This reverts a part of 365e40b. While it is not legal to clobber
the output register, it is legal and necessary to do this for inputs,
since we do not preserve them, and LLVM could have decided to
reuse them.
Thanks to @Amanieu for discovering this.
This is not actually legal (although the restriction is not
documented anywhere), and is not caught by LLVM unless the codegen
option -verify-machineinstrs is specified. This option is now used
on Travis.
While not legal, this does not seem to result in invalid output
(although it creates an unnecessary spill); however, under extremely
specific circumstances (e.g. when a register scavenger is run under
severe pressure), this will result in a codegen crash, which is
how I found it.
The core::intrinsics::unreachable() we used at the end of every naked
function is essentially pointless, as #[naked] implies that intrinsic
at the end of the function. rustc currently does not implement that
behavior (rust-lang/rust#32487), but it is a bug. On top of that,
anything except a single asm!() in naked functions is likely to be
disallowed in the future (rust-lang/rust#32490).
A nice side effect is that we avoid the core_intrinsics feature,
which will be never stabilized, though neither asm nor
naked_functions are likely to be stabilized soon.
The new design concerns itself with one thing and exactly one thing:
passing values back and forth with an extern "C" function.
This allows to simplify fringe::arch into a single primitive, swap.
Close#21
whitequark
edef
John Ericson