add autodiff inline

[Shourya742]

closes: #138920

r? @ZuseZ4

[ZuseZ4]

Thank you for looking into this!

There are two things to improve here, first we want to write that in safer Rust, and second this currently adds alwaysinline everywhere. However, we only want it for the functions we generated. I'd probably start with making it safer, that should be more straightforward, this PR is a good start: #135581

As a rule of thumb I often noticed during my autodiff work, that if you want to add something to an LLVM component, then rustc likely already has wrappers for it. If you want to get something out of an LLVM component, chances are you have to write those safe wrappers, since rustc usually just lowers things.

Here is an example I found after using rg (ripgrep) to find inline related matches in rustc_codegen_llvm:

rust/compiler/rustc_codegen_llvm/src/attributes.rs

Line 33 in 9e14530

fn inline_attr<'ll>(cx: &CodegenCx<'ll, '_>, inline: InlineAttr) -> Option<&'ll Attribute> {

SimpleCx and CodegenCx are safe wrappers around llvm modules to help you handle things safely. The CodegenCx is more powerful, but also needs more inputs to be constructed. You probably won't be able to generate a full CodegenCx, since you don't have a TypeContext (tcx) available. Instead search for the SimpleCx, you should be able to create one instead. In the next steps we can then use the SimpleCx, to safely replace the 4 LLVM functions which you are currently introducing.

(The way I usually find safe wrappers systematically is by first looking up the llvm function I need, then searching for existing uses (wrappers) in rustc, and then going up the wrapper chain till I find one that I can use.)

[bors]

☔ The latest upstream changes (presumably #139396) made this pull request unmergeable. Please resolve the merge conflicts.