[llvm-mca] Fixed a bug in the logic that checks if a memory operation is ready to execute.

Fixes PR37790.

In some (very rare) cases, the LSUnit (Load/Store unit) was wrongly marking a
load (or store) as "ready to execute" effectively bypassing older memory barrier
instructions.

To reproduce this bug, the memory barrier must be the first instruction in the
input assembly sequence, and it doesn't have to perform any register writes.

llvm-svn: 334633

1 files changed, 12 insertions, 8 deletions

diff --git a/llvm/tools/llvm-mca/LSUnit.cpp b/llvm/tools/llvm-mca/LSUnit.cpp
index dfd3e53fb49..c2f3b9dd5a0 100644
--- a/llvm/tools/llvm-mca/LSUnit.cpp
+++ b/llvm/tools/llvm-mca/LSUnit.cpp
@@ -78,18 +78,16 @@ bool LSUnit::isReady(const InstRef &IR) const {
   bool IsAStore = StoreQueue.count(Index) != 0;
   assert((IsALoad || IsAStore) && "Instruction is not in queue!");
 
-  unsigned LoadBarrierIndex = LoadBarriers.empty() ? 0 : *LoadBarriers.begin();
-  unsigned StoreBarrierIndex =
-      StoreBarriers.empty() ? 0 : *StoreBarriers.begin();
-
-  if (IsALoad && LoadBarrierIndex) {
+  if (IsALoad && !LoadBarriers.empty()) {
+    unsigned LoadBarrierIndex = *LoadBarriers.begin();
     if (Index > LoadBarrierIndex)
       return false;
     if (Index == LoadBarrierIndex && Index != *LoadQueue.begin())
       return false;
   }
 
-  if (IsAStore && StoreBarrierIndex) {
+  if (IsAStore && !StoreBarriers.empty()) {
+    unsigned StoreBarrierIndex = *StoreBarriers.begin();
     if (Index > StoreBarrierIndex)
       return false;
     if (Index == StoreBarrierIndex && Index != *StoreQueue.begin())
@@ -135,9 +133,15 @@ void LSUnit::onInstructionExecuted(const InstRef &IR) {
     StoreQueue.erase(it);
   }
 
-  if (!StoreBarriers.empty() && Index == *StoreBarriers.begin())
+  if (!StoreBarriers.empty() && Index == *StoreBarriers.begin()) {
+    LLVM_DEBUG(dbgs() << "[LSUnit]: Instruction idx=" << Index
+                      << " has been removed from the set of store barriers.\n");
     StoreBarriers.erase(StoreBarriers.begin());
-  if (!LoadBarriers.empty() && Index == *LoadBarriers.begin())
+  }
+  if (!LoadBarriers.empty() && Index == *LoadBarriers.begin()) {
+    LLVM_DEBUG(dbgs() << "[LSUnit]: Instruction idx=" << Index
+                      << " has been removed from the set of load barriers.\n");
     LoadBarriers.erase(LoadBarriers.begin());
+  }
 }
 } // namespace mca