[flang] Skip optimized bufferization on volatile refs

Memory effects on the volatile memory resource may not be attached
to a particular source, in which case the value of an effect will
be null which caused this test case to crash in the optimized
bufferization pass's safety analysis because it assumes it can get
the SSA value modified by the memory effect. This is because memory
effects on the volatile resource indicate that the operation must
not be reordered with respect to other volatile operations, but
there is not a material ssa value that can be pointed to.

This patch changes the safety checks to indicate that that memory
effects on volatile resources are not safe for optimized bufferization.

Author: ashermancinelli

flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp

@@ -608,6 +608,13 @@ ElementalAssignBufferization::findMatch(hlfir::ElementalOp elemental) {
       return std::nullopt;
     }
 
+    // Don't allow any reads to or writes from volatile memory
+    if (mlir::isa<mlir::MemoryEffects::Read, mlir::MemoryEffects::Write>(
+            effect.getEffect()) &&
+        mlir::isa<fir::VolatileMemoryResource>(effect.getResource())) {
+      return std::nullopt;
+    }
+
     // allow if and only if the reads are from the elemental indices, in order
     // => each iteration doesn't read values written by other iterations
     // don't allow reads from a different value which may alias: fir alias

flang/test/HLFIR/opt-bufferization-skip-volatile.fir

@@ -0,0 +1,49 @@
+// RUN: fir-opt --pass-pipeline="builtin.module(func.func(opt-bufferization))" %s | FileCheck %s
+
+// Ensure optimized bufferization preserves the semantics of volatile arrays
+func.func @minimal_volatile_test() {
+  %c1 = arith.constant 1 : index
+  %c200 = arith.constant 200 : index
+  
+  // Create a volatile array
+  %1 = fir.address_of(@_QMtestEarray) : !fir.ref<!fir.array<200xf32>>
+  %2 = fir.shape %c200 : (index) -> !fir.shape<1>
+  %3 = fir.volatile_cast %1 : (!fir.ref<!fir.array<200xf32>>) -> !fir.ref<!fir.array<200xf32>, volatile>
+  %4:2 = hlfir.declare %3(%2) {fortran_attrs = #fir.var_attrs<volatile>, uniq_name = "_QMtestEarray"} : (!fir.ref<!fir.array<200xf32>, volatile>, !fir.shape<1>) -> (!fir.ref<!fir.array<200xf32>, volatile>, !fir.ref<!fir.array<200xf32>, volatile>)
+  
+  // Create an elemental operation that negates each element
+  %5 = hlfir.elemental %2 unordered : (!fir.shape<1>) -> !hlfir.expr<200xf32> {
+  ^bb0(%arg1: index):
+    %6 = hlfir.designate %4#0 (%arg1) : (!fir.ref<!fir.array<200xf32>, volatile>, index) -> !fir.ref<f32, volatile>
+    %7 = fir.load %6 : !fir.ref<f32, volatile>
+    %8 = arith.negf %7 : f32
+    hlfir.yield_element %8 : f32
+  }
+  
+  // Assign the result back to the volatile array
+  hlfir.assign %5 to %4#0 : !hlfir.expr<200xf32>, !fir.ref<!fir.array<200xf32>, volatile>
+  hlfir.destroy %5 : !hlfir.expr<200xf32>
+  
+  return
+}
+
+fir.global @_QMtestEarray : !fir.array<200xf32> 
+
+// CHECK-LABEL:   func.func @minimal_volatile_test() {
+// CHECK:           %[[VAL_0:.*]] = arith.constant 200 : index
+// CHECK:           %[[VAL_1:.*]] = fir.address_of(@_QMtestEarray) : !fir.ref<!fir.array<200xf32>>
+// CHECK:           %[[VAL_2:.*]] = fir.shape %[[VAL_0]] : (index) -> !fir.shape<1>
+// CHECK:           %[[VAL_3:.*]] = fir.volatile_cast %[[VAL_1]] : (!fir.ref<!fir.array<200xf32>>) -> !fir.ref<!fir.array<200xf32>, volatile>
+// CHECK:           %[[VAL_4:.*]]:2 = hlfir.declare %[[VAL_3]](%[[VAL_2]]) {fortran_attrs = #fir.var_attrs<volatile>, uniq_name = "_QMtestEarray"} : (!fir.ref<!fir.array<200xf32>, volatile>, !fir.shape<1>) -> (!fir.ref<!fir.array<200xf32>, volatile>, !fir.ref<!fir.array<200xf32>, volatile>)
+// CHECK:           %[[VAL_5:.*]] = hlfir.elemental %[[VAL_2]] unordered : (!fir.shape<1>) -> !hlfir.expr<200xf32> {
+// CHECK:           ^bb0(%[[VAL_6:.*]]: index):
+// CHECK:             %[[VAL_7:.*]] = hlfir.designate %[[VAL_4]]#0 (%[[VAL_6]])  : (!fir.ref<!fir.array<200xf32>, volatile>, index) -> !fir.ref<f32, volatile>
+// CHECK:             %[[VAL_8:.*]] = fir.load %[[VAL_7]] : !fir.ref<f32, volatile>
+// CHECK:             %[[VAL_9:.*]] = arith.negf %[[VAL_8]] : f32
+// CHECK:             hlfir.yield_element %[[VAL_9]] : f32
+// CHECK:           }
+// CHECK:           hlfir.assign %[[VAL_5]] to %[[VAL_4]]#0 : !hlfir.expr<200xf32>, !fir.ref<!fir.array<200xf32>, volatile>
+// CHECK:           hlfir.destroy %[[VAL_5]] : !hlfir.expr<200xf32>
+// CHECK:           return
+// CHECK:         }
+// CHECK:         fir.global @_QMtestEarray : !fir.array<200xf32>