[InstCombine] Fix bug in `FoldOpIntoSelect` where we would incorrectly fold `undef` as constant

[goldstein.w.n]

Summary

D146349 Introduced the ability to use the information from the
select condition to deduce constants as we folded a binop into
select. I.e if the select cond was icmp eq %A, 10, then in the
true-arm of select, we would be able to replace usage of A with
10.

This is broken for vectors that contain undef elements. I.e with
icmp eq %A, <10, undef>, subsituting <10, undef> for A can
result in creating a more undefined result than we otherwise would
have.

We fix the issue with simply checking if the candidate constant for
substituting may contain undef elements and don't do it in that
case.

[nikic]

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

1044

I think this needs to use isGuaranteedNotToBeUndefOrPoison to handle constant expressions correctly.

[goldstein.w.n]

Use isGuaranteedNotToBeUndefOrPoison

[nikic]

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

1044

For constants, it's enough to just pass C, you can skip all the other arguments.

[goldstein.w.n]

Remove extra args

[nikic]

LGTM

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

static Constant *constantFoldOperationIntoSelectOperand(Instruction &I,
   for (Value *Op : I.operands()) {
     CmpInst::Predicate Pred;
     Constant *C = nullptr;
     if (Op == SI) {
       C = dyn_cast<Constant>(IsTrueArm ? SI->getTrueValue()
                                        : SI->getFalseValue());
     } else if (match(SI->getCondition(),
                      m_ICmp(Pred, m_Specific(Op), m_Constant(C))) &&
-               Pred == (IsTrueArm ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) {
+               Pred == (IsTrueArm ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE) &&
+               isGuaranteedNotToBeUndefOrPoison(C)) {
       // Pass
     } else {
       C = dyn_cast<Constant>(Op);
     }
     if (C == nullptr)
       return nullptr;
 
     ConstOps.push_back(C);

llvm/test/Transforms/InstCombine/binop-select.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
 declare void @use(i32)
 declare void @use_f32(float)
 declare void @use_v2f16(<2 x half>)
 declare void @use_v2i8(<2 x i8>)
 declare i32 @llvm.sadd.sat.i32(i32, i32)
+declare <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8>, <2 x i8>)
 
 define i32 @test1(i1 %c, i32 %x, i32 %y) {
 ; CHECK-LABEL: @test1(
 ; CHECK-NEXT:    [[SUB:%.*]] = sub i32 0, [[X:%.*]]
 ; CHECK-NEXT:    [[COND:%.*]] = select i1 [[C:%.*]], i32 [[SUB]], i32 [[Y:%.*]]
 ; CHECK-NEXT:    [[ADD:%.*]] = add i32 [[COND]], [[X]]
 ; CHECK-NEXT:    ret i32 [[ADD]]
 ;

 ; CHECK-NEXT:    ret i32 [[SUB]]
 ;
   %c = icmp ne i32 %x, 9
   %cond = select i1 %c, i32 %y, i32 7
   %sub = call i32 @llvm.sadd.sat.i32(i32 %x, i32 %cond)
   ret i32 %sub
 }
 
+define <2 x i8> @test_sub_dont_deduce_with_undef_cond_vec(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @test_sub_dont_deduce_with_undef_cond_vec(
+; CHECK-NEXT:    [[C_NOT:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 9, i8 undef>
+; CHECK-NEXT:    [[COND:%.*]] = select <2 x i1> [[C_NOT]], <2 x i8> <i8 7, i8 7>, <2 x i8> [[Y:%.*]]
+; CHECK-NEXT:    [[SUB:%.*]] = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> [[X]], <2 x i8> [[COND]])
+; CHECK-NEXT:    ret <2 x i8> [[SUB]]
+;
+  %c = icmp ne <2 x i8> %x, <i8 9, i8 undef>
+  %cond = select <2 x i1> %c, <2 x i8> %y, <2 x i8> <i8 7, i8 7>
+  %sub = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> %x, <2 x i8> %cond)
+  ret <2 x i8> %sub
+}
+
+define <2 x i8> @test_sub_dont_deduce_with_poison_cond_vec(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @test_sub_dont_deduce_with_poison_cond_vec(
+; CHECK-NEXT:    [[C_NOT:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 poison, i8 9>
+; CHECK-NEXT:    [[COND:%.*]] = select <2 x i1> [[C_NOT]], <2 x i8> <i8 7, i8 7>, <2 x i8> [[Y:%.*]]
+; CHECK-NEXT:    [[SUB:%.*]] = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> [[X]], <2 x i8> [[COND]])
+; CHECK-NEXT:    ret <2 x i8> [[SUB]]
+;
+  %c = icmp ne <2 x i8> %x, <i8 poison, i8 9>
+  %cond = select <2 x i1> %c, <2 x i8> %y, <2 x i8> <i8 7, i8 7>
+  %sub = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> %x, <2 x i8> %cond)
+  ret <2 x i8> %sub
+}
+
+
+define <2 x i8> @test_sub_deduce_with_undef_val_vec(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @test_sub_deduce_with_undef_val_vec(
+; CHECK-NEXT:    [[C_NOT:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 1, i8 2>
+; CHECK-NEXT:    [[TMP1:%.*]] = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> [[X]], <2 x i8> [[Y:%.*]])
+; CHECK-NEXT:    [[SUB:%.*]] = select <2 x i1> [[C_NOT]], <2 x i8> <i8 4, i8 -1>, <2 x i8> [[TMP1]]
+; CHECK-NEXT:    ret <2 x i8> [[SUB]]
+;
+  %c = icmp ne <2 x i8> %x, <i8 1, i8 2>
+  %cond = select <2 x i1> %c, <2 x i8> %y, <2 x i8> <i8 3, i8 undef>
+  %sub = call <2 x i8> @llvm.sadd.sat.v2i8(<2 x i8> %x, <2 x i8> %cond)
+  ret <2 x i8> %sub
+}
+
+
 define i32 @test6(i1 %c, i32 %x, i32 %y) {
 ; CHECK-LABEL: @test6(
 ; CHECK-NEXT:    [[COND:%.*]] = select i1 [[C:%.*]], i32 7, i32 [[X:%.*]]
 ; CHECK-NEXT:    [[AND:%.*]] = and i32 [[COND]], [[X]]
 ; CHECK-NEXT:    ret i32 [[AND]]
 ;
   %cond = select i1 %c, i32 7, i32 %x
   %and = and i32 %cond, %x