg = tls_beg;
   dtv->size = 0;
-  return dtv;
+  return nullptr;
 }
 
 DTLS *DTLS_Get() { return &dtls; }