On Wed, Jul 13, 2022 at 1:40 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > Does Rust have the equivalent of -mfunction-return=thunk-extern ? While GCC has had it for a while, Nick just landed 2 days ago the X86ReturnThunks pass in LLVM, so it will take some time to arrive in rustc. I have naively backported it to rustc and hacked it so that I set the LLVM attribute for all functions, and I am getting the rets replaced in Rust functions, e.g. (gdb) disassemble a::f Dump of assembler code for function _ZN1a1f17hdc6112b1b4a4fe99E: ... 0x0000000000008a1f <+31>: pop %rbp 0x0000000000008a20 <+32>: jmp 0x8ce0 <__x86_return_thunk> A trivial userspace program that counts the times that it goes through the return thunk also appears to work. > Related, how does Rust deal with all the various CC_HAS_ Kconfig stuff? > What if C has the relevant option but Rust does not; then we must not > have the feature enabled or there will be a mis-match. I guess that would depend on the particular option: whether it applies to Rust at all, whether it creates an incompatibility or not, etc. > Do we now have to litter everythign with RUSTC_HAS_ ? Why? Only a single `rustc` version is targeted at the moment, so it is possible to statically know what it supports. And later on, when we can declare a minimum version or when a second compiler is ready, sure, we may need to have options depending on what we want to do. Why would that be a problem? Cheers, Miguel
