[llvm] [msan] Add experimental '-msan-or-shadow-for-strict-instructions' flag to pessimize output (PR #128036)

[Vitaly Buka via llvm-commits]

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@@ -5596,13 +5606,29 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     if (ClDumpStrictInstructions)
       dumpInst(I);
     LLVM_DEBUG(dbgs() << "DEFAULT: " << I << "\n");
+
+    bool allSized = true;
     for (size_t i = 0, n = I.getNumOperands(); i < n; i++) {
       Value *Operand = I.getOperand(i);
       if (Operand->getType()->isSized())
         insertShadowCheck(Operand, &I);
+      else
+        allSized = false;
+    }
+
+    Type *RetTy = cast<Value>(I).getType();
+    if (ClOrShadowForStrictInstructions && allSized && !RetTy->isVoidTy()) {
+      // - In recover mode: the shadow will be computed instead of reset to
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vitalybuka wrote:

> and any subsequent UUM is unreported

Amount of reports will be huge, and they are irrelevant till step 2 is fixed.

https://github.com/llvm/llvm-project/pull/128036