bjoernager/rust
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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
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@ -36,11 +36,6 @@ pub fn const_alloc_to_llvm(cx: &CodegenCx<'ll, '_>, alloc: &Allocation) -> &'ll
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 @@ pub fn const_alloc_to_llvm(cx: &CodegenCx<'ll, '_>, alloc: &Allocation) -> &'ll
}
};
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));
}
}