# Bytecode
Bytecode (BC, also *P-code* , "portable code") is a type of [binary ](binary.md ) format for executable programs usually intended to be [interpreted ](interpreter.md ) or to serve as an [intermediate representation ](intermediate_representation.md ) in [compilers ](compiler.md ) (i.e. meant to be translated to some other language); it is quite similar to [machine code ](machine_code.md ), however machine code is meant to be directly run by some physical [hardware ](hardware.md ) while bytecode is more of a [virtual ](virtual.md ), machine independent code preferring things like [portability ](portability.md ), speed of interpretation, retaining meta information or being easy to translate.
TODO: moar
## Example
Let's consider a simple algorithm that tests the [Collatz conjecture ](collatz_conjecture.md ) (which says that applying a simple operation from any starting number over and over will always lead to number 1). The program reads a number (one digit for simplicity) and then prints the sequence until reaching the final number 1. The algorithm in [C ](c.md ) would look as follows:
```
// Collatz conjecture
#include <stdio.h>
int next(int n)
{
return n % 2 ? // is odd?
3 * n + 1 :
n / 2;
}
int main(void)
{
int n = getchar() - '0'; // read input ASCII digit
while (1)
{
printf("%d\n",n);
if (n == 1)
break;
n = next(n);
}
return 0;
}
```
C will be normally compiled to [machine code ](machine_code.md ), however we can take a look at some immediate representation bytecode that compilers internally use to generate the machine code. The following is [LLVM ](llvm.md ), a widely used bytecode that can be produced from the above C code with [clang ](clang.md ) compiler (e.g. as `clang -cc1 tmp.c -S -emit-llvm -o -` ):
```
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
; Function Attrs: noinline nounwind optnone
define i32 @next (i32 %n) #0 {
entry:
%n.addr = alloca i32, align 4
store i32 %n, i32* %n.addr, align 4
%0 = load i32, i32* %n.addr, align 4
%rem = srem i32 %0, 2
%tobool = icmp ne i32 %rem, 0
br i1 %tobool, label %cond.true, label %cond.false
cond.true: ; preds = %entry
%1 = load i32, i32* %n.addr, align 4
%mul = mul nsw i32 3, %1
%add = add nsw i32 %mul, 1
br label %cond.end
cond.false: ; preds = %entry
%2 = load i32, i32* %n.addr, align 4
%div = sdiv i32 %2, 2
br label %cond.end
cond.end: ; preds = %cond.false, %cond.true
%cond = phi i32 [ %add, %cond.true ], [ %div, %cond.false ]
ret i32 %cond
}
; Function Attrs: noinline nounwind optnone
define i32 @main () #0 {
entry:
%retval = alloca i32, align 4
%n = alloca i32, align 4
store i32 0, i32* %retval, align 4
%call = call i32 (...) @getchar ()
%sub = sub nsw i32 %call, 48
store i32 %sub, i32* %n, align 4
br label %while.body
while.body: ; preds = %entry, %if.end
%0 = load i32, i32* %n, align 4
%call1 = call i32 (i8*, ...) @printf (i8* ... )
%1 = load i32, i32* %n, align 4
%cmp = icmp eq i32 %1, 1
br i1 %cmp, label %if.then, label %if.end
if.then: ; preds = %while.body
br label %while.end
if.end: ; preds = %while.body
%2 = load i32, i32* %n, align 4
%call2 = call i32 @next (i32 %2)
store i32 %call2, i32* %n, align 4
br label %while.body
while.end: ; preds = %if.then
ret i32 0
}
declare i32 @getchar (...) #1
declare i32 @printf (i8*, ...) #1
attributes #0 = { ... }
attributes #1 = { ... }
!llvm.module.flags = !{!0}
!llvm.ident = !{!1}
!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)"}
```
TODO: analyze the above
Now let's rewrite the same algorithm in [comun ](comun.md ), a different language which will allow us to produce another kind of bytecode (obtained with `comun -T program.cmn` ):
```
# Collatz conjecture
next:
$0 2 % ? # is odd?
3 * 1 +
;
2 /
.
.
< - # read input ASCII digit
"0" - # convert it to number
@@
# print:
$0 10 / "0" + ->
$0 10 % "0" + ->
10 ->
$0 1 = ?
!@
.
next
.
```
Here is annotated comun bytecode this compiles to:
```
000000: DES 00 0111 # func \ next:
000001: JMA 00 0100... # 20 (#14) |
000002: COC 00 0001 |
000003: MGE 00 0000 | $0
000004: CON' 00 0010 # 2 (#2) | 2
000005: MOX 00 0000 | %
000006: DES 00 0001 # if | \ ?
000007: JNA 00 0000... # 16 (#10) | |
000008: COC 00 0001 | |
000009: CON' 00 0011 # 3 (#3) | | 3
00000a: MUX 00 0000 | | *
00000b: CON' 00 0001 # 1 (#1) | | 1
00000c: ADX 00 0000 | | +
00000d: DES 00 0010 # else | < ;
00000e: JMA 00 0011... # 19 (#13) | |
00000f: COC 00 0001 | |
000010: CON' 00 0010 # 2 (#2) | | 2
000011: DIX 00 0000 | | /
000012: DES 00 0011 # end if | / .
000013: RET 00 0000 / .
000014: INI 00 0000
000015: INP 00 0000 < -
000016: CON' 00 0000... # 48 (#30) "0"
000017: COC 00 0011
000018: SUX 00 0000 -
000019: DES 00 0100 # loop \ @@
00001a: MGE 00 0000 | $0
00001b: CON' 00 1010 # 10 (#a) | 10
00001c: DIX 00 0000 | /
00001d: CON' 00 0000... # 48 (#30) | "0"
00001e: COC 00 0011 |
00001f: ADX 00 0000 | +
000020: OUT 00 0000 | ->
000021: MGE 00 0000 | $0
000022: CON' 00 1010 # 10 (#a) | 10
000023: MOX 00 0000 | %
000024: CON' 00 0000... # 48 (#30) | "0"
000025: COC 00 0011 |
000026: ADX 00 0000 | +
000027: OUT 00 0000 | ->
000028: CON' 00 1010 # 10 (#a) | 10
000029: OUT 00 0000 | ->
00002a: MGE 00 0000 | $0
00002b: CON' 00 0001 # 1 (#1) | 1
00002c: EQX 00 0000 | =
00002d: DES 00 0001 # if | \ ?
00002e: JNA 00 0100... # 52 (#34) | |
00002f: COC 00 0011 | |
000030: DES 00 0101 # break | | !@
000031: JMA 00 1000... # 56 (#38) | |
000032: COC 00 0011 | |
000033: DES 00 0011 # end if | / .
000034: CAL 00 0011 # 3 (#3) | next
000035: DES 00 0110 # end loop / .
000036: JMA 00 1010... # 26 (#1a)
000037: COC 00 0001
000038: END 00 0000
```
TODO: analyze the above, show other bytecodes (python, java, ...)
Let's try the same in [Python ](python.md ). The code we'll examine will look like this:
```
# Collatz conjecture
def next(n):
return 3 * n + 1 if n % 2 != 0 else n / 2
n = ord(raw_input()[0]) - ord('0')
while True:
print(n)
if n == 1:
break
n = next(n)
```
And the bytecode we get (e.g. with `python -m dis program.py` ):
```
3 0 LOAD_CONST 0 (< code object next at . . . )
3 MAKE_FUNCTION 0
6 STORE_NAME 0 (next)
6 9 LOAD_NAME 1 (ord)
12 LOAD_NAME 2 (raw_input)
15 CALL_FUNCTION 0
18 LOAD_CONST 1 (0)
21 BINARY_SUBSCR
22 CALL_FUNCTION 1
25 LOAD_NAME 1 (ord)
28 LOAD_CONST 2 ('0')
31 CALL_FUNCTION 1
34 BINARY_SUBTRACT
35 STORE_NAME 3 (n)
8 38 SETUP_LOOP 43 (to 84)
>> 41 LOAD_NAME 4 (True)
44 POP_JUMP_IF_FALSE 83
9 47 LOAD_NAME 3 (n)
50 PRINT_ITEM
51 PRINT_NEWLINE
11 52 LOAD_NAME 3 (n)
55 LOAD_CONST 3 (1)
58 COMPARE_OP 2 (==)
61 POP_JUMP_IF_FALSE 68
12 64 BREAK_LOOP
65 JUMP_FORWARD 0 (to 68)
14 >> 68 LOAD_NAME 0 (next)
71 LOAD_NAME 3 (n)
74 CALL_FUNCTION 1
77 STORE_NAME 3 (n)
80 JUMP_ABSOLUTE 41
>> 83 POP_BLOCK
>> 84 LOAD_CONST 4 (None)
87 RETURN_VALUE
```
TODO: make sense of it and analyze it
TODO: web assembly
