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bf_jit.cpp
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bf_jit.cpp
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// Copyright 2014 Brian Quinlan
// See "LICENSE" file for details.
#include <cstdint>
#include <stack>
#include "bf_jit.h"
using std::stack;
// The total number of times that a loop condition (e.g. "[") must be evaluated
// before the loop is compiled.
const int kLoopCompilationThreshold = 20;
BrainfuckJIT::BrainfuckJIT() {}
bool BrainfuckJIT::init(string::const_iterator start,
string::const_iterator end) {
start_ = start;
end_ = end;
// Build the mapping from the position of the start of a block (i.e. "]") to
// a Loop struct.
stack<string::const_iterator> block_starts;
for (string::const_iterator it = start; it != end; ++it) {
if (*it == '[') {
block_starts.push(it);
} else if (*it == ']') {
if (block_starts.size() != 0) {
const string::const_iterator &loop_start = block_starts.top();
loop_start_to_loop_[loop_start] = Loop(it+1);
block_starts.pop();
}
}
}
if (block_starts.size() != 0) {
fprintf(
stderr,
"Unable to find loop end in block starting with: %s\n",
string(block_starts.top(), end).c_str());
return false;
}
return true;
}
void* BrainfuckJIT::run(BrainfuckReader reader,
void* reader_arg,
BrainfuckWriter writer,
void* writer_arg,
void* memory) {
uint8_t* byte_memory = reinterpret_cast<uint8_t *>(memory);
// When processing a "[", push the position of that command onto a stack so
// that we can quickly return to the start of the block when then "]" is
// interpreted.
stack<string::const_iterator> return_stack;
for (string::const_iterator it = start_; it != end_;) {
switch (*it) {
case '<':
--byte_memory;
++it;
break;
case '>':
++byte_memory;
++it;
break;
case '-':
*byte_memory -= 1;
++it;
break;
case '+':
*byte_memory += 1;
++it;
break;
case ',':
*byte_memory = reader(reader_arg);
++it;
break;
case '.':
writer(writer_arg, *byte_memory);
++it;
break;
case '[':
{
Loop &loop = loop_start_to_loop_[it];
if (loop.compiled == nullptr &&
loop.condition_evaluation_count > kLoopCompilationThreshold) {
shared_ptr<BrainfuckCompileAndGo> compiled(
new BrainfuckCompileAndGo());
string::const_iterator compilation_end(loop.after_end);
if (!compiled->init(it, compilation_end)) {
fprintf(stderr,
"Unable to compile code: %s\n",
string(it, compilation_end).c_str());
} else {
loop.compiled = compiled;
}
}
if (loop.compiled) {
byte_memory = reinterpret_cast<uint8_t *>(
loop.compiled->run(reader,
reader_arg,
writer,
writer_arg,
byte_memory));
it = loop_start_to_loop_[it].after_end;
} else {
++loop.condition_evaluation_count;
if (*byte_memory) {
return_stack.push(it);
++it;
} else {
it = loop_start_to_loop_[it].after_end;
}
}
}
break;
case ']':
if (return_stack.size() != 0) {
it = return_stack.top();
return_stack.pop();
} else {
++it;
}
break;
default:
++it;
break;
}
}
return byte_memory;
}