Support removal of loop carried events in airrt-to-ipu (Xilinx#470)

Check for uses before deleting airrt.wait_all and try again after unroll.

mlir/lib/Conversion/AIRRtToIpuPass.cpp

@@ -795,6 +795,10 @@ struct AIRRtToIpuPass : public impl::AIRRtToIpuBase<AIRRtToIpuPass> {
     (void)applyPatternsAndFoldGreedily(module, std::move(canoPatterns_1));
     unrollSCFFors(module);
 
+    // Purge all wait ops again after unroll, in case there were loop carried
+    // events which couldn't be purged before
+    purgeWaitAlls(module);
+
     // Purge dma ops' async tokens
     purgeDmaAsyncTokens(module);
 
@@ -901,13 +905,20 @@ struct AIRRtToIpuPass : public impl::AIRRtToIpuBase<AIRRtToIpuPass> {
   }
 
   void purgeWaitAlls(ModuleOp module) {
-    SmallVector<WaitAllOp> waits;
-    module.walk([&](WaitAllOp w) { waits.push_back(w); });
-    for (auto w : waits) {
-      w->eraseOperands(0, w->getNumOperands());
-    }
-    for (auto w : waits) {
-      w.erase();
+    int size = 0;
+    int last_size = 1;
+    while (size < last_size) {
+      SmallVector<WaitAllOp> waits;
+      module.walk([&](WaitAllOp w) { waits.push_back(w); });
+      size = waits.size();
+      last_size = size;
+      for (auto &w : waits) {
+        if (!w->use_empty())
+          continue;
+        w->eraseOperands(0, w->getNumOperands());
+        w.erase();
+        size--;
+      }
     }
   }
 

mlir/test/Conversion/AIRRtToIpu/airrt_to_ipu.mlir

@@ -602,4 +602,22 @@ module {
   }
 }
 
+// -----
+
+// Loop carried event
 
+// CHECK-LABEL: func.func @func14
+// CHECK-NEXT: return
+module {
+  func.func @func14() {
+    %c0 = arith.constant 0 : index
+    %c512 = arith.constant 512 : index
+    %c2048 = arith.constant 2048 : index
+    %9 = airrt.wait_all : !airrt.event
+    %11:1 = scf.for %arg6 = %c0 to %c2048 step %c512 iter_args(%arg7 = %9) -> (!airrt.event) {
+      %12 = airrt.wait_all : !airrt.event
+      scf.yield %12 : !airrt.event
+    }
+    return
+  }
+}