My intention was to analyze, how C-pointer-arithmetic are translated to WASM code. Assuming you have got the a C++ file foo.cpp like:
/// @file foo.cpp
#include <emscripten.h> // macro EMSCRIPTEN_KEEPALIVE
#include <stdint.h>
extern "C" {
void EMSCRIPTEN_KEEPALIVE myfunction(uint8_t *dst, uint8_t *src) {
dst[0] = src[0];
dst[1] = src[1];
}
} // extern "C"
The following code will produce the output file foo.wasm wiht the the corresponding WASM code (file size arond 255 bytes):
emcc foo.cpp -v -O3 -s ONLY_MY_CODE=1 -s WASM=1 -s SIDE_MODULE=1 -o foo.wasm
Check the content of this file using the disassembler tool wasm-dis form project binaryen:
wasm-dis foo.wasm
The output of the disassembler looks like:
(module
(type $0 (func (param i32 i32)))
(type $1 (func))
(import "env" "memoryBase" (global $import$0 i32))
(import "env" "memory" (memory $0 256))
(import "env" "table" (table 0 anyfunc))
(import "env" "tableBase" (global $import$3 i32))
(global $global$0 (mut i32) (i32.const 0))
(global $global$1 (mut i32) (i32.const 0))
(export "_myfunction" (func $0))
(export "__post_instantiate" (func $2))
(export "runPostSets" (func $1))
(func $0 (type $0) (param $var$0 i32) (param $var$1 i32)
(block $label$0
(i32.store8
(get_local $var$0)
(i32.load8_s
(get_local $var$1)
)
)
(i32.store8 offset=1
(get_local $var$0)
(i32.load8_s offset=1
(get_local $var$1)
)
)
)
)
(func $1 (type $1)
(nop)
)
(func $2 (type $1)
(block $label$0
(set_global $global$0
(get_global $import$0)
)
(set_global $global$1
(i32.add
(get_global $global$0)
(i32.const 5242880)
)
)
(call $1)
)
)
;; custom section "dylink", size 5
)
The disassembled code shows, that no dependencies to JS or other runtimes exists. Still a number of functions can be identified in the import and export section, “myfunction” does not depend on actually.
Note: Just in case, please let me know how to eliminate the imports of “memoryBase”, “memory”, “table”, “tableBase”, and eliminating the exports of “__post_instantiate”, “runPostSets”.
The following code shows the disassembled operations of ‘myfunction’ (function $0). AFAICS, the pointer parameters are of type i32, and the index is translated to a bytewise store8/load8 using the offset feature, for example ‘offset=1’.
(func $0 (type $0) (param $var$0 i32) (param $var$1 i32)
(block $label$0
(i32.store8
(get_local $var$0)
(i32.load8_s
(get_local $var$1)
)
)
(i32.store8 offset=1
(get_local $var$0)
(i32.load8_s offset=1
(get_local $var$1)
)
)
)
)