[compiler-rt] [sanitizer] Extract SANITIZER_FREEBSD version of ThreadDescriptorSizeFallback (PR #109743)

[Rainer Orth via llvm-commits]

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@@ -304,6 +293,48 @@ static uptr ThreadDescriptorSizeFallback() {
   return 1776;  // from glibc.ppc64le 2.20-8.fc21
 #    endif
 }
+#  endif  // SANITIZER_GLIBC && !SANITIZER_GO
+
+#  if SANITIZER_FREEBSD && !SANITIZER_GO
+// FIXME: Implementation is very GLIBC specific, but it's used by FreeBSD.
+static uptr ThreadDescriptorSizeFallback() {
+#    if defined(__s390__) || defined(__sparc__)
+  // The size of a prefix of TCB including pthread::{specific_1stblock,specific}
+  // suffices. Just return offsetof(struct pthread, specific_used), which hasn't
+  // changed since 2007-05. Technically this applies to i386/x86_64 as well but
+  // we call _dl_get_tls_static_info and need the precise size of struct
+  // pthread.
+  return FIRST_32_SECOND_64(524, 1552);
+#    endif
+
+#    if defined(__mips__)
+  // TODO(sagarthakur): add more values as per different glibc versions.
+  return FIRST_32_SECOND_64(1152, 1776);
+#    endif
+
+#    if SANITIZER_LOONGARCH64
+  return 1856;  // from glibc 2.36
+#    endif
+
+#    if defined(__aarch64__)
+  // The sizeof (struct pthread) is the same from GLIBC 2.17 to 2.22.
+  return 1776;
+#    endif
+
+#    if defined(__powerpc64__)
+  return 1776;  // from glibc.ppc64le 2.20-8.fc21
+#    endif
+
----------------
rorth wrote:

I have a hard time believing that that's any point in this: apart from being glibc-specific, as of FreeBSD 14.1 there isn't a s390, sparc, mips, or loongarch64 port of FreeBSD.

https://github.com/llvm/llvm-project/pull/109743