[llvm] [MachineLICM] Don't allow hoisting invariant loads across mem barrier. (PR #116987)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

The improvements in 63917e1 / #<!-- -->70796 do not check for memory barriers/unmodelled sideeffects, which means we may incorrectly hoist loads across memory barriers.

Fix this by checking any machine instruction in the loop is a load-fold barrier.

---
Full diff: https://github.com/llvm/llvm-project/pull/116987.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/MachineLICM.cpp (+1-1) 
- (modified) llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll (+2-2) 


``````````diff
diff --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index 48c901b8d06d61..00e3b5def5d108 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -1501,7 +1501,7 @@ void MachineLICMImpl::InitializeLoadsHoistableLoops() {
       if (!AllowedToHoistLoads[Loop])
         continue;
       for (auto &MI : *MBB) {
-        if (!MI.mayStore() && !MI.isCall() &&
+        if (!MI.isLoadFoldBarrier() && !MI.mayStore() && !MI.isCall() &&
             !(MI.mayLoad() && MI.hasOrderedMemoryRef()))
           continue;
         for (MachineLoop *L = Loop; L != nullptr; L = L->getParentLoop())
diff --git a/llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll b/llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll
index 932a5af264a000..17f8263560430d 100644
--- a/llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll
+++ b/llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll
@@ -499,16 +499,16 @@ for.exit:                                 ; preds = %for.body
 
 @a = external local_unnamed_addr global i32, align 4
 
-; FIXME: Load hoisted out of the loop across memory barriers.
+; Make sure the load is not hoisted out of the loop across memory barriers.
 define i32 @load_between_memory_barriers() {
 ; CHECK-LABEL: load_between_memory_barriers:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    adrp x8, :got:a
 ; CHECK-NEXT:    ldr x8, [x8, :got_lo12:a]
-; CHECK-NEXT:    ldr w0, [x8]
 ; CHECK-NEXT:  .LBB8_1: // %loop
 ; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
 ; CHECK-NEXT:    //MEMBARRIER
+; CHECK-NEXT:    ldr w0, [x8]
 ; CHECK-NEXT:    //MEMBARRIER
 ; CHECK-NEXT:    cbz w0, .LBB8_1
 ; CHECK-NEXT:  // %bb.2: // %exit

``````````

</details>


https://github.com/llvm/llvm-project/pull/116987