[InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison

[craig.topper]

Summary

We can't fold to the non-undef value unless we know it isn't poison. So check each element with isGuaranteedNotToBeUndefOrPoison. This currently rules out all constant expressions.

[lebedev.ri]

lg

llvm/lib/Analysis/InstructionSimplify.cpp

for (unsigned i = 0; i != NumElts; ++i) {
       Constant *FEltC = FalseC->getAggregateElement(i);
       if (!TEltC || !FEltC)
         break;
 
       // If the elements match (undef or not), that value is the result. If only
       // one element is undef, choose the defined element as the safe result.
       if (TEltC == FEltC)
         NewC.push_back(TEltC);
-      else if (isa<UndefValue>(TEltC))
+      else if (isa<UndefValue>(TEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(FEltC))
         NewC.push_back(FEltC);
-      else if (isa<UndefValue>(FEltC))
+      else if (isa<UndefValue>(FEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(TEltC))
         NewC.push_back(TEltC);
       else
         break;
     }
     if (NewC.size() == NumElts)
       return ConstantVector::get(NewC);
   }
 

llvm/test/Transforms/InstSimplify/select.ll

 ; CHECK-NEXT:    [[XF:%.*]] = freeze i32 [[X:%.*]]
 ; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i32 undef, i32 [[XF]]
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
   %xf = freeze i32 %x
   %s = select i1 %cond, i32 undef, i32 %xf
   ret i32 %s
 }
+
+@g = external global i32, align 1
+
+; Make sure we don't fold partial undef vectors when constexprs are involved.
+; We would need to prove the constexpr doesn't result in poison which we aren't
+; equiped to do yet.
+define <2 x i32> @false_undef_true_constextpr_vec(i1 %cond) {
+; CHECK-LABEL: @false_undef_true_constextpr_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> <i32 undef, i32 ptrtoint (i32* @g to i32)>, <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 undef>
+  ret <2 x i32> %s
+}