# 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 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 (> 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