[llvm] Reapply "[msan] Switch switch() from strict handling to (icmp eq)-style handling" (#180636) (PR #181112)

[Qinkun Bao via llvm-commits]

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@@ -2460,6 +2460,64 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     return Si;
   }
 
+  // Instrument:
+  //     switch i32 %Val, label %else [ i32 0, label %A
+  //                                    i32 1, label %B
+  //                                    i32 2, label %C ]
+  //
+  // Typically, the switch input value (%Val) is fully initialized.
+  //
+  // Sometimes the compiler may convert (icmp + br) into a switch statement.
+  // MSan allows icmp eq/ne with partly initialized inputs to still result in a
+  // fully initialized output, if there exists a bit that is initialized in
+  // both inputs with a differing value. For compatibility, we support this in
+  // the switch instrumentation as well. Note that this edge case only applies
+  // if the switch input value does not match *any* of the cases (matching any
+  // of the cases requires an exact, fully initialized match).
+  //
+  //     ShadowCases =   0
+  //                   | propagateEqualityComparison(Val, 0)
+  //                   | propagateEqualityComparison(Val, 1)
+  //                   | propagateEqualityComparison(Val, 2))
+  void visitSwitchInst(SwitchInst &SI) {
+    IRBuilder<> IRB(&SI);
+
+    Value *Val = SI.getCondition();
+    Value *ShadowVal = getShadow(Val);
+    // TODO: add fast path - if the condition is fully initialized, we know
+    // there is no UUM, without needing to consider the case values below.
+
+    // Some code (e.g., AMDGPUGenMCCodeEmitter.inc) has tens of thousands of
+    // cases. This results in an extremely long chained expression for MSan's
+    // switch instrumentation, which can cause the JumpThreadingPass to have a
+    // stack overflow or excessive runtime. We limit the number of cases
+    // considered, with the tradeoff of niche false negatives.
+    // TODO: figure out a better solution.
+    int casesToConsider = 99;
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qinkunbao wrote:

It might be better to define 99 as a named constant (e.g., kMaxShadowCases) or even a cl::opt flag to allow tuning without recompilation if needed.

https://github.com/llvm/llvm-project/pull/181112