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Merge pull request #7444 from roc-lang/better-atomic

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Improve roc atomic refcounting by using first bit as threadlocal
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Brendan Hansknecht 2025-01-01 09:31:54 -08:00 • committed by GitHub
commit 8001de5468
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1 changed files with 37 additions and 8 deletions
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45
crates/compiler/builtins/bitcode/src/utils.zig
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@ -177,6 +177,9 @@ pub const Dec = fn (?[*]u8) callconv(.C) void;
const REFCOUNT_MAX_ISIZE: isize = 0;
pub const REFCOUNT_ONE_ISIZE: isize = std.math.minInt(isize);
pub const REFCOUNT_ONE: usize = @as(usize, @bitCast(REFCOUNT_ONE_ISIZE));
// Only top bit set.
pub const REFCOUNT_IS_NOT_ATOMIC_MASK: isize = REFCOUNT_ONE_ISIZE;
pub const REFCOUNT_ONE_ATOMIC_ISIZE: isize = 1;
pub const IntWidth = enum(u8) {
U8 = 0,
@ -207,13 +210,14 @@ pub fn increfRcPtrC(ptr_to_refcount: *isize, amount: isize) callconv(.C) void {
}
// Ensure that the refcount is not whole program lifetime.
if (ptr_to_refcount.* != REFCOUNT_MAX_ISIZE) {
const refcount: isize = ptr_to_refcount.*;
if (refcount != REFCOUNT_MAX_ISIZE) {
// Note: we assume that a refcount will never overflow.
// As such, we do not need to cap incrementing.
switch (RC_TYPE) {
.normal => {
if (DEBUG_INCDEC and builtin.target.cpu.arch != .wasm32) {
const old = @as(usize, @bitCast(ptr_to_refcount.*));
const old = @as(usize, @bitCast(refcount));
const new = old + @as(usize, @intCast(amount));
const oldH = old - REFCOUNT_ONE + 1;
@ -222,10 +226,16 @@ pub fn increfRcPtrC(ptr_to_refcount: *isize, amount: isize) callconv(.C) void {
std.debug.print("{} + {} = {}!\n", .{ oldH, amount, newH });
}
ptr_to_refcount.* += amount;
ptr_to_refcount.* = refcount + amount;
},
.atomic => {
_ = @atomicRmw(isize, ptr_to_refcount, .Add, amount, .monotonic);
// If the first bit of the refcount is set, this variable is threadlocal.
// Use normal refcounting instead of atomic.
if (refcount & REFCOUNT_IS_NOT_ATOMIC_MASK != 0) {
ptr_to_refcount.* = refcount + amount;
} else {
_ = @atomicRmw(isize, ptr_to_refcount, .Add, amount, .monotonic);
}
},
.none => unreachable,
}
@ -387,9 +397,18 @@ inline fn decref_ptr_to_refcount(
}
},
.atomic => {
const last = @atomicRmw(isize, &refcount_ptr[0], .Sub, 1, .monotonic);
if (last == REFCOUNT_ONE_ISIZE) {
free_ptr_to_refcount(refcount_ptr, alignment, elements_refcounted);
// If the first bit of the refcount is set, this variable is threadlocal.
// Use normal refcounting instead of atomic.
if (refcount & REFCOUNT_IS_NOT_ATOMIC_MASK != 0) {
refcount_ptr[0] = refcount -% 1;
if (refcount == REFCOUNT_ONE_ISIZE) {
free_ptr_to_refcount(refcount_ptr, alignment, elements_refcounted);
}
} else {
const last = @atomicRmw(isize, &refcount_ptr[0], .Sub, 1, .monotonic);
if (last == REFCOUNT_ONE_ATOMIC_ISIZE) {
free_ptr_to_refcount(refcount_ptr, alignment, elements_refcounted);
}
}
},
.none => unreachable,
@ -414,7 +433,17 @@ pub fn isUnique(
std.debug.print("| is unique {*}\n", .{isizes - 1});
}
return refcount == REFCOUNT_ONE_ISIZE;
switch (RC_TYPE) {
.normal => {
return refcount == REFCOUNT_ONE_ISIZE;
},
.atomic => {
return refcount == REFCOUNT_ONE_ISIZE or refcount == REFCOUNT_ONE_ATOMIC_ISIZE;
},
.none => {
return false;
},
}
}
// We follow roughly the [fbvector](https://github.com/facebook/folly/blob/main/folly/docs/FBVector.md) when it comes to growing a RocList.