2704 backward accesses

changelog

@@ -29,6 +29,9 @@
 	10) PR #2810. Adds caching of the fparser2 parse tree to FileInfo. Is
 	disabled by default.
 
+	11) PR #2814 for #2704. Adds backward accesses capabilities to the
+	DefinitionUseChain tool.
+
 release 3.0.0 6th of December 2024
 
 	1) PR #2477 for #2463. Add support for Fortran Namelist statements.

doc/developer_guide/dependency.rst

@@ -666,12 +666,12 @@ can be parallelised:
 DefinitionUseChain
 ==================
 PSyclone also provides a DefinitionUseChain class, which can search for forward
-dependencies (backward NYI) for a given Reference inside a region of code. This
+and backward dependencies for a given Reference inside a region of code. This
 implementation differs from the DependencyTools as it is control-flow aware, so
 can find many dependencies for a single Reference in a given Routine or scope.
 
-This is primarily used to implement the `References.next_accesses` function, but can be
-used directly as follows:
+This is primarily used to implement the `Reference.next_accesses` and
+`Reference.previous_accessess` functions, but can be used directly as follows:
 
 .. code::
 

src/psyclone/psyir/nodes/reference.py

@@ -40,7 +40,7 @@
 ''' This module contains the implementation of the Reference node.'''
 
 
-from psyclone.core import AccessType, Signature, VariablesAccessInfo
+from psyclone.core import AccessType, Signature
 # We cannot import from 'nodes' directly due to circular import
 from psyclone.psyir.nodes.datanode import DataNode
 from psyclone.psyir.symbols import Symbol
@@ -233,34 +233,18 @@ def datatype(self):
             return UnresolvedType()
         return self.symbol.datatype
 
-    def previous_access(self):
+    def previous_accesses(self):
         '''
-        :returns: the previous reference to the same symbol.
-        :rtype: Optional[:py:class:`psyclone.psyir.nodes.Node`]
+        :returns: the nodes accessing the same symbol directly before this
+                  reference. It can be multiple nodes if the control flow
+                  diverges and there are multiple possible accesses.
+        :rtype: List[:py:class:`psyclone.psyir.nodes.Node`]
         '''
         # Avoid circular import
         # pylint: disable=import-outside-toplevel
-        from psyclone.psyir.nodes.routine import Routine
-        # The scope is as far as the Routine that contains this
-        # Reference.
-        routine = self.ancestor(Routine)
-        # Handle the case when this is a subtree without an ancestor
-        # Routine
-        if routine is None:
-            routine = self.root
-        var_access = VariablesAccessInfo(nodes=routine)
-        signature, _ = self.get_signature_and_indices()
-        all_accesses = var_access[signature].all_accesses
-        index = -1
-        # Find my position in the VariablesAccesInfo
-        for i, access in enumerate(all_accesses):
-            if access.node is self:
-                index = i
-                break
-
-        if index > 0:
-            return all_accesses[index-1].node
-        return None
+        from psyclone.psyir.tools import DefinitionUseChain
+        chain = DefinitionUseChain(self)
+        return chain.find_backward_accesses()
 
     def next_accesses(self):
         '''

src/psyclone/tests/psyir/nodes/reference_test.py

@@ -247,15 +247,31 @@ def test_reference_next_accesses(fortran_reader):
     end subroutine'''
     psyir = fortran_reader.psyir_from_source(code)
     routine = psyir.children[0]
-    a = routine.children[0].lhs
+    a_before_loop = routine.children[0].lhs
     loop = routine.children[1]
     b = loop.loop_body.children[0].lhs
-    a_2 = loop.loop_body.children[0].rhs
-    assert len(a.next_accesses()) == 1
-    assert a.next_accesses()[0] is loop
+    assert len(a_before_loop.next_accesses()) == 1
+    assert a_before_loop.next_accesses()[0] is loop
     assert len(b.next_accesses()) == 1
     assert b.next_accesses()[0] == b
 
+    # Check that a loop accessing a variable before
+    # the reference doesn't result in a false positive.
+    code = '''subroutine my_sub()
+    integer :: a
+    integer :: b
+    do a = 0, 10
+        b = a
+    end do
+    a = 1
+    end subroutine'''
+    psyir = fortran_reader.psyir_from_source(code)
+    routine = psyir.children[0]
+    a_after_loop = routine.children[1].lhs
+    loop = routine.children[0]
+    b = loop.loop_body.children[0].lhs
+    assert len(a_after_loop.next_accesses()) == 0
+
     # Check the function for basic structures
     code = '''subroutine my_sub()
     type :: x
@@ -359,8 +375,8 @@ def test_reference_next_accesses_with_codeblock(fortran_reader):
     assert a.next_accesses()[0] is codeblock
 
 
-def test_reference_previous_access(fortran_reader):
-    '''Test the previous_access function for a Reference'''
+def test_reference_previous_accesses(fortran_reader):
+    '''Test the previous_accesses function for a Reference'''
     code = '''subroutine my_sub()
     integer :: a
     integer :: b
@@ -374,10 +390,10 @@ def test_reference_previous_access(fortran_reader):
     b = routine.children[1].lhs
     a_2 = routine.children[2].lhs
     b_2 = routine.children[2].rhs
-    assert a.previous_access() is None
-    assert b.previous_access() is None
-    assert a_2.previous_access() is a
-    assert b_2.previous_access() is b
+    assert len(a.previous_accesses()) == 0
+    assert len(b.previous_accesses()) == 0
+    assert a_2.previous_accesses()[0] is a
+    assert b_2.previous_accesses()[0] is b
 
     code = '''subroutine my_sub()
     integer :: a
@@ -393,9 +409,11 @@ def test_reference_previous_access(fortran_reader):
     loop = routine.children[1]
     b_a = loop.loop_body.children[0].lhs
     a_2 = loop.loop_body.children[0].rhs
-    assert a.previous_access() is None
-    assert b_a.previous_access() is None
-    assert a_2.previous_access() is loop
+    assert len(a.previous_accesses()) == 0
+    assert len(b_a.previous_accesses()) == 1
+    assert b_a.previous_accesses()[0] is b_a
+    assert len(a_2.previous_accesses()) == 1
+    assert a_2.previous_accesses()[0] is loop
 
     # Check the function for basic structures
     code = '''subroutine my_sub()
@@ -415,10 +433,12 @@ def test_reference_previous_access(fortran_reader):
     b = routine.children[1].lhs
     a_2 = routine.children[2].lhs
     b_2 = routine.children[3].lhs
-    assert a.previous_access() is None
-    assert b.previous_access() is None
-    assert a_2.previous_access() is a
-    assert b_2.previous_access() is b
+    assert len(a.previous_accesses()) == 0
+    assert len(b.previous_accesses()) == 0
+    assert len(a_2.previous_accesses()) == 1
+    assert len(b_2.previous_accesses()) == 1
+    assert a_2.previous_accesses()[0] is a
+    assert b_2.previous_accesses()[0] is b
 
     # Check the function for array access
     code = '''subroutine my_sub()
@@ -430,8 +450,9 @@ def test_reference_previous_access(fortran_reader):
     routine = psyir.children[0]
     a = routine.children[0].lhs
     a_2 = routine.children[1].lhs
-    assert a.previous_access() is None
-    assert a_2.previous_access() is a
+    assert len(a.previous_accesses()) == 0
+    assert len(a_2.previous_accesses()) == 1
+    assert a_2.previous_accesses()[0] is a
 
     # Check if statements
     code = '''subroutine my_sub()
@@ -448,9 +469,12 @@ def test_reference_previous_access(fortran_reader):
     a = routine.children[0].lhs
     a_2 = routine.children[1].if_body.children[0].lhs
     a_3 = routine.children[2].lhs
-    assert a.previous_access() is None
-    assert a_2.previous_access() is a
-    assert a_3.previous_access() is a_2
+    assert len(a.previous_accesses()) == 0
+    assert len(a_2.previous_accesses()) == 1
+    assert a_2.previous_accesses()[0] is a
+    assert len(a_3.previous_accesses()) == 2
+    assert a_3.previous_accesses()[0] is a_2
+    assert a_3.previous_accesses()[1] is a
 
     # Check else block behaviour
     code = '''subroutine my_sub()
@@ -470,10 +494,15 @@ def test_reference_previous_access(fortran_reader):
     a_2 = routine.children[1].if_body.children[0].lhs
     a_3 = routine.children[1].else_body.children[0].lhs
     a_4 = routine.children[2].lhs
-    assert a.previous_access() is None
-    assert a_2.previous_access() is a
-    assert a_3.previous_access() is a_2
-    assert a_4.previous_access() is a_3
+    assert len(a.previous_accesses()) == 0
+    assert len(a_2.previous_accesses()) == 1
+    assert a_2.previous_accesses()[0] is a
+    assert len(a_2.previous_accesses()) == 1
+    assert a_3.previous_accesses()[0] is a
+    assert len(a_4.previous_accesses()) == 3
+    assert a_4.previous_accesses()[0] is a_3
+    assert a_4.previous_accesses()[1] is a_2
+    assert a_4.previous_accesses()[2] is a
 
 
 def test_reference_accesses_initialisation_statement(fortran_reader):
@@ -492,20 +521,20 @@ def test_reference_accesses_initialisation_statement(fortran_reader):
     psyir = fortran_reader.psyir_from_source(code)
     routine = psyir.children[0].children[0]
     a = routine.children[0].lhs
-    assert a.previous_access() is None
+    assert len(a.previous_accesses()) == 0
 
     sym_tab = routine.symbol_table
     symbols = sym_tab.get_symbols()
     b_sym = symbols['b']
     refs = b_sym.initial_value.walk(Reference)
     assert refs[0].next_accesses()[0] == refs[1]
-    assert refs[1].previous_access() == refs[0]
-    assert refs[0].previous_access() is None
+    assert refs[1].previous_accesses()[0] == refs[0]
+    assert len(refs[0].previous_accesses()) == 0
     assert len(refs[1].next_accesses()) == 0
 
 
-def test_reference_previous_access_with_codeblock(fortran_reader):
-    ''' Test when te previous_access is a Codeblock. '''
+def test_reference_previous_accesses_with_codeblock(fortran_reader):
+    ''' Test when te previous_accesses is a Codeblock. '''
     code = '''subroutine my_sub()
     character, dimension(100) :: a
     write(a, "A") "mytest"
@@ -516,7 +545,7 @@ def test_reference_previous_access_with_codeblock(fortran_reader):
     routine = psyir.children[0]
     a = routine.children[1].lhs
     codeblock = routine.children[1]
-    if a.previous_access() is not codeblock:
+    if a.previous_accesses() is not codeblock:
         pytest.xfail("#2271 Codeblocks don't currently support "
                      "reference_accesses")