On Thu, May 19, 2016 at 12:07:42PM +0300, Henri Sivonen wrote: > On Wed, May 18, 2016 at 8:32 PM, Benjamin Smedberg > <benja...@smedbergs.us> wrote: > > Can we require SSE2 for Mozilla builds of Firefox for Linux? Yes, I am > > comfortable making that decision today. > > Thank you! I filed > https://bugzilla.mozilla.org/show_bug.cgi?id=1274196 for this. > > > Should we also take actions that prevent anyone from supporting non-SSE2 > > codepaths? I don't know. There seem to be at least a few cases that this > > affects: > > > > Rust and its support for non-SSE2 arches > > > > This seems to be both about build-time targeting and runtime dynamic > > selection. I don't personally understand the cost/benefit tradeoff here. > > There are two levels of complication with run-time selection: > > 1) Since inline asm isn't allowed in stable-channel Rust, executing > the cpuid instruction must be done in some other way, which leads > either to having to make the crate (that could otherwise be a > standalone Rust library) depend on Gecko's existing CPU sniffing > infrastructure or to having to link some ad hoc cpuid-execution > non-Rust object to the crate that could otherwise be a simple > pure-Rust crate. > > 2) Since LLVM and, by extension, rustc currently require the compiler > instruction set targeting options to be the same throughout the > compilation unit, if the main crate is compiled without SSE2 to cater > for the non-SSE2 case, the SSE2-enabled versions of functions need to > be put in a separate crate, which is considerably less nice than what > rustc lets you do if you want to supply SSE2 and ALU-only alternatives > for a function such that the selection is done at compile time. In > particular, if you want to avoid having two copies of SSE2 code, one > in the main crate for build-time selection and the other in the > SSE2-only helper crate, you have to put the single copy in the helper > crate, which means that the preferred case (SSE2 enabled at build > time) is made more complex, too.
I don't think anyone suggested to add support for runtime selection of SSE2 for code that is not inline asm. > The problem with build-time selection is that the non-SSE2 target > isn't the target that the Rust community tests the most actively. > (Just like, evidently, Microsoft doesn't actively test the non-SSE > target of MSVC.) If distro's Rust compiler doesn't target SSE2, they'll obviously be testing that. But it's not really a concern for you to have. It's theirs. Mike _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
