add spirv target

[fee1-dead]

This is obviously a draft in an early stage, but the amount of work needed to get it working was surprisingly little.

TODO:

• Verify that core builds
• File MCP, discuss best practices and options for an initial PR
• Update src/bootstrap/download-ci-llvm-stamp
• Update build-manifest
• Update platform-support

[bjorn3]

Correct me if I'm wrong, but this would need to enable SPV_KHR_untyped_pointers, right? Rust's memory model is not compatible with typed memory and even something as simple as enums will fail (rust-gpu has a non-trivial compiler pass to try to get it to work with typed memory, but supposedly still fails on a lot of enum usage). It seems like enabling that would need -Cllvm-args=-spirv-ext=+SPV_KHR_untyped_pointers rather than -Ctarget-feature=+SPV_KHR_untyped_pointers for some reason, but I also can't find any mention of LLVM supporting this feature in the first place. Getting OpenCL SPIR-V working should be much easier. Unlike Vulkan SPIR-V, it does unconditionally support untyped pointers.

[fee1-dead]

... Yeah, I did some testing today to see if I can get core to compile. After it did (it only compiles into rlib, so mostly fine), I tried this test:

pub enum A {
    Foo(u8),
    Bar(u32),
}

#[unsafe(no_mangle)]
pub fn test(x: &mut A) -> u8 {
    match x {
        A::Foo(x) => *x,
        A::Bar(b) => *b as u8 + 2,
    }
}

Which fails with:

Unimplemented implicit down-cast from load.
UNREACHABLE executed at src/llvm-project/llvm/lib/Target/SPIRV/SPIRVLegalizePointerCast.cpp:148!

Maybe LLVM could be updated in the future such that SPIRVLegalizePointerCast can do some transforms that make it work, but I suppose any built-in rustc spirv target will have to use the OpenCL SPIR-V, so spirv64-unknown-unknown, spirv32-unknown-unknown etc. I'll push my latest draft so anyone can have a look, though.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Diff in /checkout/compiler/rustc_target/src/spec/targets/spirv_unknown_vulkan1_3.rs:1:
-use crate::spec::{
-    Arch, LinkerFlavor, Os, PanicStrategy, Target, TargetMetadata, TargetOptions
-};
+use crate::spec::{Arch, LinkerFlavor, Os, PanicStrategy, Target, TargetMetadata, TargetOptions};
 
 pub(crate) fn target() -> Target {
     Target {
Diff in /checkout/src/bootstrap/src/core/builder/cargo.rs:254:
     fn configure_linker(&mut self, builder: &Builder<'_>) -> &mut Cargo {
         let target = self.target;
         let compiler = self.compiler;
-        
+
         if target.contains("spirv") {
             return self;
         }
fmt: checked 6634 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:01:05

[ZuseZ4]

I think this should also set is_like_gpu: true?

[rust-bors]

☔ The latest upstream changes (presumably #151144) made this pull request unmergeable. Please resolve the merge conflicts.