[clang] [llvm] [IndVarSimplify] Sink unused l-invariant loads in preheader. (PR #157559)

[Sirish Pande via llvm-commits]

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@@ -1100,14 +1112,32 @@ bool IndVarSimplify::sinkUnusedInvariants(Loop *L) {
       break;
 
     // Don't move instructions which might have side effects, since the side
-    // effects need to complete before instructions inside the loop.  Also don't
-    // move instructions which might read memory, since the loop may modify
-    // memory. Note that it's okay if the instruction might have undefined
-    // behavior: LoopSimplify guarantees that the preheader dominates the exit
-    // block.
-    if (I.mayHaveSideEffects() || I.mayReadFromMemory())
+    // effects need to complete before instructions inside the loop. Note that
+    // it's okay if the instruction might have undefined behavior: LoopSimplify
+    // guarantees that the preheader dominates the exit block.
+    if (I.mayHaveSideEffects())
       continue;
 
+    // Don't sink read instruction which's memory might be modified in the loop.
+    if (I.mayReadFromMemory()) {
+      if (LoadInst *Load = dyn_cast<LoadInst>(&I)) {
+        MemoryLocation Loc = MemoryLocation::get(Load);
+        bool isModified = false;
+
+        // Check if any store instruction in the loop modifies the loaded memory
+        // location.
+        for (Instruction *S : Stores) {
+          if (isModSet(AA->getModRefInfo(S, Loc))) {
+            isModified = true;
+            break;
+          }
+        }
+        if (isModified)
+          continue;
+      } else {
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srpande wrote:

Please add comment for clarity. 

https://github.com/llvm/llvm-project/pull/157559