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don't generate partially-undef consts

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This commit is contained in:
Erik Desjardins 2021-04-27 00:15:41 -04:00
parent 4d635fdf63
commit 5e81d643d9
6 changed files with 56 additions and 24 deletions
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22
compiler/rustc_codegen_llvm/src/consts.rs
View File

@ -36,11 +36,6 @@ fn append_chunks_of_init_and_uninit_bytes<'ll, 'a, 'b>(
alloc: &'a Allocation,
range: Range<usize>,
) {
/// Allocations larger than this will only be codegen'd as entirely initialized or entirely undef.
/// This avoids compile time regressions when an alloc would have many chunks,
/// e.g. for `[(u64, u8); N]`, which has undef padding in each element.
const MAX_PARTIALLY_UNDEF_SIZE: usize = 1024;
let mut chunks = alloc
.init_mask()
.range_as_init_chunks(Size::from_bytes(range.start), Size::from_bytes(range.end));
@ -57,21 +52,30 @@ fn append_chunks_of_init_and_uninit_bytes<'ll, 'a, 'b>(
}
};
if range.len() > MAX_PARTIALLY_UNDEF_SIZE {
// Generating partially-uninit consts inhibits optimizations, so it is disabled by default.
// See https://github.com/rust-lang/rust/issues/84565.
let allow_partially_uninit =
match cx.sess().opts.debugging_opts.partially_uninit_const_threshold {
Some(max) => range.len() <= max,
None => false,
};
if allow_partially_uninit {
llvals.extend(chunks.map(chunk_to_llval));
} else {
let llval = match (chunks.next(), chunks.next()) {
(Some(chunk), None) => {
// exactly one chunk, either fully init or fully uninit
chunk_to_llval(chunk)
}
_ => {
// partially uninit
// partially uninit, codegen as if it was initialized
// (using some arbitrary value for uninit bytes)
let bytes = alloc.inspect_with_uninit_and_ptr_outside_interpreter(range);
cx.const_bytes(bytes)
}
};
llvals.push(llval);
} else {
llvals.extend(chunks.map(chunk_to_llval));
}
}

1
compiler/rustc_interface/src/tests.rs
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@ -743,6 +743,7 @@ macro_rules! tracked {
tracked!(no_profiler_runtime, true);
tracked!(osx_rpath_install_name, true);
tracked!(panic_abort_tests, true);
tracked!(partially_uninit_const_threshold, Some(123));
tracked!(plt, Some(true));
tracked!(polonius, true);
tracked!(precise_enum_drop_elaboration, false);

3
compiler/rustc_session/src/options.rs
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@ -1186,6 +1186,9 @@ mod parse {
"support compiling tests with panic=abort (default: no)"),
parse_only: bool = (false, parse_bool, [UNTRACKED],
"parse only; do not compile, assemble, or link (default: no)"),
partially_uninit_const_threshold: Option<usize> = (None, parse_opt_number, [TRACKED],
"allow generating const initializers with mixed init/uninit bytes, \
and set the maximum total size of a const allocation for which this is allowed (default: never)"),
perf_stats: bool = (false, parse_bool, [UNTRACKED],
"print some performance-related statistics (default: no)"),
plt: Option<bool> = (None, parse_opt_bool, [TRACKED],

4
src/test/codegen/consts.rs
View File

@ -43,7 +43,7 @@ pub fn inline_enum_const() -> E<i8, i16> {
#[no_mangle]
pub fn low_align_const() -> E<i16, [i16; 3]> {
// Check that low_align_const and high_align_const use the same constant
// CHECK: memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 2 %1, i8* align 2 getelementptr inbounds (<{ [4 x i8], [4 x i8] }>, <{ [4 x i8], [4 x i8] }>* [[LOW_HIGH]], i32 0, i32 0, i32 0), i{{(32|64)}} 8, i1 false)
// CHECK: memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 2 %1, i8* align 2 getelementptr inbounds (<{ [8 x i8] }>, <{ [8 x i8] }>* [[LOW_HIGH]], i32 0, i32 0, i32 0), i{{(32|64)}} 8, i1 false)
*&E::A(0)
}
@ -51,6 +51,6 @@ pub fn inline_enum_const() -> E<i8, i16> {
#[no_mangle]
pub fn high_align_const() -> E<i16, i32> {
// Check that low_align_const and high_align_const use the same constant
// CHECK: memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [4 x i8], [4 x i8] }>, <{ [4 x i8], [4 x i8] }>* [[LOW_HIGH]], i32 0, i32 0, i32 0), i{{(32|64)}} 8, i1 false)
// CHECK: memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [8 x i8] }>, <{ [8 x i8] }>* [[LOW_HIGH]], i32 0, i32 0, i32 0), i{{(32|64)}} 8, i1 false)
*&E::A(0)
}

35
src/test/codegen/uninit-consts-allow-partially-uninit.rs Normal file
View File

@ -0,0 +1,35 @@
// compile-flags: -C no-prepopulate-passes -Z partially_uninit_const_threshold=1024
// Like uninit-consts.rs, but tests that we correctly generate partially-uninit consts
// when the (disabled by default) partially_uninit_const_threshold flag is used.
#![crate_type = "lib"]
use std::mem::MaybeUninit;
pub struct PartiallyUninit {
x: u32,
y: MaybeUninit<[u8; 10]>
}
// This should be partially undef.
// CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"\EF\BE\AD\DE", [12 x i8] undef }>, align 4
// This shouldn't contain undef, since it's larger than the 1024 byte limit.
// CHECK: [[UNINIT_PADDING_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
// CHECK-LABEL: @partially_uninit
#[no_mangle]
pub const fn partially_uninit() -> PartiallyUninit {
const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeef, y: MaybeUninit::uninit() };
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [4 x i8], [12 x i8] }>, <{ [4 x i8], [12 x i8] }>* [[PARTIALLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 16, i1 false)
X
}
// CHECK-LABEL: @uninit_padding_huge
#[no_mangle]
pub const fn uninit_padding_huge() -> [(u32, u8); 4096] {
const X: [(u32, u8); 4096] = [(123, 45); 4096];
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [32768 x i8] }>, <{ [32768 x i8] }>* [[UNINIT_PADDING_HUGE]], i32 0, i32 0, i32 0), i{{(32|64)}} 32768, i1 false)
X
}

15
src/test/codegen/uninit-consts.rs
View File

@ -12,12 +12,9 @@ pub struct PartiallyUninit {
}
// CHECK: [[FULLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [10 x i8] }> undef
// CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"\EF\BE\AD\DE", [12 x i8] undef }>, align 4
// CHECK: [[PARTIALLY_UNINIT:@[0-9]+]] = private unnamed_addr constant <{ [16 x i8] }> <{ [16 x i8] c"\EF\BE\AD\DE\00\00\00\00\00\00\00\00\00\00\00\00" }>, align 4
// CHECK: [[FULLY_UNINIT_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [16384 x i8] }> undef
// This shouldn't contain undef, since generating huge partially undef constants is expensive.
// CHECK: [[UNINIT_PADDING_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
// CHECK-LABEL: @fully_uninit
#[no_mangle]
pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
@ -30,7 +27,7 @@ pub struct PartiallyUninit {
#[no_mangle]
pub const fn partially_uninit() -> PartiallyUninit {
const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeef, y: MaybeUninit::uninit() };
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [4 x i8], [12 x i8] }>, <{ [4 x i8], [12 x i8] }>* [[PARTIALLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 16, i1 false)
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [16 x i8] }>, <{ [16 x i8] }>* [[PARTIALLY_UNINIT]], i32 0, i32 0, i32 0), i{{(32|64)}} 16, i1 false)
X
}
@ -41,11 +38,3 @@ pub const fn partially_uninit() -> PartiallyUninit {
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [16384 x i8] }>, <{ [16384 x i8] }>* [[FULLY_UNINIT_HUGE]], i32 0, i32 0, i32 0), i{{(32|64)}} 16384, i1 false)
F
}
// CHECK-LABEL: @uninit_padding_huge
#[no_mangle]
pub const fn uninit_padding_huge() -> [(u32, u8); 4096] {
const X: [(u32, u8); 4096] = [(123, 45); 4096];
// CHECK: call void @llvm.memcpy.p0i8.p0i8.i{{(32|64)}}(i8* align 4 %1, i8* align 4 getelementptr inbounds (<{ [32768 x i8] }>, <{ [32768 x i8] }>* [[UNINIT_PADDING_HUGE]], i32 0, i32 0, i32 0), i{{(32|64)}} 32768, i1 false)
X
}