From 122b0640fc97202bacb630744dfc6da58f11af42 Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper@intel.com>
Date: Thu, 9 Jul 2020 11:01:11 -0700
Subject: [PATCH] [InstSimplify] Don't fold vectors of partial undef in
 SimplifySelectInst if the non-undef element value might produce poison

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.

Differential Revision: https://reviews.llvm.org/D83442
---
 llvm/lib/Analysis/InstructionSimplify.cpp   |  6 ++++--
 llvm/test/Transforms/InstSimplify/select.ll | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 8cd5d20..277e290 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4135,9 +4135,11 @@ static Value *SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
       // 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;
diff --git a/llvm/test/Transforms/InstSimplify/select.ll b/llvm/test/Transforms/InstSimplify/select.ll
index 0f43c8f6..8b69bad 100644
--- a/llvm/test/Transforms/InstSimplify/select.ll
+++ b/llvm/test/Transforms/InstSimplify/select.ll
@@ -848,3 +848,17 @@ define i32 @false_undef_false_freeze(i1 %cond, 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
+}
-- 
2.7.4