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Avoid allocating a new list when calling List.sublist

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Brendan Hansknecht 2022-10-05 08:11:02 -07:00
parent d12568d8e4
commit 4ebe1b2bce
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1 changed files with 24 additions and 9 deletions
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33
crates/compiler/builtins/bitcode/src/list.zig
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@ -517,17 +517,25 @@ pub fn listSublist(
len: usize,
dec: Dec,
) callconv(.C) RocList {
if (len == 0) {
const size = list.len();
if (len == 0 or start >= size) {
if (list.isUnique()) {
// Decrement the reference counts of all elements.
if (list.bytes) |source_ptr| {
var i: usize = 0;
while (i < size) : (i += 1) {
const element = source_ptr + i * element_width;
dec(element);
}
var output = list;
output.length = 0;
return output;
}
}
return RocList.empty();
}
if (list.bytes) |source_ptr| {
const size = list.len();
if (start >= size) {
return RocList.empty();
}
const keep_len = std.math.min(len, size - start);
const drop_start_len = start;
const drop_end_len = size - (start + keep_len);
@ -546,10 +554,17 @@ pub fn listSublist(
dec(element);
}
if (start == 0 and list.isUnique()) {
if (list.isUnique()) {
var output = list;
output.length = keep_len;
return output;
if (start == 0) {
return output;
} else {
// We want memmove due to aliasing. Zig does not expose it directly.
// Instead use copy which can write to aliases as long as the dest is before the source.
mem.copy(u8, source_ptr[0 .. keep_len * element_width], source_ptr[start * element_width .. (start + keep_len) * element_width]);
return output;
}
} else {
const output = RocList.allocate(alignment, keep_len, element_width);
const target_ptr = output.bytes orelse unreachable;