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Emit noalias on &mut parameters by default

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This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

Noalias annotations will not be emitted by default if either
-C panic=abort (as previously) or LLVM >= 6.0 (new).

-Z mutable-noalias=no is left as an escape-hatch to allow
debugging problems suspected to stem from this change.
This commit is contained in:
Nikita Popov 2018-05-14 15:20:51 +02:00
parent 90463a6bdc
commit 12308139ec
3 changed files with 14 additions and 10 deletions
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4
src/librustc/session/config.rs
View File

@ -1239,8 +1239,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
"print the result of the monomorphization collection pass"),
mir_opt_level: usize = (1, parse_uint, [TRACKED],
"set the MIR optimization level (0-3, default: 1)"),
mutable_noalias: bool = (false, parse_bool, [TRACKED],
"emit noalias metadata for mutable references"),
mutable_noalias: Option<bool> = (None, parse_opt_bool, [TRACKED],
"emit noalias metadata for mutable references (default: yes on LLVM >= 6)"),
arg_align_attributes: bool = (false, parse_bool, [TRACKED],
"emit align metadata for reference arguments"),
dump_mir: Option<String> = (None, parse_opt_string, [UNTRACKED],

10
src/librustc_codegen_llvm/type_of.rs
View File

@ -10,6 +10,7 @@
use abi::{FnType, FnTypeExt};
use common::*;
use llvm;
use rustc::hir;
use rustc::ty::{self, Ty, TypeFoldable};
use rustc::ty::layout::{self, Align, LayoutOf, Size, TyLayout};
@ -428,8 +429,13 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyLayout<'tcx> {
PointerKind::Shared
},
hir::MutMutable => {
if cx.tcx.sess.opts.debugging_opts.mutable_noalias ||
cx.tcx.sess.panic_strategy() == PanicStrategy::Abort {
// Only emit noalias annotations for LLVM >= 6 or in panic=abort
// mode, as prior versions had many bugs in conjunction with
// unwinding. See also issue #31681.
let mutable_noalias = cx.tcx.sess.opts.debugging_opts.mutable_noalias
.unwrap_or(unsafe { llvm::LLVMRustVersionMajor() >= 6 }
|| cx.tcx.sess.panic_strategy() == PanicStrategy::Abort);
if mutable_noalias {
PointerKind::UniqueBorrowed
} else {
PointerKind::Shared

10
src/test/codegen/function-arguments.rs
View File

@ -10,6 +10,7 @@
// compile-flags: -C no-prepopulate-passes
// ignore-tidy-linelength
// min-llvm-version 6.0
#![crate_type = "lib"]
#![feature(custom_attribute)]
@ -52,16 +53,14 @@ pub fn named_borrow<'r>(_: &'r i32) {
pub fn unsafe_borrow(_: &UnsafeInner) {
}
// CHECK: @mutable_unsafe_borrow(i16* dereferenceable(2) %arg0)
// CHECK: @mutable_unsafe_borrow(i16* noalias dereferenceable(2) %arg0)
// ... unless this is a mutable borrow, those never alias
// ... except that there's this LLVM bug that forces us to not use noalias, see #29485
#[no_mangle]
pub fn mutable_unsafe_borrow(_: &mut UnsafeInner) {
}
// CHECK: @mutable_borrow(i32* dereferenceable(4) %arg0)
// CHECK: @mutable_borrow(i32* noalias dereferenceable(4) %arg0)
// FIXME #25759 This should also have `nocapture`
// ... there's this LLVM bug that forces us to not use noalias, see #29485
#[no_mangle]
pub fn mutable_borrow(_: &mut i32) {
}
@ -103,9 +102,8 @@ pub fn helper(_: usize) {
pub fn slice(_: &[u8]) {
}
// CHECK: @mutable_slice([0 x i8]* nonnull %arg0.0, [[USIZE]] %arg0.1)
// CHECK: @mutable_slice([0 x i8]* noalias nonnull %arg0.0, [[USIZE]] %arg0.1)
// FIXME #25759 This should also have `nocapture`
// ... there's this LLVM bug that forces us to not use noalias, see #29485
#[no_mangle]
pub fn mutable_slice(_: &mut [u8]) {
}