re-enable check for exclusive if

src/gt4py/next/program_processors/runners/dace/gtir_dataflow.py

@@ -1637,13 +1637,87 @@ def _visit_tuple_get(
         tuple_fields = self.visit(node.args[1])
         return tuple_fields[index]
 
+    def requires_exclusive_if(self, node: gtir.FunCall) -> bool:
+        """
+        The meaning of `if_` builtin function is unclear in GTIR.
+        In some context, it corresponds to a ternary operator where, depending on
+        the condition result, only one branch or the other should be executed,
+        because one of them is invalid. The typical case is the use of `if_` to
+        decide whether it is possible or not to access a shifted iterator, for
+        example when the condition expression calls `can_deref`.
+        The ternary operator is also used in iterator view, where the field arguments
+        are not necessarily both defined on the entire output domain (this behavior
+        should not appear in field view, because there the user code should use
+        `concat_where` instead of `where` for such cases). It is difficult to catch
+        such behavior, because it would require to know the exact domain of all
+        fields, which is not known at compile time. However, the iterator view
+        behavior should only appear inside scan field operators.
+        A different usage of `if_` expressions is selecting one argument value or
+        the other, where both arguments are defined on the output domain, therefore
+        always valid.
+        In order to simplify the SDFG and facilitate the optimization stage, we
+        try to avoid the ternary operator form when not needed. The reason is that
+        exclusive branch execution is represented in the SDFG as a conditional
+        state transition, which prevents fusion.
+        """
+        assert cpm.is_call_to(node, "if_")
+        assert len(node.args) == 3
+
+        condition_vars = (
+            eve.walk_values(node.args[0])
+            .if_isinstance(gtir.SymRef)
+            .map(lambda node: str(node.id))
+            .filter(lambda x: x in self.symbol_map)
+            .to_set()
+        )
+
+        # first, check if any argument contains shift expressions that depend on the condition variables
+        for arg in node.args[1:3]:
+            shift_nodes = (
+                eve.walk_values(arg).filter(lambda node: cpm.is_applied_shift(node)).to_set()
+            )
+            for shift_node in shift_nodes:
+                shift_vars = (
+                    eve.walk_values(shift_node)
+                    .if_isinstance(gtir.SymRef)
+                    .map(lambda node: str(node.id))
+                    .filter(lambda x: x in self.symbol_map)
+                    .to_set()
+                )
+                # require exclusive branch execution if any shift expression one of
+                # the if branches accesses a variable used in the condition expression
+                depend_vars = condition_vars.intersection(shift_vars)
+                if len(depend_vars) != 0:
+                    return True
+
+        # secondly, check whether the `if_` branches access different sets of fields
+        # and this happens inside a scan field operator
+        if self.scan_carry_symbol is not None:
+            # the `if_` node is inside a scan stencil expression
+            scan_carry_var = str(self.scan_carry_symbol.id)
+            if scan_carry_var in condition_vars:
+                br1_vars, br2_vars = (
+                    eve.walk_values(arg)
+                    .if_isinstance(gtir.SymRef)
+                    .map(lambda node: str(node.id))
+                    .filter(lambda x: isinstance(self.symbol_map.get(x, None), MemletExpr))
+                    .to_set()
+                    for arg in node.args[1:3]
+                )
+                if br1_vars != br2_vars:
+                    # the two branches of the `if_` expression access different sets of fields,
+                    # depending on the scan carry value
+                    return True
+
+        return False
+
     def visit_FunCall(
         self, node: gtir.FunCall
     ) -> IteratorExpr | DataExpr | tuple[IteratorExpr | DataExpr | tuple[Any, ...], ...]:
         if cpm.is_call_to(node, "deref"):
             return self._visit_deref(node)
 
-        elif cpm.is_call_to(node, "if_"):
+        elif cpm.is_call_to(node, "if_") and self.requires_exclusive_if(node):
             return self._visit_if(node)
 
         elif cpm.is_call_to(node, "neighbors"):