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Rollup merge of rust-lang#113716 - DianQK:add-no_builtins-to-function…
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…, r=pnkfelix

Add the `no-builtins` attribute to functions when `no_builtins` is applied at the crate level.

**When `no_builtins` is applied at the crate level, we should add the `no-builtins` attribute to each function to ensure it takes effect in LTO.**

This is also the reason why no_builtins does not take effect in LTO as mentioned in rust-lang#35540.

Now, `#![no_builtins]` should be similar to `-fno-builtin` in clang/gcc, see https://clang.godbolt.org/z/z4j6Wsod5.

Next, we should make `#![no_builtins]` participate in LTO again. That makes sense, as LTO also takes into consideration function-level instruction optimizations, such as the MachineOutliner. More importantly, when a user writes a large `#![no_builtins]` crate, they would like this crate to participate in LTO as well.

We should also add a function-level no_builtins attribute to allow users to have more control over it. This is similar to Clang's `__attribute__((no_builtin))` feature, see https://clang.godbolt.org/z/Wod6KK6eq. Before implementing this feature, maybe we should discuss whether to support more fine-grained control, such as `__attribute__((no_builtin("memcpy")))`.

Related discussions:
- rust-lang#109821
- rust-lang#35540

Next (a separate pull request?):
- [ ] Revert rust-lang#35637
- [ ] Add a function-level `no_builtin` attribute?
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matthiaskrgr authored Jul 18, 2023
2 parents 4b5f7e7 + 063ddbe commit 3b07653
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Showing 8 changed files with 67 additions and 1 deletion.
4 changes: 4 additions & 0 deletions compiler/rustc_codegen_llvm/src/attributes.rs
Original file line number Diff line number Diff line change
Expand Up @@ -335,6 +335,10 @@ pub fn from_fn_attrs<'ll, 'tcx>(
to_add.extend(probestack_attr(cx));
to_add.extend(stackprotector_attr(cx));

if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_BUILTINS) {
to_add.push(llvm::CreateAttrString(cx.llcx, "no-builtins"));
}

if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {
to_add.push(AttributeKind::Cold.create_attr(cx.llcx));
}
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10 changes: 9 additions & 1 deletion compiler/rustc_codegen_ssa/src/codegen_attrs.rs
Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use rustc_ast::{ast, MetaItemKind, NestedMetaItem};
use rustc_ast::{ast, attr, MetaItemKind, NestedMetaItem};
use rustc_attr::{list_contains_name, InlineAttr, InstructionSetAttr, OptimizeAttr};
use rustc_errors::struct_span_err;
use rustc_hir as hir;
Expand Down Expand Up @@ -60,6 +60,14 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
}

// When `no_builtins` is applied at the crate level, we should add the
// `no-builtins` attribute to each function to ensure it takes effect in LTO.
let crate_attrs = tcx.hir().attrs(rustc_hir::CRATE_HIR_ID);
let no_builtins = attr::contains_name(crate_attrs, sym::no_builtins);
if no_builtins {
codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_BUILTINS;
}

let supported_target_features = tcx.supported_target_features(LOCAL_CRATE);

let mut inline_span = None;
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2 changes: 2 additions & 0 deletions compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
Original file line number Diff line number Diff line change
Expand Up @@ -100,6 +100,8 @@ bitflags! {
const REALLOCATOR = 1 << 18;
/// `#[rustc_allocator_zeroed]`: a hint to LLVM that the function only allocates zeroed memory.
const ALLOCATOR_ZEROED = 1 << 19;
/// `#[no_builtins]`: indicates that disable implicit builtin knowledge of functions for the function.
const NO_BUILTINS = 1 << 20;
}
}

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23 changes: 23 additions & 0 deletions tests/codegen/no_builtins-at-crate.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,23 @@
// compile-flags: -C opt-level=1

#![no_builtins]
#![crate_type = "lib"]

// CHECK: define void @__aeabi_memcpy
// CHECK-SAME: #0
#[no_mangle]
pub unsafe extern "C" fn __aeabi_memcpy(dest: *mut u8, src: *const u8, size: usize) {
// CHECK: call
// CHECK-SAME: @memcpy(
memcpy(dest, src, size);
}

// CHECK: declare
// CHECK-SAME: @memcpy
// CHECK-SAME: #0
extern "C" {
pub fn memcpy(dest: *mut u8, src: *const u8, n: usize) -> *mut u8;
}

// CHECK: attributes #0
// CHECK-SAME: "no-builtins"
9 changes: 9 additions & 0 deletions tests/run-make/no-builtins-attribute/Makefile
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@@ -0,0 +1,9 @@
include ../tools.mk

# We want to check if `no-builtins` is also added to the function declarations in the used crate.

all:
$(RUSTC) no_builtins.rs --emit=link
$(RUSTC) main.rs --emit=llvm-ir

cat "$(TMPDIR)"/main.ll | "$(LLVM_FILECHECK)" filecheck.main.txt
5 changes: 5 additions & 0 deletions tests/run-make/no-builtins-attribute/filecheck.main.txt
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
CHECK: declare void @foo()
CHECK-SAME: #[[ATTR_3:[0-9]+]]

CHECK: attributes #[[ATTR_3]]
CHECK-SAME: no-builtins
10 changes: 10 additions & 0 deletions tests/run-make/no-builtins-attribute/main.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
extern crate no_builtins;

#[no_mangle]
fn call_foo() {
no_builtins::foo();
}

fn main() {
call_foo();
}
5 changes: 5 additions & 0 deletions tests/run-make/no-builtins-attribute/no_builtins.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
#![crate_type = "lib"]
#![no_builtins]

#[no_mangle]
pub fn foo() {}

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