don't generate partially-undef consts

compiler/rustc_codegen_llvm/src/consts.rs

@@ -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));
         }
     }
 

compiler/rustc_interface/src/tests.rs

@@ -743,6 +743,7 @@ fn test_debugging_options_tracking_hash() {
     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);

compiler/rustc_session/src/options.rs

@@ -1186,6 +1186,9 @@ options! {
         "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],