Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -429,6 +429,17 @@
                                !OtherOpF->getType()->isVectorTy()))
     return nullptr;
 
+  // It may not be safe to sink div/rem after a select. If the select condition
+  // is poison, then the select itself is poison and div/rem may induce
+  // immediate UB with a poison operand.
+  // For example, the following transform is not safe if Cond can ever be poison
+  // because we can replace poison with zero and then we have div-by-zero that
+  // didn't exist in the original code:
+  // Cond ? x/y : x/z --> x / (Cond ? y : z)
+  auto *BO = dyn_cast<BinaryOperator>(TI);
+  if (BO && BO->isIntDivRem() && !isGuaranteedNotToBePoison(Cond))
+    return nullptr;
+
   // If we reach here, they do have operations in common.
   Value *NewSI = Builder.CreateSelect(Cond, OtherOpT, OtherOpF,
                                       SI.getName() + ".v", &SI);
Index: llvm/test/Transforms/InstCombine/select-divrem.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select-divrem.ll
+++ llvm/test/Transforms/InstCombine/select-divrem.ll
@@ -3,8 +3,9 @@
 
 define i5 @sdiv_common_divisor(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @sdiv_common_divisor(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = sdiv i5 [[SEL_V]], [[X:%.*]]
+; CHECK-NEXT:    [[R1:%.*]] = sdiv i5 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = sdiv i5 [[Z:%.*]], [[X]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = sdiv i5 %y, %x
@@ -15,8 +16,9 @@
 
 define i5 @srem_common_divisor(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @srem_common_divisor(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = srem i5 [[SEL_V]], [[X:%.*]]
+; CHECK-NEXT:    [[R1:%.*]] = srem i5 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = srem i5 [[Z:%.*]], [[X]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = srem i5 %y, %x
@@ -27,8 +29,9 @@
 
 define i5 @udiv_common_divisor(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @udiv_common_divisor(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = udiv i5 [[SEL_V]], [[X:%.*]]
+; CHECK-NEXT:    [[R1:%.*]] = udiv i5 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = udiv i5 [[Z:%.*]], [[X]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = udiv i5 %y, %x
@@ -39,8 +42,9 @@
 
 define i5 @urem_common_divisor(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @urem_common_divisor(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = urem i5 [[SEL_V]], [[X:%.*]]
+; CHECK-NEXT:    [[R1:%.*]] = urem i5 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = urem i5 [[Z:%.*]], [[X]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = urem i5 %y, %x
@@ -51,8 +55,9 @@
 
 define i5 @sdiv_common_dividend(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @sdiv_common_dividend(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = sdiv i5 [[X:%.*]], [[SEL_V]]
+; CHECK-NEXT:    [[R1:%.*]] = sdiv i5 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = sdiv i5 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = sdiv i5 %x, %y
@@ -63,8 +68,9 @@
 
 define i5 @srem_common_dividend(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @srem_common_dividend(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = srem i5 [[X:%.*]], [[SEL_V]]
+; CHECK-NEXT:    [[R1:%.*]] = srem i5 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = srem i5 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = srem i5 %x, %y
@@ -75,8 +81,9 @@
 
 define i5 @udiv_common_dividend(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @udiv_common_dividend(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = udiv i5 [[X:%.*]], [[SEL_V]]
+; CHECK-NEXT:    [[R1:%.*]] = udiv i5 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = udiv i5 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = udiv i5 %x, %y
@@ -87,8 +94,9 @@
 
 define i5 @urem_common_dividend(i1 %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @urem_common_dividend(
-; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]
-; CHECK-NEXT:    [[SEL:%.*]] = urem i5 [[X:%.*]], [[SEL_V]]
+; CHECK-NEXT:    [[R1:%.*]] = urem i5 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[R2:%.*]] = urem i5 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[B:%.*]], i5 [[R2]], i5 [[R1]]
 ; CHECK-NEXT:    ret i5 [[SEL]]
 ;
   %r1 = urem i5 %x, %y
@@ -97,6 +105,10 @@
   ret i5 %sel
 }
 
+; Repeat the above tests, but guarantee that the select
+; condition is not poison via argument attribute. That
+; makes it safe to execute the select before div/rem.
+
 define i5 @sdiv_common_divisor_defined_cond(i1 noundef %b, i5 %x, i5 %y, i5 %z) {
 ; CHECK-LABEL: @sdiv_common_divisor_defined_cond(
 ; CHECK-NEXT:    [[SEL_V:%.*]] = select i1 [[B:%.*]], i5 [[Z:%.*]], i5 [[Y:%.*]]