GNU Compiler Collection (GCC) Internals: SSA Operands

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### 12.2 SSA Operands

Almost every GIMPLE statement will contain a reference to a variable or memory location. Since statements come in different shapes and sizes, their operands are going to be located at various spots inside the statement’s tree. To facilitate access to the statement’s operands, they are organized into lists associated inside each statement’s annotation. Each element in an operand list is a pointer to a `VAR_DECL`, `PARM_DECL` or `SSA_NAME` tree node. This provides a very convenient way of examining and replacing operands.

Data flow analysis and optimization is done on all tree nodes representing variables. Any node for which `SSA_VAR_P` returns nonzero is considered when scanning statement operands. However, not all `SSA_VAR_P` variables are processed in the same way. For the purposes of optimization, we need to distinguish between references to local scalar variables and references to globals, statics, structures, arrays, aliased variables, etc. The reason is simple, the compiler can gather complete data flow information for a local scalar. On the other hand, a global variable may be modified by a function call, it may not be possible to keep track of all the elements of an array or the fields of a structure, etc.

The operand scanner gathers two kinds of operands: real and virtual. An operand for which `is_gimple_reg` returns true is considered real, otherwise it is a virtual operand. We also distinguish between uses and definitions. An operand is used if its value is loaded by the statement (e.g., the operand at the RHS of an assignment). If the statement assigns a new value to the operand, the operand is considered a definition (e.g., the operand at the LHS of an assignment).

Virtual and real operands also have very different data flow properties. Real operands are unambiguous references to the full object that they represent. For instance, given

    {
      int a, b;
      a = b
    }

Since `a` and `b` are non-aliased locals, the statement `a = b` will have one real definition and one real use because variable `a` is completely modified with the contents of variable `b`. Real definition are also known as killing definitions. Similarly, the use of `b` reads all its bits.

In contrast, virtual operands are used with variables that can have a partial or ambiguous reference. This includes structures, arrays, globals, and aliased variables. In these cases, we have two types of definitions. For globals, structures, and arrays, we can determine from a statement whether a variable of these types has a killing definition. If the variable does, then the statement is marked as having a must definition of that variable. However, if a statement is only defining a part of the variable (i.e. a field in a structure), or if we know that a statement might define the variable but we cannot say for sure, then we mark that statement as having a may definition. For instance, given

    {
      int a, b, *p;

      if (…)
        p = &a;
      else
        p = &b;
      *p = 5;
      return *p;
    }

The assignment `*p = 5` may be a definition of `a` or `b`. If we cannot determine statically where `p` is pointing to at the time of the store operation, we create virtual definitions to mark that statement as a potential definition site for `a` and `b`. Memory loads are similarly marked with virtual use operands. Virtual operands are shown in tree dumps right before the statement that contains them. To request a tree dump with virtual operands, use the `-vops` option to `-fdump-tree`:

    {
      int a, b, *p;

      if (…)
        p = &a;
      else
        p = &b;
      # a = VDEF <a>
      # b = VDEF <b>
      *p = 5;

      # VUSE <a>
      # VUSE <b>
      return *p;
    }

Notice that `VDEF` operands have two copies of the referenced variable. This indicates that this is not a killing definition of that variable. In this case we refer to it as a may definition or aliased store. The presence of the second copy of the variable in the `VDEF` operand will become important when the function is converted into SSA form. This will be used to link all the non-killing definitions to prevent optimizations from making incorrect assumptions about them.

Operands are updated as soon as the statement is finished via a call to `update_stmt`. If statement elements are changed via `SET_USE` or `SET_DEF`, then no further action is required (i.e., those macros take care of updating the statement). If changes are made by manipulating the statement’s tree directly, then a call must be made to `update_stmt` when complete. Calling one of the `bsi_insert` routines or `bsi_replace` performs an implicit call to `update_stmt`.

#### 12.2.1 Operand Iterators And Access Routines

Operands are collected by `tree-ssa-operands.c`. They are stored inside each statement’s annotation and can be accessed through either the operand iterators or an access routine.

The following access routines are available for examining operands:

1. `SINGLE_SSA_{USE,DEF,TREE}_OPERAND`: These accessors will return NULL unless there is exactly one operand matching the specified flags. If there is exactly one operand, the operand is returned as either a `tree`, `def_operand_p`, or `use_operand_p`.

       tree t = SINGLE_SSA_TREE_OPERAND (stmt, flags);
       use_operand_p u = SINGLE_SSA_USE_OPERAND (stmt, SSA_ALL_VIRTUAL_USES);
       def_operand_p d = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_ALL_DEFS);

2. `ZERO_SSA_OPERANDS`: This macro returns true if there are no operands matching the specified flags.

       if (ZERO_SSA_OPERANDS (stmt, SSA_OP_ALL_VIRTUALS))
         return;

3. `NUM_SSA_OPERANDS`: This macro Returns the number of operands matching ’flags’. This actually executes a loop to perform the count, so only use this if it is really needed.

       int count = NUM_SSA_OPERANDS (stmt, flags)

If you wish to iterate over some or all operands, use the `FOR_EACH_SSA_{USE,DEF,TREE}_OPERAND` iterator. For example, to print all the operands for a statement:

    void
    print_ops (tree stmt)
    {
      ssa_op_iter;
      tree var;

      FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS)
        print_generic_expr (stderr, var, TDF_SLIM);
    }

How to choose the appropriate iterator:

1. Determine whether you are need to see the operand pointers, or just the trees, and choose the appropriate macro:

       Need            Macro:
       ----            -------
       use_operand_p   FOR_EACH_SSA_USE_OPERAND
       def_operand_p   FOR_EACH_SSA_DEF_OPERAND
       tree            FOR_EACH_SSA_TREE_OPERAND

2. You need to declare a variable of the type you are interested in, and an ssa_op_iter structure which serves as the loop controlling variable.
3. Determine which operands you wish to use, and specify the flags of those you are interested in. They are documented in `tree-ssa-operands.h`:

       #define SSA_OP_USE              0x01    /* Real USE operands.  */
       #define SSA_OP_DEF              0x02    /* Real DEF operands.  */
       #define SSA_OP_VUSE             0x04    /* VUSE operands.  */
       #define SSA_OP_VDEF             0x08    /* VDEF operands.  */

       /* These are commonly grouped operand flags.  */
       #define SSA_OP_VIRTUAL_USES	(SSA_OP_VUSE)
       #define SSA_OP_VIRTUAL_DEFS	(SSA_OP_VDEF)
       #define SSA_OP_ALL_VIRTUALS     (SSA_OP_VIRTUAL_USES | SSA_OP_VIRTUAL_DEFS)
       #define SSA_OP_ALL_USES		(SSA_OP_VIRTUAL_USES | SSA_OP_USE)
       #define SSA_OP_ALL_DEFS		(SSA_OP_VIRTUAL_DEFS | SSA_OP_DEF)
       #define SSA_OP_ALL_OPERANDS	(SSA_OP_ALL_USES | SSA_OP_ALL_DEFS)

So if you want to look at the use pointers for all the `USE` and `VUSE` operands, you would do something like:

      use_operand_p use_p;
      ssa_op_iter iter;

      FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, (SSA_OP_USE | SSA_OP_VUSE))
        {
          process_use_ptr (use_p);
        }

The `TREE` macro is basically the same as the `USE` and `DEF` macros, only with the use or def dereferenced via `USE_FROM_PTR (use_p)` and `DEF_FROM_PTR (def_p)`. Since we aren’t using operand pointers, use and defs flags can be mixed.

      tree var;
      ssa_op_iter iter;

      FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_VUSE)
        {
           print_generic_expr (stderr, var, TDF_SLIM);
        }

`VDEF`s are broken into two flags, one for the `DEF` portion (`SSA_OP_VDEF`) and one for the USE portion (`SSA_OP_VUSE`).

There are many examples in the code, in addition to the documentation in `tree-ssa-operands.h` and `ssa-iterators.h`.

There are also a couple of variants on the stmt iterators regarding PHI nodes.

`FOR_EACH_PHI_ARG` Works exactly like `FOR_EACH_SSA_USE_OPERAND`, except it works over `PHI` arguments instead of statement operands.

    /* Look at every virtual PHI use.  */
    FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_VIRTUAL_USES)
    {
       my_code;
    }

    /* Look at every real PHI use.  */
    FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_USES)
      my_code;

    /* Look at every PHI use.  */
    FOR_EACH_PHI_ARG (use_p, phi_stmt, iter, SSA_OP_ALL_USES)
      my_code;

`FOR_EACH_PHI_OR_STMT_{USE,DEF}` works exactly like `FOR_EACH_SSA_{USE,DEF}_OPERAND`, except it will function on either a statement or a `PHI` node. These should be used when it is appropriate but they are not quite as efficient as the individual `FOR_EACH_PHI` and `FOR_EACH_SSA` routines.

    FOR_EACH_PHI_OR_STMT_USE (use_operand_p, stmt, iter, flags)
      {
         my_code;
      }

    FOR_EACH_PHI_OR_STMT_DEF (def_operand_p, phi, iter, flags)
      {
         my_code;
      }

#### 12.2.2 Immediate Uses

Immediate use information is now always available. Using the immediate use iterators, you may examine every use of any `SSA_NAME`. For instance, to change each use of `ssa_var` to `ssa_var2` and call fold_stmt on each stmt after that is done:

      use_operand_p imm_use_p;
      imm_use_iterator iterator;
      tree ssa_var, stmt;

      FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
        {
          FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
            SET_USE (imm_use_p, ssa_var_2);
          fold_stmt (stmt);
        }

There are 2 iterators which can be used. `FOR_EACH_IMM_USE_FAST` is used when the immediate uses are not changed, i.e., you are looking at the uses, but not setting them.

If they do get changed, then care must be taken that things are not changed under the iterators, so use the `FOR_EACH_IMM_USE_STMT` and `FOR_EACH_IMM_USE_ON_STMT` iterators. They attempt to preserve the sanity of the use list by moving all the uses for a statement into a controlled position, and then iterating over those uses. Then the optimization can manipulate the stmt when all the uses have been processed. This is a little slower than the FAST version since it adds a placeholder element and must sort through the list a bit for each statement. This placeholder element must be also be removed if the loop is terminated early. The macro `BREAK_FROM_IMM_USE_SAFE` is provided to do this :

      FOR_EACH_IMM_USE_STMT (stmt, iterator, ssa_var)
        {
          if (stmt == last_stmt)
            BREAK_FROM_SAFE_IMM_USE (iter);

          FOR_EACH_IMM_USE_ON_STMT (imm_use_p, iterator)
            SET_USE (imm_use_p, ssa_var_2);
          fold_stmt (stmt);
        }

There are checks in `verify_ssa` which verify that the immediate use list is up to date, as well as checking that an optimization didn’t break from the loop without using this macro. It is safe to simply ’break’; from a `FOR_EACH_IMM_USE_FAST` traverse.

Some useful functions and macros:

1. `has_zero_uses (ssa_var)` : Returns true if there are no uses of `ssa_var`.
2. `has_single_use (ssa_var)` : Returns true if there is only a single use of `ssa_var`.
3. `single_imm_use (ssa_var, use_operand_p *ptr, tree *stmt)` : Returns true if there is only a single use of `ssa_var`, and also returns the use pointer and statement it occurs in, in the second and third parameters.
4. `num_imm_uses (ssa_var)` : Returns the number of immediate uses of `ssa_var`. It is better not to use this if possible since it simply utilizes a loop to count the uses.
5. `PHI_ARG_INDEX_FROM_USE (use_p)` : Given a use within a `PHI` node, return the index number for the use. An assert is triggered if the use isn’t located in a `PHI` node.
6. `USE_STMT (use_p)` : Return the statement a use occurs in.

Note that uses are not put into an immediate use list until their statement is actually inserted into the instruction stream via a `bsi_*` routine.

It is also still possible to utilize lazy updating of statements, but this should be used only when absolutely required. Both alias analysis and the dominator optimizations currently do this.

When lazy updating is being used, the immediate use information is out of date and cannot be used reliably. Lazy updating is achieved by simply marking statements modified via calls to `mark_stmt_modified` instead of `update_stmt`. When lazy updating is no longer required, all the modified statements must have `update_stmt` called in order to bring them up to date. This must be done before the optimization is finished, or `verify_ssa` will trigger an abort.

This is done with a simple loop over the instruction stream:

      block_stmt_iterator bsi;
      basic_block bb;
      FOR_EACH_BB (bb)
        {
          for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
            update_stmt_if_modified (bsi_stmt (bsi));
        }

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