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void gen_mul(Value *dest, Value *src) {
    if (isconst(dest)) {
        gen_swap(dest, src); // this doesn't generate instructions and just swaps descriptors ("value renaming")
    }
    val_to_reg(dest); // the post-condition is that dest is allocated to a register
    if (isconst(src) && isimm32(src->ival)) {
        if (src->ival == 0) {
            int_to_val(dest, 0);
        } else if (src->ival == 1) {
            // do nothing

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Partial redundancy elimination (PRE) is an optimization where you eliminate computations that are redundant on some but not all control flow paths.

Example:

if (...) {
    // ...
    int a = x + y;
    // ...
} else {

// ...

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typedef enum {
    CMD_INT = NODE_INT,
    CMD_NEG = NODE_NEG,
    CMD_NOT = NODE_NOT,
    CMD_ADD = NODE_ADD,
    CMD_SUB = NODE_SUB,
    CMD_RET = 128,
    CMD_SETREF,
    CMD_GETREF,
} Cmd;

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# Here's a probably-not-new data structure I discovered after implementing weight-balanced trees with
# amortized subtree rebuilds (e.g. http://jeffe.cs.illinois.edu/teaching/algorithms/notes/10-scapegoat-splay.pdf)
# and realizing it was silly to turn a subtree into a sorted array with an in-order traversal only as an
# aid in constructing a balanced subtree in linear time. Why not replace the subtree by the sorted array and
# use binary search when hitting that leaf array? Then you'd defer any splitting of the array until inserts and
# deletes hit that leaf array. Only in hindsight did I realize this is similar to the rope data structure for strings.
# Unlike ropes, it's a key-value search tree for arbitrary ordered keys.
#
# The main attraction of this data structure is its simplicity (on par with a standard weight-balanced tree) and that it
# coalesces subtrees into contiguous arrays, which reduces memory overhead and boosts the performance of in-order traversals

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// Heavily based on ideas from https://github.com/LuaJIT/LuaJIT/blob/v2.1/src/lj_opt_fold.c
// The most fundamental deviation is that I eschew the big hash table and the lj_opt_fold()
// trampoline for direct tail calls. The biggest problem with a trampoline is that you lose
// the control flow context. Another problem is that there's too much short-term round-tripping
// of data through memory. It's also easier to do ad-hoc sharing between rules with my approach.
// From what I can tell, it also isn't possible to do general reassociation with LJ's fold engine
// since that requires non-tail recursion, so LJ does cases like (x + n1) + n2 => x + (n1 + n2)
// but not (x + n1) + (y + n2) => x + (y + (n1 + n2)) which is common in address generation. The
// code below has some not-so-obvious micro-optimizations for register passing and calling conventions,
// e.g. the unary_cse/binary_cse parameter order, the use of long fields in ValueRef.

pervognsen

ValueRef add(ValueRef left, ValueRef right) {
    // Canonical commutation: x + y => y + x (constants have negative pos and move to the left)
    if (left.pos > right.pos) SWAP(left, right);
    switch (left.op) {
    case NEG:
        // Strength reduction: (-x) + y => y - x
        return sub(right, getleft(left));
    case NUM:
        // Strength reduction: 0 + x => x
        if (getnum(left) == 0) return right;

pervognsen

struct Object {
    Key key; // The key is any piece of data that uniquely identifies the object.
    // ...
};

struct Handle {
    Key key;
    Index index; // This caches a speculative table index for an object with the corresponding key.
};

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// Linear-scan mark-compact collector for causal data structures, i.e. the reference graph is acyclic and the objects
// are ordered by age in the heap (which happens if you use a linear allocator) so they are automatically topologically sorted.
// This algorithm has very high memory-level parallelism which can be exploited by modern out-of-order processors, and should
// be memory throughput limited.
void collect(void) {
    // Initialize marks from roots.
    memset(marks, 0, num_nodes * sizeof(uint32_t));
    int newest = 0, oldest = num_nodes;
    for (int i = 0; i < num_roots; i++) {
        marks[roots[i]] = 1;

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// This is another take on the mark-compact collector from https://gist.github.com/pervognsen/7fe51bef8977cb249ac4c6f830f818a5
// To avoid having to do global compaction, our object indices will have two parts: a block index and a block offset.
// Within a block we have the same linear older-to-newer ordering by offset. But now blocks are allowed to have different ages.
// The block ages are defined by their position in a linked list: There's oldest_block and newest_block indices and then
// previous_block[block_index] for each block. This enables newest-to-oldest block iteration and the linked-list structure
// means that we can free an empty block by unlinking it. When a block is reused, it becomes the newest_block. Now, instead
// of only compacting within a block we will actually be coalescing across an age range of blocks. By doing so, we will usually
// be able to empty out entire blocks from the newer part of that age range, so they can be reused. This should have very similar
// performance characteri

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// The two sweetspots are 8-bit and 4-bit tags. In the latter case you can fit 14 32-bit keys in
// a cacheline-sized bucket and still have one leftover byte for metadata.

// As for how to choose tags for particular problems, Google's Swiss table and Facebook's F14 table
// both use hash bits that weren't used for the bucket indexing. This is ideal from an entropy perspective
// but it can be better to use some particular feature of the key that you'd otherwise check against anyway.
// For example, for 32-bit keys (with a usable sentinel value) you could use the 8 low bits as the tag
// while storing the remaining 24 bits in the rest of the bucket; that fits 16 keys per bucket. Or if the keys
// are strings you could store the length as the discriminator: with an 8-bit tag, 0 means an empty slot,
// 1..254 means a string of that length, and 255 means a string of length 255 or longer. With a 4-bit tag