| GNU Compiler Collection (GCC) Internals: SSA Operands |
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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.
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:
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);
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;
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:
Need Macro: ---- ------- use_operand_p FOR_EACH_SSA_USE_OPERAND def_operand_p FOR_EACH_SSA_DEF_OPERAND tree FOR_EACH_SSA_TREE_OPERAND
#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);
}
VDEFs 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;
}
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:
has_zero_uses (ssa_var) : Returns true if there are no uses of
ssa_var.
has_single_use (ssa_var) : Returns true if there is only a
single use of ssa_var.
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.
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.
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.
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 gimple_set_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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