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switch to limiting the number of init/uninit chunks

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This commit is contained in:
Erik Desjardins 2022-02-19 01:17:31 -05:00
parent b7e5597491
commit d5769e9843
5 changed files with 16 additions and 26 deletions
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28
compiler/rustc_codegen_llvm/src/consts.rs
View File

@ -37,7 +37,7 @@ fn append_chunks_of_init_and_uninit_bytes<'ll, 'a, 'b>(
alloc: &'a Allocation,
range: Range<usize>,
) {
let mut chunks = alloc
let chunks = alloc
.init_mask()
.range_as_init_chunks(Size::from_bytes(range.start), Size::from_bytes(range.end));
@ -53,32 +53,20 @@ fn append_chunks_of_init_and_uninit_bytes<'ll, 'a, 'b>(
}
};
// Generating partially-uninit consts is limited to small allocations,
// Generating partially-uninit consts is limited to small numbers of chunks,
// to avoid the cost of generating large complex const expressions.
// For example, `[(u32, u8); 1024 * 1024]` contains uninit padding in each element,
// and would result in `{ [5 x i8] zeroinitializer, [3 x i8] undef, ...repeat 1M times... }`.
let allow_partially_uninit =
match cx.sess().opts.debugging_opts.partially_uninit_const_threshold {
Some(max) => range.len() <= max,
None => false,
};
let max = cx.sess().opts.debugging_opts.uninit_const_chunk_threshold;
let allow_uninit_chunks = chunks.clone().take(max.saturating_add(1)).count() <= max;
if allow_partially_uninit {
if allow_uninit_chunks {
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, codegen as if it was initialized
// (using some arbitrary value for uninit bytes)
// If this allocation contains any uninit bytes, 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);
llvals.push(cx.const_bytes(bytes));
}
}

2
compiler/rustc_interface/src/tests.rs
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@ -758,7 +758,6 @@ macro_rules! tracked {
tracked!(osx_rpath_install_name, true);
tracked!(panic_abort_tests, true);
tracked!(panic_in_drop, PanicStrategy::Abort);
tracked!(partially_uninit_const_threshold, Some(123));
tracked!(pick_stable_methods_before_any_unstable, false);
tracked!(plt, Some(true));
tracked!(polonius, true);
@ -789,6 +788,7 @@ macro_rules! tracked {
tracked!(trap_unreachable, Some(false));
tracked!(treat_err_as_bug, NonZeroUsize::new(1));
tracked!(tune_cpu, Some(String::from("abc")));
tracked!(uninit_const_chunk_threshold, 123);
tracked!(unleash_the_miri_inside_of_you, true);
tracked!(use_ctors_section, Some(true));
tracked!(verify_llvm_ir, true);

1
compiler/rustc_middle/src/mir/interpret/allocation.rs
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@ -957,6 +957,7 @@ pub fn range_as_init_chunks(&self, start: Size, end: Size) -> InitChunkIter<'_>
}
/// Yields [`InitChunk`]s. See [`InitMask::range_as_init_chunks`].
#[derive(Clone)]
pub struct InitChunkIter<'a> {
init_mask: &'a InitMask,
/// Whether the next chunk we will return is initialized.

6
compiler/rustc_session/src/options.rs
View File

@ -1341,9 +1341,6 @@ mod parse {
"panic strategy for panics in drops"),
parse_only: bool = (false, parse_bool, [UNTRACKED],
"parse only; do not compile, assemble, or link (default: no)"),
partially_uninit_const_threshold: Option<usize> = (Some(1024), 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: 1024 bytes)"),
perf_stats: bool = (false, parse_bool, [UNTRACKED],
"print some performance-related statistics (default: no)"),
pick_stable_methods_before_any_unstable: bool = (true, parse_bool, [TRACKED],
@ -1488,6 +1485,9 @@ mod parse {
"in diagnostics, use heuristics to shorten paths referring to items"),
ui_testing: bool = (false, parse_bool, [UNTRACKED],
"emit compiler diagnostics in a form suitable for UI testing (default: no)"),
uninit_const_chunk_threshold: usize = (256, parse_number, [TRACKED],
"allow generating const initializers with mixed init/uninit chunks, \
and set the maximum number of chunks for which this is allowed (default: 256)"),
unleash_the_miri_inside_of_you: bool = (false, parse_bool, [TRACKED],
"take the brakes off const evaluation. NOTE: this is unsound (default: no)"),
unpretty: Option<String> = (None, parse_unpretty, [UNTRACKED],

3
src/test/codegen/uninit-consts.rs
View File

@ -15,7 +15,8 @@ pub struct PartiallyUninit {
// 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.
// This shouldn't contain undef, since it contains more than 256 chunks
// (the default value of uninit_const_chunk_threshold).
// CHECK: [[UNINIT_PADDING_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [32768 x i8] }> <{ [32768 x i8] c"{{.+}}" }>, align 4
// CHECK: [[FULLY_UNINIT_HUGE:@[0-9]+]] = private unnamed_addr constant <{ [16384 x i8] }> undef