1dddba649a
The x64 backend currently builds the `RealRegUniverse` in a way that is generating somewhat suboptimal code. In many blocks, we see uses of callee-save (non-volatile) registers (r12, r13, r14, rbx) first, even in very short leaf functions where there are plenty of volatiles to use. This is leading to unnecessary spills/reloads. On one (local) test program, a medium-sized C benchmark compiled to Wasm and run on Wasmtime, I am seeing a ~10% performance improvement with this change; it will be less pronounced in programs with high register pressure (there we are likely to use all registers regardless, so the prologue/epilogue will save/restore all callee-saves), or in programs with fewer calls, but this is a clear win for small functions and in many cases removes prologue/epilogue clobber-saves altogether. Separately, I think the RA's coalescing is tripping up a bit in some cases; see e.g. the filetest touched by this commit that loads a value into %rsi then moves to %rax and returns immediately. This is an orthogonal issue, though, and should be addressed (if worthwhile) in regalloc.rs.
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.