[compiler-rt] [NFC] [HWASan] clarify FIXME comment (PR #87689)

[Florian Mayer via llvm-commits]

https://github.com/fmayer created https://github.com/llvm/llvm-project/pull/87689

None

>From f8eeb741e19aa2294b2d6a4d4338856011cba6c5 Mon Sep 17 00:00:00 2001
From: Florian Mayer <fmayer at google.com>
Date: Thu, 4 Apr 2024 12:42:25 -0700
Subject: [PATCH] [NFC] [HWASan] clarify FIXME comment

---
 compiler-rt/lib/hwasan/hwasan_thread_list.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/compiler-rt/lib/hwasan/hwasan_thread_list.h b/compiler-rt/lib/hwasan/hwasan_thread_list.h
index f36d27864fc207..5eeb771cdf8203 100644
--- a/compiler-rt/lib/hwasan/hwasan_thread_list.h
+++ b/compiler-rt/lib/hwasan/hwasan_thread_list.h
@@ -55,6 +55,9 @@ static uptr RingBufferSize() {
   uptr desired_bytes = flags()->stack_history_size * sizeof(uptr);
   // FIXME: increase the limit to 8 once this bug is fixed:
   // https://bugs.llvm.org/show_bug.cgi?id=39030
+  // Note that we *cannot* do that on Android, as the runtime will indefinitely
+  // have to support code that is compield with ashr, which only works with
+  // shifts up to 6.
   for (int shift = 0; shift < 7; ++shift) {
     uptr size = 4096 * (1ULL << shift);
     if (size >= desired_bytes)