[compiler-rt] [TSan] Ignore reads if not stored early (PR #74575)

[via llvm-commits]

https://github.com/felilxtomski created https://github.com/llvm/llvm-project/pull/74575

None

>From dd23a5aed8e5d36c59823c438c3b9279f2d6adff Mon Sep 17 00:00:00 2001
From: "felix.tomski" <tomski at itc.rwth-aachen.de>
Date: Thu, 30 Nov 2023 23:23:17 +0100
Subject: [PATCH] Ignore reads if not stored early

---
 compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp b/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
index 8b20984a01000..ed1de7ed3bb17 100644
--- a/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
@@ -224,6 +224,8 @@ bool CheckRaces(ThreadState* thr, RawShadow* shadow_mem, Shadow cur,
   // the current access info, so we are done.
   if (LIKELY(stored))
     return false;
+  if (LIKELY(typ & kAccessRead))
+    return false;
   // Choose a random candidate slot and replace it.
   uptr index =
       atomic_load_relaxed(&thr->trace_pos) / sizeof(Event) % kShadowCnt;
@@ -345,8 +347,12 @@ STORE : {
     const m128 empty = _mm_cmpeq_epi32(shadow, zero);
     const int empty_mask = _mm_movemask_epi8(empty);
     index = __builtin_ffs(empty_mask);
-    if (UNLIKELY(index == 0))
+    if (UNLIKELY(index == 0)) {
+      // If we reach here, we give up storing reads
+      if (typ & kAccessRead)
+        return false;
       index = (atomic_load_relaxed(&thr->trace_pos) / 2) % 16;
+    }
   }
   StoreShadow(&shadow_mem[index / 4], cur.raw());
   // We could zero other slots determined by rewrite_mask.