1 January 2017
trampoline - closures as first-class C functions
function = alloc_trampoline_r(address, data0, data1);
is_trampoline_r(function) trampoline_r_address(function) trampoline_r_data0(function) trampoline_r_data1(function)
These functions implement closures as first-class C functions. A closure consists of a regular C function and a piece of data which gets passed to the C function when the closure is called.
Closures as first-class C functions means that they fit into a function pointer and can be called exactly like any other C function. function = alloc_trampoline_r(address, data0, data1) allocates a closure. When function gets called, it stores in a special "lexical chain register" a pointer to a storage area containing data0 in its first word and data1 in its second word and calls the C function at address. The function at address is responsible for fetching data0 and data1 off the pointer. Note that the "lexical chain register" is a call-used register, i.e. is clobbered by function calls.
This is much like gcc’s local functions, except that the GNU C local functions have dynamic extent (i.e. are deallocated when the creating function returns), while trampoline provides functions with indefinite extent: function is only deallocated when free_trampoline_r(function) is called.
is_trampoline_r(function) checks whether the C function function was produced by a call to alloc_trampoline_r. If this returns true, the arguments given to alloc_trampoline_r can be retrieved:
trampoline_r_address(function) returns address,
trampoline_r_data0(function) returns data0,
trampoline_r_data1(function) returns data1.
The way gcc builds local functions is described in the gcc source, file gcc-2.6.3/config/cpu/cpu.h.
Bruno Haible <bruno>
Many ideas were cribbed from the gcc source.