diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index 9eb3574e77b..461be98f210 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -19,6 +19,7 @@ use rustc_target::abi::call::ArgAbi;
 pub use rustc_target::abi::call::*;
 use rustc_target::abi::{self, HasDataLayout, Int};
 pub use rustc_target::spec::abi::Abi;
+use rustc_target::spec::SanitizerSet;
 
 use libc::c_uint;
 use smallvec::SmallVec;
@@ -90,6 +91,13 @@ fn get_attrs<'ll>(this: &ArgAttributes, cx: &CodegenCx<'ll, '_>) -> SmallVec<[&'
         if regular.contains(ArgAttribute::NoAliasMutRef) && should_use_mutable_noalias(cx) {
             attrs.push(llvm::AttributeKind::NoAlias.create_attr(cx.llcx));
         }
+    } else if cx.tcx.sess.opts.unstable_opts.sanitizer.contains(SanitizerSet::MEMORY) {
+        // If we're not optimising, *but* memory sanitizer is on, emit noundef, since it affects
+        // memory sanitizer's behavior.
+
+        if regular.contains(ArgAttribute::NoUndef) {
+            attrs.push(llvm::AttributeKind::NoUndef.create_attr(cx.llcx));
+        }
     }
 
     attrs
diff --git a/src/test/ui/sanitize/memory-eager.rs b/src/test/ui/sanitize/memory-eager.rs
index 0c51ed6d1c7..8a0590bf16c 100644
--- a/src/test/ui/sanitize/memory-eager.rs
+++ b/src/test/ui/sanitize/memory-eager.rs
@@ -2,7 +2,10 @@
 // needs-sanitizer-memory
 // min-llvm-version: 14.0.0
 //
-// compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins -O
+// revisions: unoptimized optimized
+//
+// [optimized]compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins -O
+// [unoptimized]compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins
 //
 // run-fail
 // error-pattern: MemorySanitizer: use-of-uninitialized-value
diff --git a/src/test/ui/sanitize/memory.rs b/src/test/ui/sanitize/memory.rs
index 85d34668d1b..ecb1a05504b 100644
--- a/src/test/ui/sanitize/memory.rs
+++ b/src/test/ui/sanitize/memory.rs
@@ -1,7 +1,10 @@
 // needs-sanitizer-support
 // needs-sanitizer-memory
 //
-// compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins -O
+// revisions: unoptimized optimized
+//
+// [optimized]compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins -O
+// [unoptimized]compile-flags: -Z sanitizer=memory -Zsanitizer-memory-track-origins
 //
 // run-fail
 // error-pattern: MemorySanitizer: use-of-uninitialized-value