[Utils][Local] Preserve !nosanitize in combineMetadata when merging instructions (#148376)

`combineMetadata` helper currently drops `!nosanitize` metadata when
merging two instructions, even if both originally carried `!nosanitize`.

This is problematic because `!nosanitize` is a key mechanism used by
sanitizer (e.g., ASan) to suppress instrumentation. Removing it can lead
to unintended sanitizer behavior.

This patch adds `nosanitize` to the whitelist in combineMetadata,
preserving it only if both instructions carry `!nosanitize`; otherwise,
it is dropped. This patch also adds corresponding tests in a test file
and regenerates it.

---
### Details

**Example (see [Godbolt](https://godbolt.org/z/83P5eWczx) for
details**):

```llvm
%v1 = load i32, ptr %p, !nosanitize
%v2 = load i32, ptr %p, !nosanitize
```
When merged via `combineMetadata(%v1, %v2, ...)`, the resulting
instruction loses its `!nosanitize` metadata.

Tools such as UBSan and AFL rely on `nosanitize` to prevent unwanted
transformations or checks. However, the current implementation of
combineMetadata mistakenly drops !nosanitize. This may lead to
unintended behavior during optimization.

For example, under `-fsanitize=address,undefined -O2`, IR emitted by
UBSan may lose its `!nosanitize` metadata due to the incorrect metadata
merging in optimization. As a result, ASan could unexpectedly instrument
those instructions.
> Note: due to the current UBSan handlers having relatively
coarse-grained attributes, this specific case is difficult to reproduce
end-to-end from source code—UBSan currently inhibits such optimizations
(refer to #135135 for details).

Still, I believe it's necessary to fix this now, to support future
versions of UBSan that might allow such optimizations, and to support
third-party tools (such as AFL-based fuzzers) that rely on the presence
of !nosanitize.

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index f5208d5..81d8537 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -3381,7 +3381,11 @@ static void combineMetadata(Instruction *K, const Instruction *J,
           K->setMetadata(Kind,
                          MDNode::getMostGenericNoaliasAddrspace(JMD, KMD));
         break;
-    }
+      case LLVMContext::MD_nosanitize:
+        // Preserve !nosanitize if both K and J have it.
+        K->setMetadata(Kind, JMD);
+        break;
+      }
   }
   // Set !invariant.group from J if J has it. If both instructions have it
   // then we will just pick it from J - even when they are different.