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Box::new(uninitialized) creates a big alloca #58201
Description
The following code
pub fn box_uninitialized2() -> Box<MaybeUninit<[usize; 1024*1024]>> { unsafe {
// CHECK-LABEL: @box_uninitialized2
// CHECK-NOT: store
// CHECK-NOT: alloca
// CHECK-NOT: memcpy
// CHECK-NOT: memset
Box::new(std::mem::uninitialized())
} }generates some really awful LLVM IR:
start:
%_1.sroa.0 = alloca [1048576 x i64], align 8
%_1.sroa.0.0.sroa_cast3 = bitcast [1048576 x i64]* %_1.sroa.0 to i8*
call void @llvm.lifetime.start.p0i8(i64 8388608, i8* nonnull %_1.sroa.0.0.sroa_cast3)
%0 = tail call i8* @__rust_alloc(i64 8388608, i64 8) #3, !noalias !1
%1 = icmp eq i8* %0, null
br i1 %1, label %bb7.i.i, label %"_ZN35_$LT$alloc..boxed..Box$LT$T$GT$$GT$3new17h9460275a68e769e0E.exit"
bb7.i.i: ; preds = %start
; call alloc::alloc::handle_alloc_error
tail call void @_ZN5alloc5alloc18handle_alloc_error17h3b023122fefb532cE(i64 8388608, i64 8) #3, !noalias !1
unreachable
"_ZN35_$LT$alloc..boxed..Box$LT$T$GT$$GT$3new17h9460275a68e769e0E.exit": ; preds = %start
%2 = bitcast i8* %0 to %"core::mem::MaybeUninit<[usize; 1048576]>"*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 8 %0, i8* nonnull align 8 %_1.sroa.0.0.sroa_cast3, i64 8388608, i1 false)
call void @llvm.lifetime.end.p0i8(i64 8388608, i8* nonnull %_1.sroa.0.0.sroa_cast3)
ret %"core::mem::MaybeUninit<[usize; 1048576]>"* %2
The same happens with std::mem::MaybeUninit::uninitialized().
Cc @arielb1, who made me write these tests, and @rust-lang/wg-codegen
Activity
arielb1 commentedon Feb 6, 2019
Feels like an LLVM limitation to me. It should be replacing the
memcpywith amemset(%0, 8388608, undef)(or amemsetof poison when that becomes available).nikic commentedon Feb 7, 2019
For reference: https://rust.godbolt.org/z/6ea0aL
LLVM is capable of optimizing a memcpy of undef alloca away, this just runs into the usual limitation that all of memcpyopt doesn't work across BBs.
arielb1 commentedon Feb 7, 2019
This is such a long-standing issue. Seems like it might be wise to have someone working on fixing this.
nikic commentedon Feb 7, 2019
@arielb1 Someone has already worked on this, see https://reviews.llvm.org/D38374. It's a bit hard to track, but unfortunately this got reverted again (multiple times). I suspect that this is only going to get really resolved once memcpyopt moves to memssa -- which someone has also worked on in the past (https://reviews.llvm.org/D26739), but which never really went anywhere.
nikic commentedon Mar 12, 2021
The alloca and memcpy are no longer present on nightly, as a result of #82806.
Noratrieb commentedon Dec 30, 2022
The FIXME in the test is still there, needs a test.
Rollup merge of rust-lang#106292 - Nilstrieb:box-uninit-test, r=RalfJung