[JumpThreading] A bug fix for stale loop info after unfold select

[hjyamauchi]

Summary

The return value of a TryToUnfoldSelect call was not checked, which led to an
incorrectly preserved loop info and some crash.

The original crash was reported on https://reviews.llvm.org/D59514.

[brzycki]

LGTM (but I'm not listed as a reviewer...)

[davidxl]

lgtm

[fhahn]

test/Transforms/JumpThreading/stale-loop-info-after-unfold-select.ll

1

you could just use verify<loops> to verify the loop info is correct after jt.

[hjyamauchi]

Use 'verify<loops>' instead.

lib/Transforms/Scalar/JumpThreading.cpp

if (CondBr && CondConst) {
       // We did not manage to simplify this branch, try to see whether
       // CondCmp depends on a known phi-select pattern.
       if (TryToUnfoldSelect(CondCmp, BB))
         return true;
     }
   }
 
   if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
-    TryToUnfoldSelect(SI, BB);
+    if (TryToUnfoldSelect(SI, BB))
+      return true;
 
   // Check for some cases that are worth simplifying.  Right now we want to look
   // for loads that are used by a switch or by the condition for the branch.  If
   // we see one, check to see if it's partially redundant.  If so, insert a PHI
   // which can then be used to thread the values.
   Value *SimplifyValue = CondInst;
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
     if (isa<Constant>(CondCmp->getOperand(1)))

test/Transforms/JumpThreading/stale-loop-info-after-unfold-select.ll

+; RUN: opt -passes='require<loops>,jump-threading,print<loops>' -S < %s 2>&1 | FileCheck %s
+
+; CHECK: Loop at depth 1 containing: %while.cond<header>,%if.end.i,%func1.exit<latch>,%select.unfold<latch>
+
+%"type1" = type { i8 }
+%"type2" = type opaque
+
+define dso_local i16* @func2(%"type1"* %this, %"type2"*) {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %func1.exit, %entry
+  %month.0 = phi i32 [ undef, %entry ], [ %month.0.be, %func1.exit ]
+  switch i32 %month.0, label %if.end.i [
+    i32 4, label %func1.exit
+    i32 1, label %func1.exit
+  ]
+
+if.end.i:                                         ; preds = %while.cond
+  br label %func1.exit
+
+func1.exit:                  ; preds = %if.end.i, %while.cond, %while.cond
+  %retval.0.i = phi i32 [ 9, %if.end.i ], [ 0, %while.cond ], [ 0, %while.cond ]
+  %call2 = tail call signext i32 @func3(i32 signext %retval.0.i, i32 signext 1, i32 signext 3)
+  %cmp = icmp slt i32 %call2, 1
+  %add = add nsw i32 %call2, 2
+  %month.0.be = select i1 %cmp, i32 %add, i32 %call2
+  br label %while.cond
+}
+
+declare i32 @func3(i32, i32, i32)
+