[llvm] [GVN] MemorySSA for GVN: embed the memory state in symbolic expressions (PR #123218)

[Alina Sbirlea via llvm-commits]

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@@ -595,15 +608,48 @@ uint32_t GVNPass::ValueTable::lookupOrAddCall(CallInst *C) {
     return V;
   }
 
+  if (MSSA && AA->onlyReadsMemory(C)) {
+    Expression Exp = createExpr(C);
+    addMemoryStateToExp(C, Exp);
+    auto [V, _] = assignExpNewValueNum(Exp);
+    ValueNumbering[C] = V;
+    return V;
+  }
+
   ValueNumbering[C] = NextValueNumber;
   return NextValueNumber++;
 }
 
+/// Returns the value number for the specified load or store instruction.
+uint32_t GVNPass::ValueTable::lookupOrAddLoadStore(Instruction *I) {
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alinas wrote:

Why is this called `lookupOrAddLoadStore`? AFAICT, the "default" (no MSSA) will always reassign to the next value number, not do a lookup. Should it do a lookup in that case or, if not, why not?
I am missing the reasoning here.

https://github.com/llvm/llvm-project/pull/123218