Example of a QBE pass

input.qbe

function w $add(w %a, w %b) {              # Define a function add
@start
    %c =w add %a, %b                   # Adds the 2 arguments
    ret %c                             # Return the result
}

function w $sum(w %n) {
@entry
    %s =w add 0, 0

@header
    jnz %n, @body, @exit

@faux1
    jmp @faux2

@body
    %s =w add %s, %n
    %n =w sub %n, 1
    jmp @header

@exit
    ret %s

@faux2
    jmp @faux1
}

export function w $main() {
@start
    %r =w call $sum(w 3)
    call $printf(l $fmt, w %r, ...)
    ret 0
}
data $fmt = { b "%d\n", b 0 }

pass.c

#include <qbe/all.h>

#include <stdbool.h>
#include <stdio.h>


void remove_dead_code (Fn *fn) {
  if (!fn->nblk) {
    return;
  }

  bool live_blocks[fn->nblk];
  memset(live_blocks, false, sizeof live_blocks);
  live_blocks[0] = true;

  for (bool changed = true; changed; ) {
    changed = false;

    for (Blk *blk = fn->start; blk; blk = blk->link) {
      if (!live_blocks[blk->id]) {
        continue;
      }

      Blk *succs[] = { blk->s1, blk->s2, NULL };

      for (Blk **psucc = succs; *psucc; ++psucc) {
        Blk *succ = *psucc;

        changed |= !live_blocks[succ->id];
        live_blocks[succ->id] = true;
      }
    }
  }

  for (Blk **pblk = &fn->start; *pblk; ) {
    Blk *blk = *pblk;

    if (live_blocks[blk->id]) {
      pblk = &blk->link;
    }
    else {
      edgedel(blk, &blk->s1);
      edgedel(blk, &blk->s2);
      *pblk = blk->link;
    }
  }
}

void read_fn (Fn *fn) {
  remove_dead_code(fn);
  printfn(fn, stdout);
}

void read_dat (Dat *dat) {
  static bool s_comma = false;

  switch (dat->type) {
    case DStart:
      break;

    case DEnd:
      printf(" }\n");
      s_comma = false;
      break;

    case DName:
      printf("data $%s = { ", dat->u.ref.nam);
      break;

    case DB:
      if (s_comma) {
        printf(", ");
      }
      else {
        s_comma = true;
      }
      printf("b ");
      if (dat->u.num) {
        printf("\"%s\"", dat->u.str);
      }
      else {
        printf("0");
      }
      break;

    default:
      die("Unsupported Dat type");
  }
}

int main () {
  parse(stdin, "stdin", read_dat, read_fn);
}