[PATCH] D153932: [AliasAnalysis] Mark fences as not Mod'ing unescaped local values

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Analysis/AliasAnalysis.cpp:513
                                     AAQueryInfo &AAQI) {
-  // All we know about a fence instruction is what we get from the ModRef
-  // mask: if Loc is a constant memory location, the fence definitely could
-  // not modify it.
-  if (Loc.Ptr)
+  // All we know about a fence instruction is what we get from the ModRef mask:
+  // if Loc is a constant memory location or cannot have a pointer outside this
----------------
The bit about the ModRef mask doesn't make sense in the rewritten comment.


================
Comment at: llvm/lib/Analysis/AliasAnalysis.cpp:518
+    if (isIdentifiedFunctionLocal(Loc.Ptr) &&
+        AAQI.CI->isNotCapturedBeforeOrAt(Loc.Ptr, S)) {
+      return ModRefInfo::Ref;
----------------
"before" in isNotCapturedBeforeOrAt is not happens-before... so if the value is captured after the fence, operations could appear to happen on it before the fence.  That might mean reordering read operations across the fence isn't actually legal?  Not sure.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153932/new/

https://reviews.llvm.org/D153932