[all-commits] [llvm/llvm-project] a6e170: [Utils][Local] Preserve !nosanitize in combineMeta...

[Kunqiu Chen via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a6e1700fa622f75436c3c3ead23b1eefb67e88ea
      https://github.com/llvm/llvm-project/commit/a6e1700fa622f75436c3c3ead23b1eefb67e88ea
  Author: Kunqiu Chen <camsyn at foxmail.com>
  Date:   2025-07-14 (Mon, 14 Jul 2025)

  Changed paths:
    M llvm/lib/Transforms/Utils/Local.cpp
    M llvm/test/Transforms/GVN/metadata.ll

  Log Message:
  -----------
  [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.



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