Auto merge of #94130 - erikdesjardins:partially, r=nikic

Use undef for (some) partially-uninit constants

There needs to be some limit to avoid perf regressions on large arrays
with undef in each element (see comment in the code).

Fixes: #84565
Original PR: #83698

Depends on LLVM 14: #93577

4 files changed, 24 insertions, 26 deletions

diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index 51223697dbd..c98720944c9 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -2,6 +2,7 @@ use crate::base;
 use crate::common::CodegenCx;
 use crate::debuginfo;
 use crate::llvm::{self, True};
+use crate::llvm_util;
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
@@ -37,7 +38,7 @@ pub fn const_alloc_to_llvm<'ll>(cx: &CodegenCx<'ll, '_>, alloc: &Allocation) ->
         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));
 
@@ -53,30 +54,26 @@ pub fn const_alloc_to_llvm<'ll>(cx: &CodegenCx<'ll, '_>, alloc: &Allocation) ->
             }
         };
 
-        // 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,
-            };
+        // 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 max = if llvm_util::get_version() < (14, 0, 0) {
+            // Generating partially-uninit consts inhibits optimizations in LLVM < 14.
+            // See https://github.com/rust-lang/rust/issues/84565.
+            1
+        } else {
+            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)
-                    let bytes = alloc.inspect_with_uninit_and_ptr_outside_interpreter(range);
-                    cx.const_bytes(bytes)
-                }
-            };
-            llvals.push(llval);
+            // 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);
+            llvals.push(cx.const_bytes(bytes));
         }
     }
 
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 9ab138c1b12..def5c30411b 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -758,7 +758,6 @@ fn test_debugging_options_tracking_hash() {
     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 @@ fn test_debugging_options_tracking_hash() {
     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);
diff --git a/compiler/rustc_middle/src/mir/interpret/allocation.rs b/compiler/rustc_middle/src/mir/interpret/allocation.rs
index a36c9b6ed73..5de119f9562 100644
--- a/compiler/rustc_middle/src/mir/interpret/allocation.rs
+++ b/compiler/rustc_middle/src/mir/interpret/allocation.rs
@@ -957,6 +957,7 @@ impl InitMask {
 }
 
 /// 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.
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 9a92258f809..359e00718bf 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1336,9 +1336,6 @@ options! {
         "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> = (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)"),
     pick_stable_methods_before_any_unstable: bool = (true, parse_bool, [TRACKED],
@@ -1483,6 +1480,9 @@ options! {
         "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 = (16, 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: 16)"),
     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],