https://github.com/rust-lang/rust http://git.dreamy.place/mirrors/rust/atom?h=perf-tmp 2018-10-23T07:59:14+00:00 2018-10-23T07:59:14+00:00 Alex Crichton alex@alexcrichton.com 2018-10-23T07:59:14+00:00 urn:sha1:086f5a55be73dff71ce5d93f16cdf5652d833090 This reverts commit 3cc8f738d4247a9b475d8e074b621e602ac2b7be. 2018-10-19T09:35:00+00:00 Alex Crichton alex@alexcrichton.com 2018-10-14T19:27:22+00:00 urn:sha1:3cc8f738d4247a9b475d8e074b621e602ac2b7be The issue of passing around SIMD types as values between functions has seen [quite a lot] of [discussion], and although we thought [we fixed it][quite a lot] it [wasn't]! This PR is a change to rustc to, again, try to fix this issue. The fundamental problem here remains the same, if a SIMD vector argument is passed by-value in LLVM's function type, then if the caller and callee disagree on target features a miscompile happens. We solve this by never passing SIMD vectors by-value, but LLVM will still thwart us with its argument promotion pass to promote by-ref SIMD arguments to by-val SIMD arguments. This commit is an attempt to thwart LLVM thwarting us. We, just before codegen, will take yet another look at the LLVM module and demote any by-value SIMD arguments we see. This is a very manual attempt by us to ensure the codegen for a module keeps working, and it unfortunately is likely producing suboptimal code, even in release mode. The saving grace for this, in theory, is that if SIMD types are passed by-value across a boundary in release mode it's pretty unlikely to be performance sensitive (as it's already doing a load/store, and otherwise perf-sensitive bits should be inlined). The implementation here is basically a big wad of C++. It was largely copied from LLVM's own argument promotion pass, only doing the reverse. In local testing this... Closes #50154 Closes #52636 Closes #54583 Closes #55059 [quite a lot]: https://github.com/rust-lang/rust/pull/47743 [discussion]: https://github.com/rust-lang/rust/issues/44367 [wasn't]: https://github.com/rust-lang/rust/issues/50154