[Local] Consider atomic loads from constant global as dead

Per the guidance in
https://llvm.org/docs/Atomics.html#atomics-and-ir-optimization,
an atomic load from a constant global can be dropped, as there can
be no stores to synchronize with. Any write to the constant global
would be UB.

IPSCCP will already drop such loads, but the main helper in Local
doesn't recognize this currently. This is motivated by D118387.

Differential Revision: https://reviews.llvm.org/D124241

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 7a9a272..e72e3ce 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -500,6 +500,13 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
     if (isMathLibCallNoop(Call, TLI))
       return true;
 
+  // Non-volatile atomic loads from constants can be removed.
+  if (auto *LI = dyn_cast<LoadInst>(I))
+    if (auto *GV = dyn_cast<GlobalVariable>(
+            LI->getPointerOperand()->stripPointerCasts()))
+      if (!LI->isVolatile() && GV->isConstant())
+        return true;
+
   return false;
 }