[LICM] Fix PR35342

[junbuml]

Summary

This change fix PR35342 by rebuilding user_iterator which is invalidated after replacing users with undef in unreachable blocks.

[efriedma]

lib/Transforms/Scalar/LICM.cpp

909

Could we solve the problem by instead changing this call to replaceUsesOfWith with "U = UndefValue::get(I.getType());"? We shouldn't be resetting the iterators like this if we can avoid it.

[junbuml]

lib/Transforms/Scalar/LICM.cpp

909

It seems to be a better fix.

Thanks Eli.

[efriedma]

LGTM

lib/Transforms/Scalar/LICM.cpp

for (Value::user_iterator UI = I.user_begin(), UE = I.user_end(); UI != UE;) {
     auto *User = cast<Instruction>(*UI);
     Use &U = UI.getUse();
     ++UI;
 
     if (VisitedUsers.count(User))
       continue;
 
     if (!DT->isReachableFromEntry(User->getParent())) {
       User->replaceUsesOfWith(&I, UndefValue::get(I.getType()));
+      UI = I.user_begin();
+      UE = I.user_end();
       continue;
     }
 
     // The user must be a PHI node.
     PHINode *PN = cast<PHINode>(User);
 
     // Surprisingly, instructions can be used outside of loops without any
     // exits.  This can only happen in PHI nodes if the incoming block is
     // unreachable.
     BasicBlock *BB = PN->getIncomingBlock(U);
     if (!DT->isReachableFromEntry(BB)) {
       U = UndefValue::get(I.getType());
+      UI = I.user_begin();
+      UE = I.user_end();
       continue;
     }
 
     VisitedUsers.insert(PN);
     if (isTriviallyReplacablePHI(*PN, I))
       continue;
 
     if (!canSplitPredecessors(PN))

test/Transforms/LICM/pr35342.ll

+; RUN: opt -licm -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: @f1
+; CHECK-LABEL: bci_524:
+; CHECK: add i32 undef, undef
+define void @f1(i32 %v) {
+not_zero.lr.ph:
+  br label %not_zero
+
+not_zero:
+  br i1 undef, label %bci_748 ,  label %bci_314
+
+bci_314:
+  %0 = select i1 undef, i32 undef, i32 undef
+  br label %not_zero
+
+bci_524:                   ; No predecessors!
+  %add = add i32 %0, %0
+  br label %bci_748
+
+bci_748:
+  ret void
+}
+