[PATCH] D123531: [GlobalsModRef][FIX] Ensure we honor synchronizing effects of intrinsics

[Johannes Doerfert via Phabricator via llvm-commits]

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This is a long standing problem that resurfaces once in a while [0].
There might actually be two problems because I'm not 100% sure if the
issue underlying https://reviews.llvm.org/D115302 would be solved by
this or not. Anyway.

In 2008 we thought intrinsics do not read/write globals passed to them:
https://github.com/llvm/llvm-project/commit/d4133ac31535ce5176f97e9fc81825af8a808760
This is not correct given that intrinsics can synchronize threads and
cause effects to effectively become visible.

NOTE: I did not yet modify any tests but only tried out the reproducer
      of https://github.com/llvm/llvm-project/issues/54851.

Fixes: https://github.com/llvm/llvm-project/issues/54851

[0] https://discourse.llvm.org/t/bug-gvn-memdep-bug-in-the-presence-of-intrinsics/59402


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123531

Files:
  llvm/lib/Analysis/GlobalsModRef.cpp


Index: llvm/lib/Analysis/GlobalsModRef.cpp
===================================================================
--- llvm/lib/Analysis/GlobalsModRef.cpp
+++ llvm/lib/Analysis/GlobalsModRef.cpp
@@ -533,7 +533,8 @@
           FI.addModRefInfo(ModRefInfo::ModRef);
           if (!F->onlyAccessesArgMemory())
             FI.setMayReadAnyGlobal();
-          if (!F->isIntrinsic()) {
+          // Intrinsics, like any other synchronizing function, can make effects of other threads visible. Without nosync we know nothing really.
+          if (!F->hasNoSync()) {
             KnowNothing = true;
             break;
           }


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