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change refcounts to assume overflows are impossible

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Brendan Hansknecht 2022-08-01 22:01:53 -07:00
parent ed9b7da4a3
commit afaf59a050
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1 changed files with 19 additions and 21 deletions
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40
crates/compiler/builtins/bitcode/src/utils.zig
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@ -131,18 +131,16 @@ const RC_TYPE = Refcount.normal;
pub fn increfC(ptr_to_refcount: *isize, amount: isize) callconv(.C) void {
if (RC_TYPE == Refcount.none) return;
var refcount = ptr_to_refcount.*;
if (refcount < REFCOUNT_MAX_ISIZE) {
// Ensure that the refcount is not whole program lifetime.
if (ptr_to_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) {
Refcount.normal => {
ptr_to_refcount.* = std.math.min(refcount + amount, REFCOUNT_MAX_ISIZE);
ptr_to_refcount.* += amount;
},
Refcount.atomic => {
var next = std.math.min(refcount + amount, REFCOUNT_MAX_ISIZE);
while (@cmpxchgWeak(isize, ptr_to_refcount, refcount, next, Monotonic, Monotonic)) |found| {
refcount = found;
next = std.math.min(refcount + amount, REFCOUNT_MAX_ISIZE);
}
_ = @atomicRmw(isize, ptr_to_refcount, std.builtin.AtomicRmwOp.Add, amount, Monotonic);
},
Refcount.none => unreachable,
}
@ -194,24 +192,24 @@ inline fn decref_ptr_to_refcount(
) void {
if (RC_TYPE == Refcount.none) return;
const extra_bytes = std.math.max(alignment, @sizeOf(usize));
switch (RC_TYPE) {
Refcount.normal => {
const refcount: isize = refcount_ptr[0];
if (refcount == REFCOUNT_ONE_ISIZE) {
dealloc(@ptrCast([*]u8, refcount_ptr) - (extra_bytes - @sizeOf(usize)), alignment);
} else if (refcount < REFCOUNT_MAX_ISIZE) {
// Ensure that the refcount is not whole program lifetime.
const refcount: isize = refcount_ptr[0];
if (refcount != REFCOUNT_MAX_ISIZE) {
switch (RC_TYPE) {
Refcount.normal => {
refcount_ptr[0] = refcount - 1;
}
},
Refcount.atomic => {
if (refcount_ptr[0] < REFCOUNT_MAX_ISIZE) {
if (refcount == REFCOUNT_ONE_ISIZE) {
dealloc(@ptrCast([*]u8, refcount_ptr) - (extra_bytes - @sizeOf(usize)), alignment);
}
},
Refcount.atomic => {
var last = @atomicRmw(isize, &refcount_ptr[0], std.builtin.AtomicRmwOp.Sub, 1, Monotonic);
if (last == REFCOUNT_ONE_ISIZE) {
dealloc(@ptrCast([*]u8, refcount_ptr) - (extra_bytes - @sizeOf(usize)), alignment);
}
}
},
Refcount.none => unreachable,
},
Refcount.none => unreachable,
}
}
}