[InstCombine] Sanitize undef vector constant to 1 in X*(2^C) with X << C (PR47133)

While x*undef is undef, shift-by-undef is poison,
which we must avoid introducing.

Also log2(iN undef) is *NOT* iN undef, because log2(iN undef) u< N.

See https://bugs.llvm.org/show_bug.cgi?id=47133

(cherry picked from commit 12d93a2)

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -216,7 +216,11 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
 
     if (match(&I, m_Mul(m_Value(NewOp), m_Constant(C1)))) {
       // Replace X*(2^C) with X << C, where C is either a scalar or a vector.
-      if (Constant *NewCst = getLogBase2(NewOp->getType(), C1)) {
+      // Note that we need to sanitize undef multipliers to 1,
+      // to avoid introducing poison.
+      Constant *SafeC1 = Constant::replaceUndefsWith(
+          C1, ConstantInt::get(C1->getType()->getScalarType(), 1));
+      if (Constant *NewCst = getLogBase2(NewOp->getType(), SafeC1)) {
         BinaryOperator *Shl = BinaryOperator::CreateShl(NewOp, NewCst);
 
         if (I.hasNoUnsignedWrap())

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -216,7 +216,7 @@ define <2 x i1> @test13_vector(<2 x i64> %X, <2 x %S*> %P) nounwind {
 define <2 x i1> @test13_vector2(i64 %X, <2 x %S*> %P) nounwind {
 ; CHECK-LABEL: @test13_vector2(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> undef, i64 [[X:%.*]], i32 0
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 0>
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq <2 x i64> [[TMP1]], <i64 -4, i64 undef>
 ; CHECK-NEXT:    [[C:%.*]] = shufflevector <2 x i1> [[TMP2]], <2 x i1> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
@@ -231,7 +231,7 @@ define <2 x i1> @test13_vector2(i64 %X, <2 x %S*> %P) nounwind {
 define <2 x i1> @test13_vector3(i64 %X, <2 x %S*> %P) nounwind {
 ; CHECK-LABEL: @test13_vector3(
 ; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> undef, i64 [[X:%.*]], i32 0
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 2, i64 0>
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq <2 x i64> [[TMP1]], <i64 4, i64 undef>
 ; CHECK-NEXT:    [[C:%.*]] = shufflevector <2 x i1> [[TMP2]], <2 x i1> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i1> [[C]]