[has_globals]
module builtin

// With -prealloc, V calls libc's malloc to get chunks, each at least 16MB
// in size, as needed. Once a chunk is available, all malloc() calls within
// V code, that can fit inside the chunk, will use it instead, each bumping a
// pointer, till the chunk is filled. Once a chunk is filled, a new chunk will
// be allocated by calling libc's malloc, and the process continues.
// Each new chunk has a pointer to the old one, and at the end of the program,
// the entire linked list of chunks is freed.
// The goal of all this is to amortize the cost of calling libc's malloc,
// trading higher memory usage for a compiler (or any single threaded batch
// mode program), for a ~8-10% speed increase.
// Note: `-prealloc` is NOT safe to be used for multithreaded programs!

// size of the preallocated chunk
const prealloc_block_size = 16 * 1024 * 1024

__global g_memory_block &VMemoryBlock
[heap]
struct VMemoryBlock {
mut:
    id        int
    cap       isize
    start     &u8 = 0
    previous  &VMemoryBlock = 0
    remaining isize
    current   &u8 = 0
    mallocs   int
}

[unsafe]
fn vmemory_block_new(prev &VMemoryBlock, at_least isize) &VMemoryBlock {
    mut v := unsafe { &VMemoryBlock(C.calloc(1, sizeof(VMemoryBlock))) }
    if unsafe { prev != 0 } {
        v.id = prev.id + 1
    }

    v.previous = prev
    block_size := if at_least < prealloc_block_size { prealloc_block_size } else { at_least }
    v.start = unsafe { C.malloc(block_size) }
    v.cap = block_size
    v.remaining = block_size
    v.current = v.start
    return v
}

[unsafe]
fn vmemory_block_malloc(n isize) &u8 {
    unsafe {
        if g_memory_block.remaining < n {
            g_memory_block = vmemory_block_new(g_memory_block, n)
        }
        mut res := &u8(0)
        res = g_memory_block.current
        g_memory_block.remaining -= n
        g_memory_block.mallocs++
        g_memory_block.current += n
        return res
    }
}

/////////////////////////////////////////////////

[unsafe]
fn prealloc_vinit() {
    unsafe {
        g_memory_block = vmemory_block_new(nil, prealloc_block_size)
        $if !freestanding {
            C.atexit(prealloc_vcleanup)
        }
    }
}

[unsafe]
fn prealloc_vcleanup() {
    $if prealloc_stats ? {
        // Note: we do 2 loops here, because string interpolation
        // in the first loop may still use g_memory_block
        // The second loop however should *not* allocate at all.
        mut nr_mallocs := i64(0)
        mut mb := g_memory_block
        for unsafe { mb != 0 } {
            nr_mallocs += mb.mallocs
            eprintln('> freeing mb.id: ${mb.id:3} | cap: ${mb.cap:7} | rem: ${mb.remaining:7} | start: ${voidptr(mb.start)} | current: ${voidptr(mb.current)} | diff: ${u64(mb.current) - u64(mb.start):7} bytes | mallocs: ${mb.mallocs}')
            mb = mb.previous
        }
        eprintln('> nr_mallocs: ${nr_mallocs}')
    }
    unsafe {
        for g_memory_block != 0 {
            C.free(g_memory_block.start)
            g_memory_block = g_memory_block.previous
        }
    }
}

[unsafe]
fn prealloc_malloc(n isize) &u8 {
    return unsafe { vmemory_block_malloc(n) }
}

[unsafe]
fn prealloc_realloc(old_data &u8, old_size isize, new_size isize) &u8 {
    new_ptr := unsafe { vmemory_block_malloc(new_size) }
    min_size := if old_size < new_size { old_size } else { new_size }
    unsafe { C.memcpy(new_ptr, old_data, min_size) }
    return new_ptr
}

[unsafe]
fn prealloc_calloc(n isize) &u8 {
    new_ptr := unsafe { vmemory_block_malloc(n) }
    unsafe { C.memset(new_ptr, 0, n) }
    return new_ptr
}