Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -499,6 +499,11 @@
   if (TCmp == FCmp)
     return TCmp;
 
+  // It is not safe to propagate poison to the compare unless both arms of the
+  // select were poison.
+  if (isa<PoisonValue>(TCmp) || isa<PoisonValue>(FCmp))
+    return nullptr;
+
   // The remaining cases only make sense if the select condition has the same
   // type as the result of the comparison, so bail out if this is not so.
   if (Cond->getType()->isVectorTy() == RHS->getType()->isVectorTy())
Index: llvm/test/Transforms/InstCombine/icmp.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp.ll
+++ llvm/test/Transforms/InstCombine/icmp.ll
@@ -3935,7 +3935,7 @@
 
 define i1 @thread_cmp_over_select_with_poison_trueval(i1 %b) {
 ; CHECK-LABEL: @thread_cmp_over_select_with_poison_trueval(
-; CHECK-NEXT:    ret i1 poison
+; CHECK-NEXT:    ret i1 false
 ;
   %s = select i1 %b, i32 poison, i32 0
   %tobool = icmp ne i32 %s, 0
@@ -3944,7 +3944,7 @@
 
 define i1 @thread_cmp_over_select_with_poison_falseval(i1 %b) {
 ; CHECK-LABEL: @thread_cmp_over_select_with_poison_falseval(
-; CHECK-NEXT:    ret i1 poison
+; CHECK-NEXT:    ret i1 true
 ;
   %s = select i1 %b, i32 1, i32 poison
   %tobool = icmp ne i32 %s, 0