From 11d0889dc0532c287cb7b2b5b8cfc6269a36e5f7 Mon Sep 17 00:00:00 2001 From: Simonas Kazlauskas Date: Tue, 13 Jan 2026 16:52:02 +0200 Subject: [PATCH] abi: add a rust-preserve-none calling convention This is the conceptual opposite of the rust-cold calling convention and is particularly useful in combination with the new `explicit_tail_calls` feature. For relatively tight loops implemented with tail calling (`become`) each of the function with the regular calling convention is still responsible for restoring the initial value of the preserved registers. So it is not unusual to end up with a situation where each step in the tail call loop is spilling and reloading registers, along the lines of: foo: push r12 ; do things pop r12 jmp next_step This adds up quickly, especially when most of the clobberable registers are already used to pass arguments or other uses. I was thinking of making the name of this ABI a little less LLVM-derived and more like a conceptual inverse of `rust-cold`, but could not come with a great name (`rust-cold` is itself not a great name: cold in what context? from which perspective? is it supposed to mean that the function is rarely called?) --- src/abi/mod.rs | 3 +++ 1 file changed, 3 insertions(+) diff --git a/src/abi/mod.rs b/src/abi/mod.rs index 09d71f5dd557..5a46f79e2ba0 100644 --- a/src/abi/mod.rs +++ b/src/abi/mod.rs @@ -56,6 +56,9 @@ pub(crate) fn conv_to_call_conv( CanonAbi::Rust | CanonAbi::C => default_call_conv, CanonAbi::RustCold => CallConv::Cold, + // Cranelift doesn't currently have anything for this. + CanonAbi::RustPreserveNone => default_call_conv, + // Functions with this calling convention can only be called from assembly, but it is // possible to declare an `extern "custom"` block, so the backend still needs a calling // convention for declaring foreign functions.