diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index 0ce161d7e75..26f5225f6b4 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -19,6 +19,7 @@
 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 should_use_mutable_noalias(cx: &CodegenCx<'_, '_>) -> bool {
         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/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index bc49dfe7eae..24e18826048 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -134,7 +134,12 @@ extern "C" LLVMPassRef LLVMRustCreateMemorySanitizerPass(int TrackOrigins, bool
   const bool CompileKernel = false;
 
   return wrap(createMemorySanitizerLegacyPassPass(
-      MemorySanitizerOptions{TrackOrigins, Recover, CompileKernel}));
+#if LLVM_VERSION_GE(14, 0)
+      MemorySanitizerOptions{TrackOrigins, Recover, CompileKernel, /*EagerChecks=*/true}
+#else
+      MemorySanitizerOptions{TrackOrigins, Recover, CompileKernel}
+#endif
+  ));
 #else
   report_fatal_error("Legacy PM not supported with LLVM 15");
 #endif
@@ -930,10 +935,18 @@ LLVMRustOptimizeWithNewPassManager(
 
   if (SanitizerOptions) {
     if (SanitizerOptions->SanitizeMemory) {
+#if LLVM_VERSION_GE(14, 0)
+      MemorySanitizerOptions Options(
+          SanitizerOptions->SanitizeMemoryTrackOrigins,
+          SanitizerOptions->SanitizeMemoryRecover,
+          /*CompileKernel=*/false,
+          /*EagerChecks=*/true);
+#else
       MemorySanitizerOptions Options(
           SanitizerOptions->SanitizeMemoryTrackOrigins,
           SanitizerOptions->SanitizeMemoryRecover,
           /*CompileKernel=*/false);
+#endif
       OptimizerLastEPCallbacks.push_back(
         [Options](ModulePassManager &MPM, OptimizationLevel Level) {
 #if LLVM_VERSION_GE(14, 0) && LLVM_VERSION_LT(16, 0)
diff --git a/src/test/ui/sanitize/memory-eager.rs b/src/test/ui/sanitize/memory-eager.rs
new file mode 100644
index 00000000000..8a0590bf16c
--- /dev/null
+++ b/src/test/ui/sanitize/memory-eager.rs
@@ -0,0 +1,38 @@
+// needs-sanitizer-support
+// needs-sanitizer-memory
+// min-llvm-version: 14.0.0
+//
+// 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
+// error-pattern: Uninitialized value was created by an allocation
+// error-pattern: in the stack frame of function 'random'
+//
+// This test case intentionally limits the usage of the std,
+// since it will be linked with an uninstrumented version of it.
+
+#![feature(core_intrinsics)]
+#![feature(start)]
+#![feature(bench_black_box)]
+
+use std::hint::black_box;
+use std::mem::MaybeUninit;
+
+#[inline(never)]
+#[no_mangle]
+#[allow(invalid_value)]
+fn random() -> char {
+    let r = unsafe { MaybeUninit::uninit().assume_init() };
+    // Avoid optimizing everything out.
+    black_box(r)
+}
+
+#[start]
+fn main(_: isize, _: *const *const u8) -> isize {
+    random();
+    0
+}
diff --git a/src/test/ui/sanitize/memory.rs b/src/test/ui/sanitize/memory.rs
index adda51f6be0..14d4de65dd3 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
@@ -22,9 +25,9 @@
 #[inline(never)]
 #[no_mangle]
 fn random() -> [isize; 32] {
-    let r = unsafe { MaybeUninit::uninit().assume_init() };
+    let r = MaybeUninit::uninit();
     // Avoid optimizing everything out.
-    black_box(r)
+    unsafe { std::intrinsics::volatile_load(r.as_ptr()) }
 }
 
 #[inline(never)]
@@ -39,6 +42,6 @@ fn xor(a: &[isize]) -> isize {
 
 #[start]
 fn main(_: isize, _: *const *const u8) -> isize {
-    let r = random();
+    let r = black_box(random as fn() -> [isize; 32])();
     xor(&r)
 }