The hint module

Hints to the optimizer about memory a loop will soon read or write.

Bring it in with import hint. A hint never changes what a program computes, only how fast: a prefetch starts loading a cache line early. Where the CPU has no prefetch instruction it does nothing.

To say which way a branch usually goes, write [Likely] or [Unlikely] above the if or match arm; likely and unlikely here are deprecated.

Example

import hint;

main(): int {
    var text = "a long text that a loop will read";
    hint.prefetch_read(text.c_str());
    assert(text.length > 0, "a prefetch changes nothing");
    return 0;
}

Source: runtime/hint/module.pep

likely

[Deprecated("write '[Likely]' above the 'if' instead")] likely(condition: bool): bool

Deprecated: write [Likely] above the if instead.

Returns condition, telling the optimizer it is usually true (llvm.expect).

Parameters

• condition — the condition to test

Returns

• condition

unlikely

[Deprecated("write '[Unlikely]' above the 'if' instead")] unlikely(condition: bool): bool

Deprecated: write [Unlikely] above the if instead.

Returns condition, telling the optimizer it is usually false (llvm.expect).

Parameters

• condition — the condition to test

Returns

• condition

prefetch_read

prefetch_read(address: __ptr<void>)

Starts loading the cache line holding address for reading, so a read of it soon after does not wait on memory (llvm.prefetch). It never faults, even for an address that is not mapped, and does nothing where the CPU has no prefetch instruction.

Parameters

• address — memory that will be read soon

Example

hint.prefetch_read(text.c_str());

prefetch_write

prefetch_write(address: __ptr<void>)

Starts loading the cache line holding address for writing, so a store to it soon after does not wait on memory (llvm.prefetch). It never faults, even for an address that is not mapped, and does nothing where the CPU has no prefetch instruction.

Parameters

• address — memory that will be written soon