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9.4 Statement 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 arrays associated inside each
statement's annotation. Each element in an operand array 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 b is completely modified with the contents of
variable a. Real definition are also known as killing
definitions. Similarly, the use of a 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 = V_MAY_DEF <a>
# b = V_MAY_DEF <b>
*p = 5;
# VUSE <a>
# VUSE <b>
return *p;
}
Notice that V_MAY_DEF 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 V_MAY_DEF 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 collected by tree-ssa-operands.c. They are stored
inside each statement's annotation and can be accessed with
DEF_OPS, USE_OPS, V_MAY_DEF_OPS,
V_MUST_DEF_OPS and VUSE_OPS. The following are all the
accessor macros available to access USE operands. To access all the
other operand arrays, just change the name accordingly. Note that
this interface to the operands is deprecated, and is slated for
removal in a future version of gcc. The preferred interface is the
operand iterator interface. Unless you need to discover the number of
operands of a given type on a statement, you are strongly urged not to
use this interface.
The following function shows how to print all the operands of a given statement:
void
print_ops (tree stmt)
{
vuse_optype vuses;
v_may_def_optype v_may_defs;
v_must_def_optype v_must_defs;
def_optype defs;
use_optype uses;
stmt_ann_t ann;
size_t i;
get_stmt_operands (stmt);
ann = stmt_ann (stmt);
defs = DEF_OPS (ann);
for (i = 0; i < NUM_DEFS (defs); i++)
print_generic_expr (stderr, DEF_OP (defs, i), 0);
uses = USE_OPS (ann);
for (i = 0; i < NUM_USES (uses); i++)
print_generic_expr (stderr, USE_OP (uses, i), 0);
v_may_defs = V_MAY_DEF_OPS (ann);
for (i = 0; i < NUM_V_MAY_DEFS (v_may_defs); i++)
{
print_generic_expr (stderr, V_MAY_DEF_OP (v_may_defs, i), 0);
print_generic_expr (stderr, V_MAY_DEF_RESULT (v_may_defs, i), 0);
}
v_must_defs = V_MUST_DEF_OPS (ann);
for (i = 0; i < NUM_V_MUST_DEFS (v_must_defs); i++)
print_generic_expr (stderr, V_MUST_DEF_OP (v_must_defs, i), 0);
vuses = VUSE_OPS (ann);
for (i = 0; i < NUM_VUSES (vuses); i++)
print_generic_expr (stderr, VUSE_OP (vuses, i), 0);
}
To collect the operands, you first need to call
get_stmt_operands. Since that is a potentially expensive
operation, statements are only scanned if they have been marked
modified by a call to modify_stmt. So, if your pass replaces
operands in a statement, make sure to call modify_stmt.
9.4.1 Operand Iterators
There is an alternative to iterating over the operands in a statement. It is especially useful when you wish to perform the same operation on more than one type of operand. The previous example could be rewritten as follows:
void
print_ops (tree stmt)
{
ssa_op_iter;
tree var;
get_stmt_operands (stmt);
FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_OPERANDS)
print_generic_expr (stderr, var, 0);
}
- 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 - 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.
- 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_VMAYUSE 0x08 /* USE portion of V_MAY_DEFS. */ #define SSA_OP_VMAYDEF 0x10 /* DEF portion of V_MAY_DEFS. */ #define SSA_OP_VMUSTDEF 0x20 /* V_MUST_DEF definitions. */ /* These are commonly grouped operand flags. */ #define SSA_OP_VIRTUAL_USES (SSA_OP_VUSE | SSA_OP_VMAYUSE) #define SSA_OP_VIRTUAL_DEFS (SSA_OP_VMAYDEF | SSA_OP_VMUSTDEF) #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 | SSA_OP_VMUSTDEF)
{
print_generic_expr (stderr, var, TDF_SLIM);
}
V_MAY_DEFs are broken into two flags, one for the
DEF portion (SSA_OP_VMAYDEF) and one for the USE portion
(SSA_OP_VMAYUSE). If all you want to look at are the
V_MAY_DEFs together, there is a fourth iterator macro for this,
which returns both a def_operand_p and a use_operand_p for each
V_MAY_DEF in the statement. Note that you don't need any flags for
this one.
use_operand_p use_p;
def_operand_p def_p;
ssa_op_iter iter;
FOR_EACH_SSA_MAYDEF_OPERAND (def_p, use_p, stmt, iter)
{
my_code;
}
V_MUST_DEFs are broken into two flags, one for the
DEF portion (SSA_OP_VMUSTDEF) and one for the kill portion
(SSA_OP_VMUSTDEFKILL). If all you want to look at are the
V_MUST_DEFs together, there is a fourth iterator macro for this,
which returns both a def_operand_p and a use_operand_p for each
V_MUST_DEF in the statement. Note that you don't need any flags for
this one.
use_operand_p kill_p;
def_operand_p def_p;
ssa_op_iter iter;
FOR_EACH_SSA_MUSTDEF_OPERAND (def_p, kill_p, stmt, iter)
{
my_code;
}
There are many examples in the code as well, as well as the documentation in tree-ssa-operands.h.