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Use small code model for UEFI targets

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* Since the code model only applies to the code and not the data and the code model
only applies to functions you call through using `call`, `jmp` and data with `lea`, etc…

If you are calling functions using the function pointers from the UEFI structures the code
model does not apply in that case. It’s just related to the address space size of your own binary.
Since UEFI (uefi is all relocatable) uses relocatable PEs (relocatable code does not care about the
code model) so, we use the small code model here.

* Since applications don't usually take gigabytes of memory, setting the
target to use the small code model should result in better codegen (comparable
with majority of other targets).

Large code models are also known for generating horrible code, for
example 16 bytes of code to load a single 8-byte value.

* Use the LLVM default code model for the architecture for the
x86_64-unknown-uefi targets. For reference small is the default
code model on x86 in LLVM: <7de2173c2a/llvm/lib/Target/X86/X86TargetMachine.cpp (L204)>

* Remove the comments too as they are not UEFI-specific and applies
to pretty much any target. I added them before as I was explicitily
setting the code model to small.

Signed-off-by: Andy-Python-Programmer <andypythonappdeveloper@gmail.com>
This commit is contained in:
Andy-Python-Programmer 2021-07-14 19:19:27 +10:00
parent a08f25a7ef
commit db1e49257e
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1 changed files with 2 additions and 6 deletions
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8
compiler/rustc_target/src/spec/x86_64_unknown_uefi.rs
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@ -5,7 +5,7 @@
// 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 crate::spec::{CodeModel, Target};
use crate::spec::Target;
pub fn target() -> Target {
let mut base = super::uefi_msvc_base::opts();
@ -19,15 +19,11 @@ pub fn target() -> Target {
// 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".to_string();
// UEFI systems run without a host OS, hence we cannot assume any code locality. We must tell
// LLVM to expect code to reference any address in the address-space. The "large" code-model
// places no locality-restrictions, so it fits well here.
base.code_model = Some(CodeModel::Large);
Target {
llvm_target: "x86_64-unknown-windows".to_string(),
pointer_width: 64,