[clang] 92dfb7a - [msan] Correctly handle llvm::fake_use (as a no-op) (#229591)

[via cfe-commits]

Author: Thurston Dang
Date: 2026-10-06T23:49:27Z
New Revision: 92dfb7a5b82c63895be374da73e24a9aabbd5309

URL: https://github.com/llvm/llvm-project/commit/92dfb7a5b82c63895be374da73e24a9aabbd5309
DIFF: https://github.com/llvm/llvm-project/commit/92dfb7a5b82c63895be374da73e24a9aabbd5309.diff

LOG: [msan] Correctly handle llvm::fake_use (as a no-op) (#229591)

The fake_use intrinsic (used by -fextend-variable-liveness) was being
strictly handled (i.e., check that the parameter is fully initialized),
which led to false positives
(https://github.com/llvm/llvm-project/issues/225425). This patch solves
the issue by silently not instrumenting fake_use, since they are, by
definition, not real uses and therefore cannot lead to
use-of-uninitialized-memory.

Fixes: #225425

Added: 
    

Modified: 
    clang/test/CodeGen/fake-use-sanitizer.cpp
    llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp

Removed: 
    


################################################################################
diff  --git a/clang/test/CodeGen/fake-use-sanitizer.cpp b/clang/test/CodeGen/fake-use-sanitizer.cpp
index 68a702b132663..3f6e4e10337e7 100644
--- a/clang/test/CodeGen/fake-use-sanitizer.cpp
+++ b/clang/test/CodeGen/fake-use-sanitizer.cpp
@@ -65,7 +65,7 @@ void foo()
 // MSan regression test from https://github.com/llvm/llvm-project/issues/225425
 // This test (not the tests above) was generated with update_cc_test_checks.py.
 //
-// A fake_use will be inserted for x. Currently, MSan erroneously checks it.
+// A fake_use will be inserted for x. MSan shouldn't check its parameter.
 //
 // MEMORY-LABEL: define dso_local noundef i32 @main(
 // MEMORY-SAME: ) #[[ATTR0:[0-9]+]] {
@@ -92,12 +92,6 @@ void foo()
 // MEMORY-NEXT:    [[TMP10:%.*]] = xor i64 [[TMP9]], 87960930222080
 // MEMORY-NEXT:    [[TMP11:%.*]] = inttoptr i64 [[TMP10]] to ptr
 // MEMORY-NEXT:    [[_MSLD:%.*]] = load i8, ptr [[TMP11]], align 1
-// MEMORY-NEXT:    [[_MSCMP:%.*]] = icmp ne i8 [[_MSLD]], 0
-// MEMORY-NEXT:    br i1 [[_MSCMP]], label %[[BB12:.*]], label %[[BB13:.*]], !prof [[PROF2:![0-9]+]]
-// MEMORY:       [[BB12]]:
-// MEMORY-NEXT:    call void @__msan_warning_noreturn() #[[ATTR7:[0-9]+]]
-// MEMORY-NEXT:    unreachable
-// MEMORY:       [[BB13]]:
 // MEMORY-NEXT:    notail call void (...) @llvm.fake.use(i8 [[FAKE_USE]]) #[[ATTR6]]
 // MEMORY-NEXT:    call void @llvm.lifetime.end.p0(ptr [[X]]) #[[ATTR6]]
 // MEMORY-NEXT:    ret i32 0

diff  --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index dfae300d56bef..360c071bc95f1 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -6125,6 +6125,17 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       handleGenericVectorConvertIntrinsic(I, /*FixedPoint=*/false);
       break;
 
+    // e.g.,
+    //     notail call void (...) @llvm.fake.use(i64 %x)
+    //     notail call void (...) @llvm.fake.use(i32 %y)
+    //     notail call void (...) @llvm.fake.use(ptr %z)
+    case Intrinsic::fake_use:
+      assert(I.getType()->isVoidTy());
+      // fake_uses aren't real, they can't hurt you. If the use isn't real, it
+      // can't be a real use-of-uninitialized memory. Silently skip over
+      // fake_use.
+      return true;
+
     default:
       return false;
     }