Statements#

Most statements in GIMPLE are assignment statements, represented by GIMPLE_ASSIGN. No other C expressions can appear at statement level; a reference to a volatile object is converted into a GIMPLE_ASSIGN.

There are also several varieties of complex statements.

Basic Statements#

ASM_EXPR#

Used to represent an inline assembly statement. For an inline assembly statement like:

asm ("mov x, y");

The ASM_STRING macro will return a STRING_CST node for "mov x, y". If the original statement made use of the extended-assembly syntax, then ASM_OUTPUTS, ASM_INPUTS, and ASM_CLOBBERS will be the outputs, inputs, and clobbers for the statement, represented as STRING_CST nodes. The extended-assembly syntax looks like:

asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

The first string is the ASM_STRING, containing the instruction template. The next two strings are the output and inputs, respectively; this statement has no clobbers. As this example indicates, ‘plain’ assembly statements are merely a special case of extended assembly statements; they have no cv-qualifiers, outputs, inputs, or clobbers. All of the strings will be NUL -terminated, and will contain no embedded NUL -characters.

If the assembly statement is declared volatile, or if the statement was not an extended assembly statement, and is therefore implicitly volatile, then the predicate ASM_VOLATILE_P will hold of the ASM_EXPR.

DECL_EXPR#: Used to represent a local declaration. The DECL_EXPR_DECL macro can be used to obtain the entity declared. This declaration may be a LABEL_DECL, indicating that the label declared is a local label. (As an extension, GCC allows the declaration of labels with scope.) In C, this declaration may be a FUNCTION_DECL, indicating the use of the GCC nested function extension. For more information, see Functions.

LABEL_EXPR#: Used to represent a label. The LABEL_DECL declared by this statement can be obtained with the LABEL_EXPR_LABEL macro. The IDENTIFIER_NODE giving the name of the label can be obtained from the LABEL_DECL with DECL_NAME.

GOTO_EXPR#: Used to represent a goto statement. The GOTO_DESTINATION will usually be a LABEL_DECL. However, if the ‘computed goto’ extension has been used, the GOTO_DESTINATION will be an arbitrary expression indicating the destination. This expression will always have pointer type.

RETURN_EXPR#

Used to represent a return statement. Operand 0 represents the value to return. It should either be the RESULT_DECL for the containing function, or a MODIFY_EXPR or INIT_EXPR setting the function’s RESULT_DECL. It will be NULL_TREE if the statement was just

return;

LOOP_EXPR#: These nodes represent ‘infinite’ loops. The LOOP_EXPR_BODY represents the body of the loop. It should be executed forever, unless an EXIT_EXPR is encountered.

EXIT_EXPR#: These nodes represent conditional exits from the nearest enclosing LOOP_EXPR. The single operand is the condition; if it is nonzero, then the loop should be exited. An EXIT_EXPR will only appear within a LOOP_EXPR.

SWITCH_EXPR#

Used to represent a switch statement. The SWITCH_COND is the expression on which the switch is occurring. The SWITCH_BODY is the body of the switch statement. SWITCH_ALL_CASES_P is true if the switch includes a default label or the case label ranges cover all possible values of the condition expression.

Note that TREE_TYPE for a SWITCH_EXPR represents the original type of switch expression as given in the source, before any compiler conversions, instead of the type of the switch expression itself (which is not meaningful).

CASE_LABEL_EXPR#

Use to represent a case label, range of case labels, or a default label. If CASE_LOW is NULL_TREE, then this is a default label. Otherwise, if CASE_HIGH is NULL_TREE, then this is an ordinary case label. In this case, CASE_LOW is an expression giving the value of the label. Both CASE_LOW and CASE_HIGH are INTEGER_CST nodes. These values will have the same type as the condition expression in the switch statement.

Otherwise, if both CASE_LOW and CASE_HIGH are defined, the statement is a range of case labels. Such statements originate with the extension that allows users to write things of the form:

case 2 ... 5:

The first value will be CASE_LOW, while the second will be CASE_HIGH.

DEBUG_BEGIN_STMT#: Marks the beginning of a source statement, for purposes of debug information generation.

Blocks#

Block scopes and the variables they declare in GENERIC are expressed using the BIND_EXPR code, which in previous versions of GCC was primarily used for the C statement-expression extension.

Variables in a block are collected into BIND_EXPR_VARS in declaration order through their TREE_CHAIN field. Any runtime initialization is moved out of DECL_INITIAL and into a statement in the controlled block. When gimplifying from C or C++, this initialization replaces the DECL_STMT. These variables will never require cleanups. The scope of these variables is just the body

Variable-length arrays (VLAs) complicate this process, as their size often refers to variables initialized earlier in the block and their initialization involves an explicit stack allocation. To handle this, we add an indirection and replace them with a pointer to stack space allocated by means of alloca. In most cases, we also arrange for this space to be reclaimed when the enclosing BIND_EXPR is exited, the exception to this being when there is an explicit call to alloca in the source code, in which case the stack is left depressed on exit of the BIND_EXPR.

A C++ program will usually contain more BIND_EXPR s than there are syntactic blocks in the source code, since several C++ constructs have implicit scopes associated with them. On the other hand, although the C++ front end uses pseudo-scopes to handle cleanups for objects with destructors, these don’t translate into the GIMPLE form; multiple declarations at the same level use the same BIND_EXPR.

Statement Sequences#

Multiple statements at the same nesting level are collected into a STATEMENT_LIST. Statement lists are modified and traversed using the interface in tree-iterator.h.

Empty Statements#

Whenever possible, statements with no effect are discarded. But if they are nested within another construct which cannot be discarded for some reason, they are instead replaced with an empty statement, generated by build_empty_stmt. Initially, all empty statements were shared, after the pattern of the Java front end, but this caused a lot of trouble in practice.

An empty statement is represented as (void)0.

Jumps#

Other jumps are expressed by either GOTO_EXPR or RETURN_EXPR.

The operand of a GOTO_EXPR must be either a label or a variable containing the address to jump to.

The operand of a RETURN_EXPR is either NULL_TREE, RESULT_DECL, or a MODIFY_EXPR which sets the return value. It would be nice to move the MODIFY_EXPR into a separate statement, but the special return semantics in expand_return make that difficult. It may still happen in the future, perhaps by moving most of that logic into expand_assignment.

Cleanups#

Destructors for local C++ objects and similar dynamic cleanups are represented in GIMPLE by a TRY_FINALLY_EXPR. TRY_FINALLY_EXPR has two operands, both of which are a sequence of statements to execute. The first sequence is executed. When it completes the second sequence is executed.

The first sequence may complete in the following ways:

Execute the last statement in the sequence and fall off the end.
Execute a goto statement (GOTO_EXPR) to an ordinary label outside the sequence.
Execute a return statement (RETURN_EXPR).
Throw an exception. This is currently not explicitly represented in GIMPLE.

The second sequence is not executed if the first sequence completes by calling setjmp or exit or any other function that does not return. The second sequence is also not executed if the first sequence completes via a non-local goto or a computed goto (in general the compiler does not know whether such a goto statement exits the first sequence or not, so we assume that it doesn’t).

After the second sequence is executed, if it completes normally by falling off the end, execution continues wherever the first sequence would have continued, by falling off the end, or doing a goto, etc.

If the second sequence is an EH_ELSE_EXPR selector, then the sequence in its first operand is used when the first sequence completes normally, and that in its second operand is used for exceptional cleanups, i.e., when an exception propagates out of the first sequence.

TRY_FINALLY_EXPR complicates the flow graph, since the cleanup needs to appear on every edge out of the controlled block; this reduces the freedom to move code across these edges. Therefore, the EH lowering pass which runs before most of the optimization passes eliminates these expressions by explicitly adding the cleanup to each edge. Rethrowing the exception is represented using RESX_EXPR.

OpenMP#

All the statements starting with OMP_ represent directives and clauses used by the OpenMP API https://www.openmp.org.

OMP_PARALLEL#

Represents #pragma omp parallel [clause1 ... clauseN]. It has four operands:

Operand OMP_PARALLEL_BODY is valid while in GENERIC and High GIMPLE forms. It contains the body of code to be executed by all the threads. During GIMPLE lowering, this operand becomes NULL and the body is emitted linearly after OMP_PARALLEL.

Operand OMP_PARALLEL_CLAUSES is the list of clauses associated with the directive.

Operand OMP_PARALLEL_FN is created by pass_lower_omp, it contains the FUNCTION_DECL for the function that will contain the body of the parallel region.

Operand OMP_PARALLEL_DATA_ARG is also created by pass_lower_omp. If there are shared variables to be communicated to the children threads, this operand will contain the VAR_DECL that contains all the shared values and variables.

OMP_FOR#

Represents #pragma omp for [clause1 ... clauseN]. It has six operands:

Operand OMP_FOR_BODY contains the loop body.

Operand OMP_FOR_CLAUSES is the list of clauses associated with the directive.

Operand OMP_FOR_INIT is the loop initialization code of the form VAR = N1.

Operand OMP_FOR_COND is the loop conditional expression of the form VAR {<,>,<=,>=} N2.

Operand OMP_FOR_INCR is the loop index increment of the form VAR {+=,-=} INCR.

Operand OMP_FOR_PRE_BODY contains side effect code from operands OMP_FOR_INIT, OMP_FOR_COND and OMP_FOR_INC. These side effects are part of the OMP_FOR block but must be evaluated before the start of loop body.

The loop index variable VAR must be a signed integer variable, which is implicitly private to each thread. Bounds N1 and N2 and the increment expression INCR are required to be loop invariant integer expressions that are evaluated without any synchronization. The evaluation order, frequency of evaluation and side effects are unspecified by the standard.

OMP_SECTIONS#

Represents #pragma omp sections [clause1 ... clauseN].

Operand OMP_SECTIONS_BODY contains the sections body, which in turn contains a set of OMP_SECTION nodes for each of the concurrent sections delimited by #pragma omp section.

Operand OMP_SECTIONS_CLAUSES is the list of clauses associated with the directive.

OMP_SECTION#: Section delimiter for OMP_SECTIONS.

OMP_SINGLE#

Represents #pragma omp single.

Operand OMP_SINGLE_BODY contains the body of code to be executed by a single thread.

Operand OMP_SINGLE_CLAUSES is the list of clauses associated with the directive.

OMP_MASTER#

Represents #pragma omp master.

Operand OMP_MASTER_BODY contains the body of code to be executed by the master thread.

OMP_ORDERED#

Represents #pragma omp ordered.

Operand OMP_ORDERED_BODY contains the body of code to be executed in the sequential order dictated by the loop index variable.

OMP_CRITICAL#

Represents #pragma omp critical [name].

Operand OMP_CRITICAL_BODY is the critical section.

Operand OMP_CRITICAL_NAME is an optional identifier to label the critical section.

OMP_RETURN#: This does not represent any OpenMP directive, it is an artificial marker to indicate the end of the body of an OpenMP. It is used by the flow graph (tree-cfg.cc) and OpenMP region building code (omp-low.cc).

OMP_CONTINUE#

Similarly, this instruction does not represent an OpenMP directive, it is used by OMP_FOR (and similar codes) as well as OMP_SECTIONS to mark the place where the code needs to loop to the next iteration, or the next section, respectively.

In some cases, OMP_CONTINUE is placed right before OMP_RETURN. But if there are cleanups that need to occur right after the looping body, it will be emitted between OMP_CONTINUE and OMP_RETURN.

OMP_ATOMIC#

Represents #pragma omp atomic.

Operand 0 is the address at which the atomic operation is to be performed.

Operand 1 is the expression to evaluate. The gimplifier tries three alternative code generation strategies. Whenever possible, an atomic update built-in is used. If that fails, a compare-and-swap loop is attempted. If that also fails, a regular critical section around the expression is used.

OMP_CLAUSE#

Represents clauses associated with one of the OMP_ directives. Clauses are represented by separate subcodes defined in tree.h. Clauses codes can be one of: OMP_CLAUSE_PRIVATE, OMP_CLAUSE_SHARED, OMP_CLAUSE_FIRSTPRIVATE, OMP_CLAUSE_LASTPRIVATE, OMP_CLAUSE_COPYIN, OMP_CLAUSE_COPYPRIVATE, OMP_CLAUSE_IF, OMP_CLAUSE_NUM_THREADS, OMP_CLAUSE_SCHEDULE, OMP_CLAUSE_NOWAIT, OMP_CLAUSE_ORDERED, OMP_CLAUSE_DEFAULT, OMP_CLAUSE_REDUCTION, OMP_CLAUSE_COLLAPSE, OMP_CLAUSE_UNTIED, OMP_CLAUSE_FINAL, and OMP_CLAUSE_MERGEABLE. Each code represents the corresponding OpenMP clause.

Clauses associated with the same directive are chained together via OMP_CLAUSE_CHAIN. Those clauses that accept a list of variables are restricted to exactly one, accessed with OMP_CLAUSE_VAR. Therefore, multiple variables under the same clause C need to be represented as multiple C clauses chained together. This facilitates adding new clauses during compilation.

OpenACC#

All the statements starting with OACC_ represent directives and clauses used by the OpenACC API https://www.openacc.org.

OACC_CACHE#: Represents #pragma acc cache (var ...).

OACC_DATA#: Represents #pragma acc data [clause1 ... clauseN].

OACC_DECLARE#: Represents #pragma acc declare [clause1 ... clauseN].

OACC_ENTER_DATA#: Represents #pragma acc enter data [clause1 ... clauseN].

OACC_EXIT_DATA#: Represents #pragma acc exit data [clause1 ... clauseN].

OACC_HOST_DATA#: Represents #pragma acc host_data [clause1 ... clauseN].

OACC_KERNELS#: Represents #pragma acc kernels [clause1 ... clauseN].

OACC_LOOP#

Represents #pragma acc loop [clause1 ... clauseN].

See the description of the OMP_FOR code.

OACC_PARALLEL#: Represents #pragma acc parallel [clause1 ... clauseN].

OACC_SERIAL#: Represents #pragma acc serial [clause1 ... clauseN].

OACC_UPDATE#: Represents #pragma acc update [clause1 ... clauseN].