[MLIR][EmitC] Don't translate expressions inline if user is emitc.subscript (#91087)

This change updates the logic that determines whether an `emitc.expression`
result is translated into a dedicated variable assignment. Due to how
the translation of `emitc.subscript` currently works, a previously
inline-able `emitc.expression` would produce incorrect C++ if its single user 
was a `emitc.subscript` operation.

Author: christopherbate

mlir/lib/Target/Cpp/TranslateToCpp.cpp

@@ -293,9 +293,16 @@ static bool shouldBeInlined(ExpressionOp expressionOp) {
   if (!result.hasOneUse())
     return false;
 
+  Operation *user = *result.getUsers().begin();
+
+  // Do not inline expressions used by subscript operations, since the
+  // way the subscript operation translation is implemented requires that
+  // variables be materialized.
+  if (isa<emitc::SubscriptOp>(user))
+    return false;
+
   // Do not inline expressions used by other expressions, as any desired
   // expression folding was taken care of by transformations.
-  Operation *user = *result.getUsers().begin();
   return !user->getParentOfType<ExpressionOp>();
 }
 

mlir/test/Target/Cpp/expressions.mlir

@@ -210,3 +210,18 @@ func.func @expression_with_address_taken(%arg0: i32, %arg1: i32, %arg2: !emitc.p
   }
   return %c : i1
 }
+
+// CPP-DEFAULT: int32_t expression_with_subscript_user(void* [[VAL_1:v.+]])
+// CPP-DEFAULT-NEXT:   int64_t [[VAL_2:v.+]] = 0;
+// CPP-DEFAULT-NEXT:   int32_t* [[VAL_3:v.+]] = (int32_t*) [[VAL_1]];
+// CPP-DEFAULT-NEXT:   return [[VAL_3]][[[VAL_2]]];
+
+func.func @expression_with_subscript_user(%arg0: !emitc.ptr<!emitc.opaque<"void">>) -> i32 {
+  %c0 = "emitc.constant"() {value = 0 : i64} : () -> i64
+  %0 = emitc.expression : !emitc.ptr<i32> {
+    %0 = emitc.cast %arg0 : !emitc.ptr<!emitc.opaque<"void">> to !emitc.ptr<i32>
+    emitc.yield %0 : !emitc.ptr<i32>
+  }
+  %1 = emitc.subscript %0[%c0] : (!emitc.ptr<i32>, i64) -> i32
+  return %1 : i32
+}