GCC Middle and Back End API Reference

Data Structures  
struct  use_pred_info 
struct  norm_cond 
Typedefs  
typedef struct use_pred_info *  use_pred_info_t 
typedef struct norm_cond *  norm_cond_t 
Functions  
static int  get_mask_first_set_bit () 
static bool  has_undefined_value_p () 
static bool  uninit_undefined_value_p (tree t) 
static void  warn_uninit (enum opt_code wc, tree t, tree expr, tree var, const char *gmsgid, void *data) 
static unsigned int  warn_uninitialized_vars () 
static bool  can_skip_redundant_opnd () 
static unsigned  compute_uninit_opnds_pos () 
static basic_block  find_pdom () 
static basic_block  find_dom () 
static bool  is_non_loop_exit_postdominating () 
static basic_block  find_control_equiv_block () 
static bool  compute_control_dep_chain (basic_block bb, basic_block dep_bb, vec< edge > *cd_chains, size_t *num_chains, vec< edge > *cur_cd_chain) 
static bool  convert_control_dep_chain_into_preds (vec< edge > *dep_chains, size_t num_chains, vec< use_pred_info_t > **preds, size_t *num_preds) 
static bool  find_predicates (vec< use_pred_info_t > **preds, size_t *num_preds, basic_block phi_bb, basic_block use_bb) 
static void  collect_phi_def_edges (gimple phi, basic_block cd_root, vec< edge > *edges, struct pointer_set_t *visited_phis) 
static bool  find_def_preds (vec< use_pred_info_t > **preds, size_t *num_preds, gimple phi) 
static void  dump_predicates (gimple usestmt, size_t num_preds, vec< use_pred_info_t > *preds, const char *msg) 
static void  destroy_predicate_vecs (size_t n, vec< use_pred_info_t > *preds) 
static enum tree_code  get_cmp_code (enum tree_code orig_cmp_code, bool swap_cond, bool invert) 
static bool  is_value_included_in () 
static bool  find_matching_predicate_in_rest_chains (use_pred_info_t pred, vec< use_pred_info_t > *preds, size_t num_pred_chains) 
static bool  is_use_properly_guarded (gimple use_stmt, basic_block use_bb, gimple phi, unsigned uninit_opnds, struct pointer_set_t *visited_phis) 
static bool  prune_uninit_phi_opnds_in_unrealizable_paths (gimple phi, unsigned uninit_opnds, gimple flag_def, tree boundary_cst, enum tree_code cmp_code, struct pointer_set_t *visited_phis, bitmap *visited_flag_phis) 
static bool  use_pred_not_overlap_with_undef_path_pred (size_t num_preds, vec< use_pred_info_t > *preds, gimple phi, unsigned uninit_opnds, struct pointer_set_t *visited_phis) 
static bool  is_and_or_or () 
static void  normalize_cond_1 (gimple cond, norm_cond_t norm_cond, enum tree_code cond_code) 
static void  normalize_cond () 
static bool  is_gcond_subset_of (gimple cond1, bool invert1, gimple cond2, bool invert2, bool reverse) 
static bool  is_subset_of_any (gimple cond, bool invert, norm_cond_t norm_cond, bool reverse) 
static bool  is_or_set_subset_of (norm_cond_t norm_cond1, norm_cond_t norm_cond2) 
static bool  is_and_set_subset_of (norm_cond_t norm_cond1, norm_cond_t norm_cond2) 
static bool  is_norm_cond_subset_of (norm_cond_t norm_cond1, norm_cond_t norm_cond2) 
static bool  is_pred_expr_subset_of (use_pred_info_t expr1, use_pred_info_t expr2) 
static bool  is_pred_chain_subset_of (vec< use_pred_info_t > pred1, vec< use_pred_info_t > pred2) 
static bool  is_included_in (vec< use_pred_info_t > one_pred, vec< use_pred_info_t > *preds, size_t n) 
static bool  is_superset_of (vec< use_pred_info_t > *preds1, size_t n1, vec< use_pred_info_t > *preds2, size_t n2) 
static int  pred_chain_length_cmp () 
static bool  normalize_preds () 
static gimple  find_uninit_use (gimple phi, unsigned uninit_opnds, vec< gimple > *worklist, struct pointer_set_t *added_to_worklist) 
static void  warn_uninitialized_phi (gimple phi, vec< gimple > *worklist, struct pointer_set_t *added_to_worklist) 
static unsigned int  execute_late_warn_uninitialized () 
static bool  gate_warn_uninitialized () 
gimple_opt_pass *  make_pass_late_warn_uninitialized () 
static unsigned int  execute_early_warn_uninitialized () 
gimple_opt_pass *  make_pass_early_warn_uninitialized () 
Variables  
static struct pointer_set_t *  possibly_undefined_names = 0 
typedef struct norm_cond * norm_cond_t 
typedef struct use_pred_info * use_pred_info_t 

static 
Checks if the operand OPND of PHI is defined by another phi with one operand defined by this PHI, but the rest operands are all defined. If yes, returns true to skip this this operand as being redundant. Can be enhanced to be more general.

static 
Computes the set of incoming edges of PHI that have non empty definitions of a phi chain. The collection will be done recursively on operands that are defined by phis. CD_ROOT is the control dependence root. *EDGES holds the result, and VISITED_PHIS is a pointer set for detecting cycles.
References dump_file, dump_flags, and print_gimple_stmt().

static 
Computes the control dependence chains (paths of edges) for DEP_BB up to the dominating basic block BB (the head node of a chain should be dominated by it). CD_CHAINS is pointer to a dynamic array holding the result chains. CUR_CD_CHAIN is the current chain being computed. *NUM_CHAINS is total number of chains. The function returns true if the information is successfully computed, return false if there is no control dependence or not computed.
Could use a set instead.
cycle detected.
Found a direct control dependence.
check path from next edge.
Now check if DEP_BB is indirectly control dependent on BB.

static 
Returns a bit mask holding the positions of arguments in PHI that have empty (or possibly empty) definitions.
Bail out for phi with too many args.
Ignore SSA_NAMEs that appear on abnormal edges somewhere.
References function::calls_setjmp, cfun, and function::has_nonlocal_label.

static 
Converts the chains of control dependence edges into a set of predicates. A control dependence chain is represented by a vector edges. DEP_CHAINS points to an array of dependence chains. NUM_CHAINS is the size of the chain array. One edge in a dependence chain is mapped to predicate expression represented by use_pred_info_t type. One dependence chain is converted to a composite predicate that is the result of AND operation of use_pred_info_t mapped to each edge. A composite predicate is presented by a vector of use_pred_info_t. On return, *PREDS points to the resulting array of composite predicates. *NUM_PREDS is the number of composite predictes.
Now convert the control dep chain into a set of predicates.
Ignore EH edge. Can add assertion on the other edge's flag.
Skip if there is essentially one succesor.

static 
Destroys the predicate set *PREDS.

static 
Dumps the predicates (PREDS) for USESTMT.
do some dumping here:

static 
Currently, this pass runs always but execute_late_warn_uninitialized only runs with optimization. With optimization we want to warn about possible uninitialized as late as possible, thus don't do it here. However, without optimization we need to warn here about "may be uninitialized".
Postdominator information can not be reliably updated. Free it after the use.

static 
Entry point to the late uninitialized warning pass.
Redo the plain uninitialized variable check, as optimization may have straightened control flow. Do this first so that we don't accidentally get a "may be" warning when we'd have seen an "is" warning later.
Initialize worklist
Don't look at virtual operands.
References gate_warn_uninitialized().

inlinestatic 
Find the closest postdominator of a specified BB, which is control equivalent to BB.
Skip the postdominating bb that is also loop exit.
References basic_block_def::succs.

static 
For each use edge of PHI, computes all control dependence chains. The control dependence chains are then converted to an array of composite predicates pointed to by PREDS.
First find the closest dominating bb to be the control dependence root
Free individual chain
Now update the newly added chains with the phi operand edge:

inlinestatic 
Find the immediate DOM of the specified basic block BLOCK.
References CDI_POST_DOMINATORS, dominated_by_p(), single_pred_p(), and single_succ_p().

static 
Returns true if PRED is common among all the predicate chains (PREDS) (and therefore can be factored out). NUM_PRED_CHAIN is the size of array PREDS.
trival case
can relax the condition comparison to not use address comparison. However, the most common case is that multiple control dependent paths share a common path prefix, so address comparison should be ok.
References bitmap_bit_p(), bitmap_set_bit(), gimple_phi_arg_def(), gimple_phi_num_args(), gimple_phi_result(), and is_gimple_constant().

inlinestatic 
Find the immediate postdominator PDOM of the specified basic block BLOCK.
References CDI_DOMINATORS, and get_immediate_dominator().

static 
Computes all control dependence chains for USE_BB. The control dependence chains are then converted to an array of composite predicates pointed to by PREDS. PHI_BB is the basic block of the phi whose result is used in USE_BB.
First find the closest bb that is control equivalent to PHI_BB that also dominates USE_BB.
Free individual chain

static 
Searches through all uses of a potentially uninitialized variable defined by PHI and returns a use statement if the use is not properly guarded. It returns NULL if all uses are guarded. UNINIT_OPNDS is a bitvector holding the position(s) of uninit PHI operands. WORKLIST is the vector of candidate phis that may be updated by this function. ADDED_TO_WORKLIST is the pointer set tracking if the new phi is already in the worklist.
Found one real use, return.
Found a phi use that is not guarded, add the phi to the worklist.
References dump_file, and print_gimple_stmt().

static 
Referenced by execute_late_warn_uninitialized().

static 
Computes the 'normalized' conditional code with operand swapping and condition inversion.
References invert_tree_comparison().

static 
Returns the first bit position (starting from LSB) in mask that is non zero. Returns 1 if the mask is empty.

static 
Return true if T, an SSA_NAME, has an undefined value.
Referenced by uninit_undefined_value_p().

inlinestatic 
Returns true if TC is AND or OR
References normalize_cond_1().

static 
NORM_COND1 and NORM_COND2 are normalized logical AND expressions (formed by following UD chains not control dependence chains). The function returns true of domain of and expression NORM_COND1 is a subset of NORM_COND2's.
References norm_cond::conds, norm_cond::invert, is_gcond_subset_of(), and is_subset_of_any().
Referenced by is_subset_of_any().

static 
Returns true if the domain for condition COND1 is a subset of COND2. REVERSE is a flag. when it is true the function checks if COND1 is a superset of COND2. INVERT1 and INVERT2 are flags to indicate if COND1 and COND2 need to be inverted or not.
Take the short cut.
Assuming const operands have been swapped to the rhs at this point of the analysis.
Referenced by is_and_set_subset_of().

static 
Returns true if the domain defined by one pred chain ONE_PRED is a subset of the domain of *PREDS. It returns false if ONE_PRED's domain is not a subset of any of the subdomains of PREDS ( corresponding to each individual chains in it), even though it may be still be a subset of whole domain of PREDS which is the union (ORed) of all its subdomains. In other words, the result is conservative.
References pred_chain_length_cmp(), and vNULL.
Referenced by is_pred_chain_subset_of().

static 
Returns true if BB1 is postdominating BB2 and BB1 is not a loop exit bb. The loop exit bb check is simple and does not cover all cases.

static 
Returns true of the domain if NORM_COND1 is a subset of that of NORM_COND2. Returns false if it can not be proved to be so.
Both conditions are AND expressions.
NORM_COND1 is an AND expression, and NORM_COND2 is an OR expression. In this case, returns true if any subexpression of NORM_COND1 is a subset of any subexpression of NORM_COND2.
NORM_COND1 is an OR expression
Conservatively returns false if NORM_COND1 is nondecomposible and NORM_COND2 is an AND expression.
References use_pred_info::cond, norm_cond::conds, gimple_cond_code(), gimple_cond_lhs(), gimple_cond_rhs(), use_pred_info::invert, invert_tree_comparison(), and normalize_cond().

static 
NORM_COND1 and NORM_COND2 are normalized logical/BIT OR expressions (formed by following UD chains not control dependence chains). The function returns true of domain of and expression NORM_COND1 is a subset of NORM_COND2's. The implementation is conservative, and it returns false if it the inclusion relationship may not hold.

static 
Returns true if the domain of PRED1 is a subset of that of PRED2. Returns false if it can not be proved so.
References is_included_in().
Referenced by is_pred_expr_subset_of().

static 
Returns true of the domain of single predicate expression EXPR1 is a subset of that of EXPR2. Returns false if it can not be proved.
Fast path  match exactly
Normalize conditions. To keep NE_EXPR, do not invert with both need inversion.
Free memory
References is_pred_chain_subset_of().

static 
Returns true if the domain of the condition expression in COND is a subset of any of the subconditions of the normalized condtion NORM_COND. INVERT is a flag to indicate of the COND needs to be inverted. REVERSE is a flag. When it is true, the check is reversed  it returns true if COND is a superset of any of the subconditions of NORM_COND.
References norm_cond::cond_code, norm_cond::conds, and is_and_set_subset_of().
Referenced by is_and_set_subset_of().

static 
compares two predicate sets PREDS1 and PREDS2 and returns true if the domain defined by PREDS1 is a superset of PREDS2's domain. N1 and N2 are array sizes of PREDS1 and PREDS2 respectively. The implementation chooses not to build generic trees (and relying on the folding capability of the compiler), but instead performs brute force comparison of individual predicate chains (won't be a compile time problem as the chains are pretty short). When the function returns false, it does not necessarily mean *PREDS1 is not a superset of *PREDS2, but mean it may not be so since the analysis can not prove it. In such cases, false warnings may still be emitted.

static 
Forward declaration.
Computes the predicates that guard the use and checks if the incoming paths that have empty (or possibly empty) definition can be pruned/filtered. The function returns true if it can be determined that the use of PHI's def in USE_STMT is guarded with a predicate set not overlapping with predicate sets of all runtime paths that do not have a definition. Returns false if it is not or it can not be determined. USE_BB is the bb of the use (for phi operand use, the bb is not the bb of the phi stmt, but the src bb of the operand edge). UNINIT_OPNDS is a bit vector. If an operand of PHI is uninitialized, the corresponding bit in the vector is 1. VISIED_PHIS is a pointer set of phis being visted.
further prune the dead incoming phi edges.
References dump_file, dump_flags, gimple_bb(), gimple_phi_arg_edge(), is_gimple_debug(), pointer_set_create(), pointer_set_destroy(), pointer_set_insert(), print_gimple_stmt(), and edge_def::src.

static 
Returns true if VAL falls in the range defined by BOUNDARY and CMPC, i.e. all values in the range satisfies (x CMPC BOUNDARY) == true.
Only handle integer constant here.
gimple_opt_pass* make_pass_early_warn_uninitialized  (  ) 
gimple_opt_pass* make_pass_late_warn_uninitialized  (  ) 

static 
See normalize_cond_1 for details. INVERT is a flag to indicate if COND needs to be inverted or not.
Referenced by is_norm_cond_subset_of().

static 
Normalizes gimple condition COND. The normalization follows UD chains to form larger condition expression trees. NORM_COND holds the normalized result. COND_CODE is the logical opcode (AND or OR) of the normalized tree.
Referenced by is_and_or_or().

static 
x OR (!x AND y) is equivalent to x OR y. This function normalizes x1 OR (!x1 AND x2) OR (!x1 AND !x2 AND x3) into x1 OR x2 OR x3. PREDS is the predicate chains, and N is the number of chains. Returns true if normalization happens.
First sort the chains in ascending order of lengths.
See if simplification x AND y OR x AND !y is possible.
Now merge the first two chains.
The loop extracts x1, x2, x3, etc from chains x1 OR (!x1 AND x2) OR (!x1 AND !x2 AND x3) OR ...
Check if nxj is !xj
Now normalize the pred chains using the extraced x1, x2, x3 etc.
A new chain.

static 
Comparison function used by qsort. It is used to sort predicate chains to allow predicate simplification.
Allow predicates with similar prefix come together.
Referenced by is_included_in().

static 
@verbatim
Returns true if all uninitialized opnds are pruned. Returns false otherwise. PHI is the phi node with uninitialized operands, UNINIT_OPNDS is the bitmap of the uninitialize operand positions, FLAG_DEF is the statement defining the flag guarding the use of the PHI output, BOUNDARY_CST is the const value used in the predicate associated with the flag, CMP_CODE is the comparison code used in the predicate, VISITED_PHIS is the pointer set of phis visited, and VISITED_FLAG_PHIS is the pointer to the pointer set of flag definitions that are also phis.
Example scenario:
BB1: flag_1 = phi <0, 1> // (1) var_1 = phi <undef, some_val>
BB2: flag_2 = phi <0, flag_1, flag_1> // (2) var_2 = phi <undef, var_1, var_1> if (flag_2 == 1) goto BB3;
BB3: use of var_2 // (3)
Because some flag arg in (1) is not constant, if we do not look into the flag phis recursively, it is conservatively treated as unknown and var_1 is thought to be flowed into use at (3). Since var_1 is potentially uninitialized a false warning will be emitted. Checking recursively into (1), the compiler can find out that only some_val (which is defined) can flow into (3) which is OK.
Now recursively prune the uninitialized phi args.
Now check if the constant is in the guarded range.
Now that we know that this undefined edge is not pruned. If the operand is defined by another phi, we can further prune the incoming edges of that phi by checking the predicates of this operands.

inlinestatic 
Like has_undefined_value_p, but don't return true if TREE_NO_WARNING is set on SSA_NAME_VAR.
References has_undefined_value_p().

static 
A helper function that determines if the predicate set of the use is not overlapping with that of the uninit paths. The most common senario of guarded use is in Example 1: Example 1: if (some_cond) { x = ...; flag = true; } ... some code ... if (flag) use (x); The real world examples are usually more complicated, but similar and usually result from inlining: bool init_func (int * x) { if (some_cond) return false; *x = .. return true; } void foo(..) { int x; if (!init_func(&x)) return; .. some_code ... use (x); } Another possible use scenario is in the following trivial example: Example 2: if (n > 0) x = 1; ... if (n > 0) { if (m < 2) .. = x; } Predicate analysis needs to compute the composite predicate: 1) 'x' use predicate: (n > 0) .AND. (m < 2) 2) 'x' default value (nondef) predicate: .NOT. (n > 0) (the predicate chain for phi operand defs can be computed starting from a bb that is control equivalent to the phi's bb and is dominating the operand def.) and check overlapping: (n > 0) .AND. (m < 2) .AND. (.NOT. (n > 0)) <==> false This implementation provides framework that can handle scenarios. (Note that many simple cases are handled properly without the predicate analysis  this is due to jump threading transformation which eliminates the merge point thus makes path sensitive analysis unnecessary.) NUM_PREDS is the number is the number predicate chains, PREDS is the array of chains, PHI is the phi node whose incoming (undefined) paths need to be pruned, and UNINIT_OPNDS is the bitmap holding uninit operand positions. VISITED_PHIS is the pointer set of phi stmts being checked.
Find within the common prefix of multiple predicate chains a predicate that is a comparison of a flag variable against a constant.
Now check all the uninit incoming edge has a constant flag value that is in conflict with the use guard/predicate.

static 
Emit warnings for uninitialized variables. This is done in two passes. The first pass notices real uses of SSA names with undefined values. Such uses are unconditionally uninitialized, and we can be certain that such a use is a mistake. This pass is run before most optimizations, so that we catch as many as we can. The second pass follows PHI nodes to find uses that are potentially uninitialized. In this case we can't necessarily prove that the use is really uninitialized. This pass is run after most optimizations, so that we thread as many jumps and possible, and delete as much dead code as possible, in order to reduce false positives. We also look again for plain uninitialized variables, since optimization may have changed conditionally uninitialized to unconditionally uninitialized.
Emit a warning for EXPR based on variable VAR at the point in the program T, an SSA_NAME, is used being uninitialized. The exact warning text is in MSGID and LOCUS may contain a location or be null. WC is the warning code.
TREE_NO_WARNING either means we already warned, or the front end wishes to suppress the warning.
Referenced by warn_uninitialized_vars().

static 
Look for inputs to PHI that are SSA_NAMEs that have empty definitions and gives warning if there exists a runtime path from the entry to a use of the PHI def that does not contain a definition. In other words, the warning is on the real use. The more dead paths that can be pruned by the compiler, the fewer false positives the warning is. WORKLIST is a vector of candidate phis to be examined. ADDED_TO_WORKLIST is a pointer set tracking if the new phi is added to the worklist or not.
Don't look at virtual operands.
Now check if we have any use of the value without proper guard.
All uses are properly guarded.

static 
We only do data flow with SSA_NAMEs, so that's all we can warn about.
For memory the only cheap thing we can do is see if we have a use of the default def of the virtual operand. ??? Note that at O0 we do not have virtual operands. ??? Not so cheap would be to use the alias oracle via walk_aliased_vdefs, if we don't find any aliasing vdef warn as isuseduninitialized, if we don't find an aliasing vdef that kills our use (stmt_kills_ref_p), warn as maybeuseduninitialized. But this walk is quadratic and so must be limited which means we would miss warning opportunities.
Do not warn if it can be initialized outside this function.
References get_base_address(), gimple_assign_rhs1(), gimple_assign_single_p(), gimple_vdef(), gimple_vuse(), gsi_stmt(), is_gimple_debug(), is_global_var(), and warn_uninit().

static 
@verbatim
Predicate aware uninitialized variable warning. Copyright (C) 20012013 Free Software Foundation, Inc. Contributed by Xinliang David Li david xl@g oogle .com
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.
GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/.
This implements the pass that does predicate aware warning on uses of possibly uninitialized variables. The pass first collects the set of possibly uninitialized SSA names. For each such name, it walks through all its immediate uses. For each immediate use, it rebuilds the condition expression (the predicate) that guards the use. The predicate is then examined to see if the variable is always defined under that same condition. This is done either by pruning the unrealizable paths that lead to the default definitions or by checking if the predicate set that guards the defining paths is a superset of the use predicate.
Pointer set of potentially undefined ssa names, i.e., ssa names that are defined by phi with operands that are not defined or potentially undefined.