compiler/rustc_target/src/spec/targets/x86_64_unknown_uefi.rs

// This defines the amd64 target for UEFI systems as described in the UEFI specification. See the
// uefi-base module for generic UEFI options. On x86_64 systems (mostly called "x64" in the spec)
// UEFI systems always run in long-mode, have the interrupt-controller pre-configured and force a
// single-CPU execution.
// The win64 ABI is used. It differs from the sysv64 ABI, so we must use a windows target with
// LLVM. "x86_64-unknown-windows" is used to get the minimal subset of windows-specific features.

use rustc_abi::{CanonAbi, X86Call};

use crate::spec::{Arch, RustcAbi, Target, TargetMetadata, base};

pub(crate) fn target() -> Target {
    let mut base = base::uefi_msvc::opts();
    base.cpu = "x86-64".into();
    base.plt_by_default = false;
    base.max_atomic_width = Some(64);
    base.entry_abi = CanonAbi::X86(X86Call::Win64);

    // We disable MMX and SSE for now, even though UEFI allows using them. Problem is, you have to
    // enable these CPU features explicitly before their first use, otherwise their instructions
    // will trigger an exception. Rust does not inject any code that enables AVX/MMX/SSE
    // instruction sets, so this must be done by the firmware. However, existing firmware is known
    // to leave these uninitialized, thus triggering exceptions if we make use of them. Which is
    // why we avoid them and instead use soft-floats. This is also what GRUB and friends did so
    // far.
    //
    // If you initialize FP units yourself, you can override these flags with custom linker
    // arguments, thus giving you access to full MMX/SSE acceleration.
    base.features = "-mmx,-sse,+soft-float".into();
    base.rustc_abi = Some(RustcAbi::Softfloat);

    Target {
        llvm_target: "x86_64-unknown-windows".into(),
        metadata: TargetMetadata {
            description: Some("64-bit UEFI".into()),
            tier: Some(2),
            host_tools: Some(false),
            std: None, // ?
        },
        pointer_width: 64,
        data_layout:
            "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128".into(),
        arch: Arch::X86_64,

        options: base,
    }
}