third_party/boringssl/src/crypto/perlasm/readme

The perl scripts in this directory are my 'hack' to generate
multiple different assembler formats via the one origional script.

The way to use this library is to start with adding the path to this directory
and then include it.

push(@INC,"perlasm","../../perlasm");
require "x86asm.pl";

The first thing we do is setup the file and type of assembler

&asm_init($ARGV[0]);

The first argument is the 'type'.  Currently
'cpp', 'sol', 'a.out', 'elf' or 'win32'.
Argument 2 is the file name.

The reciprocal function is
&asm_finish() which should be called at the end.

There are 2 main 'packages'. x86ms.pl, which is the Microsoft assembler,
and x86unix.pl which is the unix (gas) version.

Functions of interest are:
&external_label("des_SPtrans");	declare and external variable
&LB(reg);			Low byte for a register
&HB(reg);			High byte for a register
&BP(off,base,index,scale)	Byte pointer addressing
&DWP(off,base,index,scale)	Word pointer addressing
&stack_push(num)		Basically a 'sub esp, num*4' with extra
&stack_pop(num)			inverse of stack_push
&function_begin(name,extra)	Start a function with pushing of
				edi, esi, ebx and ebp.  extra is extra win32
				external info that may be required.
&function_begin_B(name,extra)	Same as normal function_begin but no pushing.
&function_end(name)		Call at end of function.
&function_end_A(name)		Standard pop and ret, for use inside functions
&function_end_B(name)		Call at end but with poping or 'ret'.
&swtmp(num)			Address on stack temp word.
&wparam(num)			Parameter number num, that was push
				in C convention.  This all works over pushes
				and pops.
&comment("hello there")		Put in a comment.
&label("loop")			Refer to a label, normally a jmp target.
&set_label("loop")		Set a label at this point.
&data_word(word)		Put in a word of data.

So how does this all hold together?  Given

int calc(int len, int *data)
	{
	int i,j=0;

	for (i=0; i<len; i++)
		{
		j+=other(data[i]);
		}
	}

So a very simple version of this function could be coded as

	push(@INC,"perlasm","../../perlasm");
	require "x86asm.pl";
	
	&asm_init($ARGV[0]);

	&external_label("other");

	$tmp1=	"eax";
	$j=	"edi";
	$data=	"esi";
	$i=	"ebp";

	&comment("a simple function");
	&function_begin("calc");
	&mov(	$data,		&wparam(1)); # data
	&xor(	$j,		$j);
	&xor(	$i,		$i);

	&set_label("loop");
	&cmp(	$i,		&wparam(0));
	&jge(	&label("end"));

	&mov(	$tmp1,		&DWP(0,$data,$i,4));
	&push(	$tmp1);
	&call(	"other");
	&add(	$j,		"eax");
	&pop(	$tmp1);
	&inc(	$i);
	&jmp(	&label("loop"));

	&set_label("end");
	&mov(	"eax",		$j);

	&function_end("calc");

	&asm_finish();

The above example is very very unoptimised but gives an idea of how
things work.