Bail out of a SimplifyCFG switch table opt at undef values.

[JesperAntonsson]

Summary

A true or false result is expected from a comparison, but it seems the possibility of undef was overlooked, which could lead to a failed assert. This is fixed by this patch by bailing out if we encounter undef.

The bug is old and the assert has been there since the end of 2014, so it seems this is unusual enough to forego optimization.

[hans]

Looks good to me.

[uabelho]

I submitted this for Jesper in r319537 but then reverted it in r319539 since it broke some build bots.

Probably the test case needs to be updated or moved.

[JesperAntonsson]

The testcase could fail on other targets, since the label names could become different. Therefore, the checks were relaxed, since the test really only needs to verify that it doesn't crash and emits some code.

lib/Transforms/Utils/SimplifyCFG.cpp

static void reuseTableCompare(
   if (DefaultConst != TrueConst && DefaultConst != FalseConst)
     return;
 
   // Check if the compare with the case values is distinct from the default
   // compare result.
   for (auto ValuePair : Values) {
     Constant *CaseConst = ConstantExpr::getICmp(CmpInst->getPredicate(),
                                                 ValuePair.second, CmpOp1, true);
-    if (!CaseConst || CaseConst == DefaultConst)
+    if (!CaseConst || CaseConst == DefaultConst || isa<UndefValue>(CaseConst))
       return;
     assert((CaseConst == TrueConst || CaseConst == FalseConst) &&
            "Expect true or false as compare result.");
   }
 
   // Check if the branch instruction dominates the phi node. It's a simple
   // dominance check, but sufficient for our needs.
   // Although this check is invariant in the calling loops, it's better to do it

test/Transforms/SimplifyCFG/switch_undef.ll

+; RUN: opt %s -keep-loops=false -switch-to-lookup=true -simplifycfg -S | FileCheck %s
+
+define void @f6() #0 {
+; CHECK-LABEL: entry:
+
+entry:
+  br label %for.cond.i
+
+for.cond.i:                                       ; preds = %f1.exit.i, %entry
+  switch i16 undef, label %f1.exit.i [
+    i16 -1, label %cond.false.i3.i
+    i16 1, label %cond.false.i3.i
+    i16 0, label %cond.false.i3.i
+  ]
+
+cond.false.i3.i:                                  ; preds = %for.cond.i, %for.cond.i, %for.cond.i
+  br label %f1.exit.i
+
+f1.exit.i:                                        ; preds = %cond.false.i3.i, %for.cond.i
+  %cond.i4.i = phi i16 [ undef, %cond.false.i3.i ], [ 1, %for.cond.i ]
+  %tobool7.i = icmp ne i16 %cond.i4.i, 0
+  br label %for.cond.i
+}