279 lines
7.6 KiB
Markdown
279 lines
7.6 KiB
Markdown
# Bytecode
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TODO
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## Example
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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:
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```
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// Collatz conjecture
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#include <stdio.h>
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int next(int n)
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{
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return n % 2 ? // is odd?
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3 * n + 1 :
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n / 2;
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}
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int main(void)
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{
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int n = getchar() - '0'; // read input ASCII digit
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while (1)
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{
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printf("%d\n",n);
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if (n == 1)
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break;
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n = next(n);
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}
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return 0;
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}
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```
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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 -`):
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```
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target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-pc-linux-gnu"
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@.str = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
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; Function Attrs: noinline nounwind optnone
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define i32 @next(i32 %n) #0 {
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entry:
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%n.addr = alloca i32, align 4
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store i32 %n, i32* %n.addr, align 4
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%0 = load i32, i32* %n.addr, align 4
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%rem = srem i32 %0, 2
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%tobool = icmp ne i32 %rem, 0
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br i1 %tobool, label %cond.true, label %cond.false
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cond.true: ; preds = %entry
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%1 = load i32, i32* %n.addr, align 4
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%mul = mul nsw i32 3, %1
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%add = add nsw i32 %mul, 1
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br label %cond.end
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cond.false: ; preds = %entry
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%2 = load i32, i32* %n.addr, align 4
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%div = sdiv i32 %2, 2
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br label %cond.end
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cond.end: ; preds = %cond.false, %cond.true
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%cond = phi i32 [ %add, %cond.true ], [ %div, %cond.false ]
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ret i32 %cond
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}
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; Function Attrs: noinline nounwind optnone
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define i32 @main() #0 {
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entry:
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%retval = alloca i32, align 4
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%n = alloca i32, align 4
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store i32 0, i32* %retval, align 4
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%call = call i32 (...) @getchar()
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%sub = sub nsw i32 %call, 48
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store i32 %sub, i32* %n, align 4
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br label %while.body
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while.body: ; preds = %entry, %if.end
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%0 = load i32, i32* %n, align 4
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%call1 = call i32 (i8*, ...) @printf(i8* ... )
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%1 = load i32, i32* %n, align 4
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%cmp = icmp eq i32 %1, 1
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br i1 %cmp, label %if.then, label %if.end
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if.then: ; preds = %while.body
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br label %while.end
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if.end: ; preds = %while.body
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%2 = load i32, i32* %n, align 4
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%call2 = call i32 @next(i32 %2)
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store i32 %call2, i32* %n, align 4
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br label %while.body
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while.end: ; preds = %if.then
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ret i32 0
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}
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declare i32 @getchar(...) #1
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declare i32 @printf(i8*, ...) #1
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attributes #0 = { ... }
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attributes #1 = { ... }
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!llvm.module.flags = !{!0}
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!llvm.ident = !{!1}
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!0 = !{i32 1, !"wchar_size", i32 4}
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!1 = !{!"clang version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)"}
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```
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TODO: analyze the above
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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`):
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```
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# Collatz conjecture
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next:
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$0 2 % ? # is odd?
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3 * 1 +
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;
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2 /
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.
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.
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<- # read input ASCII digit
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"0" - # convert it to number
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@@
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# print:
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$0 10 / "0" + ->
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$0 10 % "0" + ->
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10 ->
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$0 1 = ?
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!@
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.
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next
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.
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```
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Here is annotated comun bytecode this compiles to:
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```
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000000: DES 00 0111 # func \ next:
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000001: JMA 00 0100... # 20 (#14) |
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000002: COC 00 0001 |
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000003: MGE 00 0000 | $0
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000004: CON' 00 0010 # 2 (#2) | 2
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000005: MOX 00 0000 | %
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000006: DES 00 0001 # if | \ ?
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000007: JNA 00 0000... # 16 (#10) | |
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000008: COC 00 0001 | |
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000009: CON' 00 0011 # 3 (#3) | | 3
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00000a: MUX 00 0000 | | *
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00000b: CON' 00 0001 # 1 (#1) | | 1
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00000c: ADX 00 0000 | | +
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00000d: DES 00 0010 # else | < ;
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00000e: JMA 00 0011... # 19 (#13) | |
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00000f: COC 00 0001 | |
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000010: CON' 00 0010 # 2 (#2) | | 2
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000011: DIX 00 0000 | | /
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000012: DES 00 0011 # end if | / .
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000013: RET 00 0000 / .
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000014: INI 00 0000
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000015: INP 00 0000 <-
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000016: CON' 00 0000... # 48 (#30) "0"
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000017: COC 00 0011
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000018: SUX 00 0000 -
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000019: DES 00 0100 # loop \ @@
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00001a: MGE 00 0000 | $0
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00001b: CON' 00 1010 # 10 (#a) | 10
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00001c: DIX 00 0000 | /
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00001d: CON' 00 0000... # 48 (#30) | "0"
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00001e: COC 00 0011 |
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00001f: ADX 00 0000 | +
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000020: OUT 00 0000 | ->
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000021: MGE 00 0000 | $0
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000022: CON' 00 1010 # 10 (#a) | 10
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000023: MOX 00 0000 | %
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000024: CON' 00 0000... # 48 (#30) | "0"
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000025: COC 00 0011 |
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000026: ADX 00 0000 | +
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000027: OUT 00 0000 | ->
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000028: CON' 00 1010 # 10 (#a) | 10
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000029: OUT 00 0000 | ->
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00002a: MGE 00 0000 | $0
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00002b: CON' 00 0001 # 1 (#1) | 1
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00002c: EQX 00 0000 | =
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00002d: DES 00 0001 # if | \ ?
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00002e: JNA 00 0100... # 52 (#34) | |
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00002f: COC 00 0011 | |
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000030: DES 00 0101 # break | | !@
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000031: JMA 00 1000... # 56 (#38) | |
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000032: COC 00 0011 | |
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000033: DES 00 0011 # end if | / .
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000034: CAL 00 0011 # 3 (#3) | next
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000035: DES 00 0110 # end loop / .
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000036: JMA 00 1010... # 26 (#1a)
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000037: COC 00 0001
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000038: END 00 0000
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```
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TODO: analyze the above, show other bytecodes (python, java, ...)
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Let's try the same in [Python](python.md). The code we'll examine will look like this:
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```
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# Collatz conjecture
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def next(n):
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return 3 * n + 1 if n % 2 != 0 else n / 2
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n = ord(raw_input()[0]) - ord('0')
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while True:
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print(n)
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if n == 1:
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break
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n = next(n)
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```
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And the bytecode we get (e.g. with `python -m dis program.py`):
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```
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3 0 LOAD_CONST 0 (<code object next at ...)
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3 MAKE_FUNCTION 0
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6 STORE_NAME 0 (next)
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6 9 LOAD_NAME 1 (ord)
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12 LOAD_NAME 2 (raw_input)
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15 CALL_FUNCTION 0
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18 LOAD_CONST 1 (0)
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21 BINARY_SUBSCR
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22 CALL_FUNCTION 1
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25 LOAD_NAME 1 (ord)
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28 LOAD_CONST 2 ('0')
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31 CALL_FUNCTION 1
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34 BINARY_SUBTRACT
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35 STORE_NAME 3 (n)
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8 38 SETUP_LOOP 43 (to 84)
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>> 41 LOAD_NAME 4 (True)
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44 POP_JUMP_IF_FALSE 83
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9 47 LOAD_NAME 3 (n)
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50 PRINT_ITEM
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51 PRINT_NEWLINE
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11 52 LOAD_NAME 3 (n)
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55 LOAD_CONST 3 (1)
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58 COMPARE_OP 2 (==)
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61 POP_JUMP_IF_FALSE 68
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12 64 BREAK_LOOP
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65 JUMP_FORWARD 0 (to 68)
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14 >> 68 LOAD_NAME 0 (next)
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71 LOAD_NAME 3 (n)
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74 CALL_FUNCTION 1
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77 STORE_NAME 3 (n)
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80 JUMP_ABSOLUTE 41
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>> 83 POP_BLOCK
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>> 84 LOAD_CONST 4 (None)
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87 RETURN_VALUE
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```
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TODO: make sense of it and analyze it
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TODO: web assembly |