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langref: document @prefetch() builtin

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Isaac Freund 2021-12-10 23:06:08 +01:00
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@ -8674,6 +8674,50 @@ test "@wasmMemoryGrow" {
{#see_also|@ctz|@clz#}
{#header_close#}
{#header_open|@prefetch#}
<pre>{#syntax#}@prefetch(ptr: anytype, comptime options: std.builtin.PrefetchOptions){#endsyntax#}</pre>
<p>
This builtin tells the compiler to emit a prefetch instruction if supported by the
target CPU. If the target CPU does not support the requested prefetch instruction,
this builtin is a noop. This function has no effect on the behavior of the program,
only on the performance characteristics.
</p>
<p>
The {#syntax#}ptr{#endsyntax#} argument may be any pointer type and determines the memory
address to prefetch. This function does not dereference the pointer, it is perfectly legal
to pass a pointer to invalid memory to this function and no illegal behavior will result.
</p>
<p>
The {#syntax#}options{#endsyntax#} argument is the following struct:
</p>
{#code_begin|syntax|builtin#}
/// This data structure is used by the Zig language code generation and
/// therefore must be kept in sync with the compiler implementation.
pub const PrefetchOptions = struct {
/// Whether the prefetch should prepare for a read or a write.
rw: Rw = .read,
/// 0 means no temporal locality. That is, the data can be immediately
/// dropped from the cache after it is accessed.
///
/// 3 means high temporal locality. That is, the data should be kept in
/// the cache as it is likely to be accessed again soon.
locality: u2 = 3,
/// The cache that the prefetch should be preformed on.
cache: Cache = .data,
pub const Rw = enum {
read,
write,
};
pub const Cache = enum {
instruction,
data,
};
};
{#code_end#}
{#header_close#}
{#header_open|@ptrCast#}
<pre>{#syntax#}@ptrCast(comptime DestType: type, value: anytype) DestType{#endsyntax#}</pre>
<p>