The top-level of the API is the gcc_jit_context type.
A gcc_jit_context instance encapsulates the state of a compilation.
You can set up options on it, and add types, functions and code. Invoking gcc_jit_context_compile() on it gives you a gcc_jit_result.
Contexts are the unit of lifetime-management within the API: objects have their lifetime bounded by the context they are created within, and cleanup of such objects is done for you when the context is released.
This function acquires a new gcc_jit_object * instance, which is independent of any others that may be present within this process.
This function releases all resources associated with the given context. Both the context itself and all of its gcc_jit_object * instances are cleaned up. It should be called exactly once on a given context.
It is invalid to use the context or any of its “contextual” objects after calling this.
gcc_jit_context_release (ctxt);
Given an existing JIT context, create a child context.
The child inherits a copy of all option-settings from the parent.
The child can reference objects created within the parent, but not vice-versa.
The lifetime of the child context must be bounded by that of the parent: you should release a child context before releasing the parent context.
If you use a function from a parent context within a child context, you have to compile the parent context before you can compile the child context, and the gcc_jit_result of the parent context must outlive the gcc_jit_result of the child context.
This allows caching of shared initializations. For example, you could create types and declarations of global functions in a parent context once within a process, and then create child contexts whenever a function or loop becomes hot. Each such child context can be used for JIT-compiling just one function or loop, but can reference types and helper functions created within the parent context.
Contexts can be arbitrarily nested, provided the above rules are followed, but it’s probably not worth going above 2 or 3 levels, and there will likely be a performance hit for such nesting.
Instances of gcc_jit_context * created via gcc_jit_context_acquire() are independent from each other: only one thread may use a given context at once, but multiple threads could each have their own contexts without needing locks.
Contexts created via gcc_jit_context_new_child_context() are related to their parent context. They can be partitioned by their ultimate ancestor into independent “family trees”. Only one thread within a process may use a given “family tree” of such contexts at once, and if you’re using multiple threads you should provide your own locking around entire such context partitions.
Various kinds of errors are possible when using the API, such as mismatched types in an assignment. You can only compile and get code from a context if no errors occur.
Errors are printed on stderr and can be queried using gcc_jit_context_get_first_error().
They typically contain the name of the API entrypoint where the error occurred, and pertinent information on the problem:
./buggy-program: error: gcc_jit_block_add_assignment: mismatching types: assignment to i (type: int) from "hello world" (type: const char *)
In general, if an error occurs when using an API entrypoint, the entrypoint returns NULL. You don’t have to check everywhere for NULL results, since the API handles a NULL being passed in for any argument by issuing another error. This typically leads to a cascade of followup error messages, but is safe (albeit verbose). The first error message is usually the one to pay attention to, since it is likely to be responsible for all of the rest:
Returns the first error message that occurred on the context.
The returned string is valid for the rest of the lifetime of the context.
If no errors occurred, this will be NULL.
If you are wrapping the C API for a higher-level language that supports exception-handling, you may instead by interested in the last error that occurred on the context, so that you can embed this in an exception:
Returns the last error message that occurred on the context.
The returned string is valid for the rest of the lifetime of the context.
If no errors occurred, this will be NULL.
To help with debugging: dump a C-like representation to the given path, describing what’s been set up on the context.
If “update_locations” is true, then also set up gcc_jit_location information throughout the context, pointing at the dump file as if it were a source file. This may be of use in conjunction with GCC_JIT_BOOL_OPTION_DEBUGINFO to allow stepping through the code in a debugger.
To help with debugging; enable ongoing logging of the context’s activity to the given file.
For example, the following will enable logging to stderr.
gcc_jit_context_set_logfile (ctxt, stderr, 0, 0);
Examples of information logged include:
An example of a log can be seen here, though the precise format and kinds of information logged is subject to change.
The caller remains responsible for closing logfile, and it must not be closed until all users are released. In particular, note that child contexts and gcc_jit_result instances created by the context will use the logfile.
There may a performance cost for logging.
You can turn off logging on ctxt by passing NULL for logfile. Doing so only affects the context; it does not affect child contexts or gcc_jit_result instances already created by the context.
The parameters “flags” and “verbosity” are reserved for future expansion, and must be zero for now.
To contrast the above: gcc_jit_context_dump_to_file() dumps the current state of a context to the given path, whereas gcc_jit_context_set_logfile() enables on-going logging of future activies on a context to the given FILE *.
Write C source code into path that can be compiled into a self-contained executable (i.e. with libgccjit as the only dependency). The generated code will attempt to replay the API calls that have been made into the given context.
This may be useful when debugging the library or client code, for reducing a complicated recipe for reproducing a bug into a simpler form. For example, consider client code that parses some source file into some internal representation, and then walks this IR, calling into libgccjit. If this encounters a bug, a call to gcc_jit_context_dump_reproducer_to_file will write out C code for a much simpler executable that performs the equivalent calls into libgccjit, without needing the client code and its data.
Typically you need to supply -Wno-unused-variable when compiling the generated file (since the result of each API call is assigned to a unique variable within the generated C source, and not all are necessarily then used).
Enable the dumping of a specific set of internal state from the compilation, capturing the result in-memory as a buffer.
Parameter “dumpname” corresponds to the equivalent gcc command-line option, without the “-fdump-” prefix. For example, to get the equivalent of -fdump-tree-vrp1, supply "tree-vrp1":
static char *dump_vrp1;
void
create_code (gcc_jit_context *ctxt)
{
gcc_jit_context_enable_dump (ctxt, "tree-vrp1", &dump_vrp1);
/* (other API calls omitted for brevity) */
}
The context directly stores the dumpname as a (const char *), so the passed string must outlive the context.
gcc_jit_context_compile() will capture the dump as a dynamically-allocated buffer, writing it to *out_ptr.
The caller becomes responsible for calling:
free (*out_ptr)
each time that gcc_jit_context_compile() is called. *out_ptr will be written to, either with the address of a buffer, or with NULL if an error occurred.
Warning
This API entrypoint is likely to be less stable than the others. In particular, both the precise dumpnames, and the format and content of the dumps are subject to change.
It exists primarily for writing the library’s own test suite.
Set a string option of the context.
There is currently just one string option:
The name of the program, for use as a prefix when printing error messages to stderr. If NULL, or default, “libgccjit.so” is used.
Set a boolean option of the context. Zero is “false” (the default), non-zero is “true”.
If true, gcc_jit_context_compile() will attempt to do the right thing so that if you attach a debugger to the process, it will be able to inspect variables and step through your code.
Note that you can’t step through code unless you set up source location information for the code (by creating and passing in gcc_jit_location instances).
If true, gcc_jit_context_compile() will dump its initial “tree” representation of your code to stderr (before any optimizations).
Here’s some sample output (from the square example):
<statement_list 0x7f4875a62cc0
type <void_type 0x7f4875a64bd0 VOID
align 8 symtab 0 alias set -1 canonical type 0x7f4875a64bd0
pointer_to_this <pointer_type 0x7f4875a64c78>>
side-effects head 0x7f4875a761e0 tail 0x7f4875a761f8 stmts 0x7f4875a62d20 0x7f4875a62d00
stmt <label_expr 0x7f4875a62d20 type <void_type 0x7f4875a64bd0>
side-effects
arg 0 <label_decl 0x7f4875a79080 entry type <void_type 0x7f4875a64bd0>
VOID file (null) line 0 col 0
align 1 context <function_decl 0x7f4875a77500 square>>>
stmt <return_expr 0x7f4875a62d00
type <integer_type 0x7f4875a645e8 public SI
size <integer_cst 0x7f4875a623a0 constant 32>
unit size <integer_cst 0x7f4875a623c0 constant 4>
align 32 symtab 0 alias set -1 canonical type 0x7f4875a645e8 precision 32 min <integer_cst 0x7f4875a62340 -2147483648> max <integer_cst 0x7f4875a62360 2147483647>
pointer_to_this <pointer_type 0x7f4875a6b348>>
side-effects
arg 0 <modify_expr 0x7f4875a72a78 type <integer_type 0x7f4875a645e8>
side-effects arg 0 <result_decl 0x7f4875a7a000 D.54>
arg 1 <mult_expr 0x7f4875a72a50 type <integer_type 0x7f4875a645e8>
arg 0 <parm_decl 0x7f4875a79000 i> arg 1 <parm_decl 0x7f4875a79000 i>>>>>
If true, gcc_jit_context_compile() will dump the “gimple” representation of your code to stderr, before any optimizations are performed. The dump resembles C code:
square (signed int i)
{
signed int D.56;
entry:
D.56 = i * i;
return D.56;
}
If true, gcc_jit_context_compile() will dump the final generated code to stderr, in the form of assembly language:
.file "fake.c"
.text
.globl square
.type square, @function
square:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl %edi, -4(%rbp)
.L2:
movl -4(%rbp), %eax
imull -4(%rbp), %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size square, .-square
.ident "GCC: (GNU) 4.9.0 20131023 (Red Hat 0.1-%{gcc_release})"
.section .note.GNU-stack,"",@progbits
If true, gcc_jit_context_compile() will print information to stderr on the actions it is performing, followed by a profile showing the time taken and memory usage of each phase.
If true, gcc_jit_context_compile() will dump copious amount of information on what it’s doing to various files within a temporary directory. Use GCC_JIT_BOOL_OPTION_KEEP_INTERMEDIATES (see below) to see the results. The files are intended to be human-readable, but the exact files and their formats are subject to change.
If true, libgccjit will aggressively run its garbage collector, to shake out bugs (greatly slowing down the compile). This is likely to only be of interest to developers of the library. It is used when running the selftest suite.
If true, the gcc_jit_context will not clean up intermediate files written to the filesystem, and will display their location on stderr.
Set an integer option of the context.
There is currently just one integer option:
How much to optimize the code.
Valid values are 0-3, corresponding to GCC’s command-line options -O0 through -O3.
The default value is 0 (unoptimized).