|GNU Compiler Collection (GCC) Internals: SSA|
Most of the tree optimizers rely on the data flow information provided by the Static Single Assignment (SSA) form. We implement the SSA form as described in R. Cytron, J. Ferrante, B. Rosen, M. Wegman, and K. Zadeck. Efficiently Computing Static Single Assignment Form and the Control Dependence Graph. ACM Transactions on Programming Languages and Systems, 13(4):451-490, October 1991.
The SSA form is based on the premise that program variables are assigned in exactly one location in the program. Multiple assignments to the same variable create new versions of that variable. Naturally, actual programs are seldom in SSA form initially because variables tend to be assigned multiple times. The compiler modifies the program representation so that every time a variable is assigned in the code, a new version of the variable is created. Different versions of the same variable are distinguished by subscripting the variable name with its version number. Variables used in the right-hand side of expressions are renamed so that their version number matches that of the most recent assignment.
We represent variable versions using
SSA_NAME nodes. The
renaming process in tree-ssa.c wraps every real and
virtual operand with an
SSA_NAME node which contains
the version number and the statement that created the
SSA_NAME. Only definitions and virtual definitions may
Sometimes, flow of control makes it impossible to determine the most recent version of a variable. In these cases, the compiler inserts an artificial definition for that variable called PHI function or PHI node. This new definition merges all the incoming versions of the variable to create a new name for it. For instance,
if (…) a_1 = 5; else if (…) a_2 = 2; else a_3 = 13; # a_4 = PHI <a_1, a_2, a_3> return a_4;
Since it is not possible to determine which of the three branches
will be taken at runtime, we don’t know which of
a_3 to use at the return statement. So, the
SSA renamer creates a new version
a_4 which is assigned
the result of “merging”
Hence, PHI nodes mean “one of these operands. I don’t know
The following functions can be used to examine PHI nodes
Returns the number of arguments in phi. This number is exactly the number of incoming edges to the basic block holding phi.
Some optimization passes make changes to the function that invalidate the SSA property. This can happen when a pass has added new symbols or changed the program so that variables that were previously aliased aren’t anymore. Whenever something like this happens, the affected symbols must be renamed into SSA form again. Transformations that emit new code or replicate existing statements will also need to update the SSA form.
Since GCC implements two different SSA forms for register and virtual variables, keeping the SSA form up to date depends on whether you are updating register or virtual names. In both cases, the general idea behind incremental SSA updates is similar: when new SSA names are created, they typically are meant to replace other existing names in the program.
For instance, given the following code:
1 L0: 2 x_1 = PHI (0, x_5) 3 if (x_1 < 10) 4 if (x_1 > 7) 5 y_2 = 0 6 else 7 y_3 = x_1 + x_7 8 endif 9 x_5 = x_1 + 1 10 goto L0; 11 endif
Suppose that we insert new names
1 L0: 2 x_1 = PHI (0, x_5) 3 if (x_1 < 10) 4 x_10 = … 5 if (x_1 > 7) 6 y_2 = 0 7 else 8 x_11 = … 9 y_3 = x_1 + x_7 10 endif 11 x_5 = x_1 + 1 12 goto L0; 13 endif
We want to replace all the uses of
x_1 with the new definitions
x_11. Note that the only uses that should
be replaced are those at lines
Also, the use of
x_7 at line
9 should not be
replaced (this is why we cannot just mark symbol
Additionally, we may need to insert a PHI node at line
because that is a merge point for
x_11. So the
x_1 at line
11 will be replaced with the new PHI
node. The insertion of PHI nodes is optional. They are not strictly
necessary to preserve the SSA form, and depending on what the caller
inserted, they may not even be useful for the optimizers.
Updating the SSA form is a two step process. First, the pass has to
identify which names need to be updated and/or which symbols need to
be renamed into SSA form for the first time. When new names are
introduced to replace existing names in the program, the mapping
between the old and the new names are registered by calling
register_new_name_mapping (note that if your pass creates new
code by duplicating basic blocks, the call to
will set up the necessary mappings automatically).
After the replacement mappings have been registered and new symbols
marked for renaming, a call to
update_ssa makes the registered
changes. This can be done with an explicit call or by creating
TODO flags in the
tree_opt_pass structure for your pass.
There are several
TODO flags that control the behavior of
TODO_update_ssa. Update the SSA form inserting PHI nodes for newly exposed symbols and virtual names marked for updating. When updating real names, only insert PHI nodes for a real name
O_jin blocks reached by all the new and old definitions for
O_j. If the iterated dominance frontier for
O_jis not pruned, we may end up inserting PHI nodes in blocks that have one or more edges with no incoming definition for
O_j. This would lead to uninitialized warnings for
TODO_update_ssa_no_phi. Update the SSA form without inserting any new PHI nodes at all. This is used by passes that have either inserted all the PHI nodes themselves or passes that need only to patch use-def and def-def chains for virtuals (e.g., DCE).
TODO_update_ssa_full_phi. Insert PHI nodes everywhere they are needed. No pruning of the IDF is done. This is used by passes that need the PHI nodes for
O_jeven if it means that some arguments will come from the default definition of
O_j’s symbol (e.g.,
WARNING: If you need to use this flag, chances are that your pass may be doing something wrong. Inserting PHI nodes for an old name where not all edges carry a new replacement may lead to silent codegen errors or spurious uninitialized warnings.
TODO_update_ssa_only_virtuals. Passes that update the SSA form on their own may want to delegate the updating of virtual names to the generic updater. Since FUD chains are easier to maintain, this simplifies the work they need to do. NOTE: If this flag is used, any OLD->NEW mappings for real names are explicitly destroyed and only the symbols marked for renaming are processed.
The following macros can be used to examine
Returns the statement s that creates the
var. If s is an empty statement (i.e.,
true), it means that the first reference to
this variable is a USE or a VUSE.
This function walks the dominator tree for the current CFG calling a set of callback functions defined in struct dom_walk_data in domwalk.h. The call back functions you need to define give you hooks to execute custom code at various points during traversal: