[InstCombine] Fold select -> and/or using impliesPoison

We can fold a ? b : false to a & b if is_poison(b) implies that
is_poison(a), at which point we're able to reuse all the usual fold
on ands. In particular, this covers the very common case of
icmp X, C && icmp X, C'. The same applies to ors.

This currently only has an effect if the
-instcombine-unsafe-select-transform=0 option is set.

Differential Revision: https://reviews.llvm.org/D94550

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -2572,11 +2572,13 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
 
   if (SelType->isIntOrIntVectorTy(1) &&
       TrueVal->getType() == CondVal->getType()) {
-    if (EnableUnsafeSelectTransform && match(TrueVal, m_One())) {
+    if (match(TrueVal, m_One()) &&
+        (EnableUnsafeSelectTransform || impliesPoison(FalseVal, CondVal))) {
       // Change: A = select B, true, C --> A = or B, C
       return BinaryOperator::CreateOr(CondVal, FalseVal);
     }
-    if (EnableUnsafeSelectTransform && match(FalseVal, m_Zero())) {
+    if (match(FalseVal, m_Zero()) &&
+        (EnableUnsafeSelectTransform || impliesPoison(TrueVal, CondVal))) {
       // Change: A = select B, C, false --> A = and B, C
       return BinaryOperator::CreateAnd(CondVal, TrueVal);
     }

llvm/test/Transforms/InstCombine/select-and-or.ll

@@ -91,7 +91,7 @@ define i1 @logical_or_implies(i32 %x) {
 ; CHECK-LABEL: @logical_or_implies(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp eq i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[C2:%.*]] = icmp eq i32 [[X]], 42
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 true, i1 [[C2]]
+; CHECK-NEXT:    [[RES:%.*]] = or i1 [[C1]], [[C2]]
 ; CHECK-NEXT:    ret i1 [[RES]]
 ;
   %c1 = icmp eq i32 %x, 0
@@ -103,10 +103,7 @@ define i1 @logical_or_implies(i32 %x) {
 ; Will fold after conversion to or.
 define i1 @logical_or_implies_folds(i32 %x) {
 ; CHECK-LABEL: @logical_or_implies_folds(
-; CHECK-NEXT:    [[C1:%.*]] = icmp slt i32 [[X:%.*]], 0
-; CHECK-NEXT:    [[C2:%.*]] = icmp sgt i32 [[X]], -1
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 true, i1 [[C2]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 true
 ;
   %c1 = icmp slt i32 %x, 0
   %c2 = icmp sge i32 %x, 0
@@ -119,7 +116,7 @@ define i1 @logical_and_implies(i32 %x) {
 ; CHECK-LABEL: @logical_and_implies(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ne i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[C2:%.*]] = icmp ne i32 [[X]], 42
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 [[C2]], i1 false
+; CHECK-NEXT:    [[RES:%.*]] = and i1 [[C1]], [[C2]]
 ; CHECK-NEXT:    ret i1 [[RES]]
 ;
   %c1 = icmp ne i32 %x, 0
@@ -132,9 +129,7 @@ define i1 @logical_and_implies(i32 %x) {
 define i1 @logical_and_implies_folds(i32 %x) {
 ; CHECK-LABEL: @logical_and_implies_folds(
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ugt i32 [[X:%.*]], 42
-; CHECK-NEXT:    [[C2:%.*]] = icmp ne i32 [[X]], 0
-; CHECK-NEXT:    [[RES:%.*]] = select i1 [[C1]], i1 [[C2]], i1 false
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 [[C1]]
 ;
   %c1 = icmp ugt i32 %x, 42
   %c2 = icmp ne i32 %x, 0
@@ -153,6 +148,7 @@ define i1 @logical_or_noundef_a(i1 noundef %a, i1 %b) {
 }
 
 ; Noundef on false value allows conversion to or.
+; TODO: impliesPoison doesn't handle this yet.
 define i1 @logical_or_noundef_b(i1 %a, i1 noundef %b) {
 ; CHECK-LABEL: @logical_or_noundef_b(
 ; CHECK-NEXT:    [[RES:%.*]] = select i1 [[A:%.*]], i1 true, i1 [[B:%.*]]
@@ -173,6 +169,7 @@ define i1 @logical_and_noundef_a(i1 noundef %a, i1 %b) {
 }
 
 ; Noundef on false value allows conversion to and.
+; TODO: impliesPoison doesn't handle this yet.
 define i1 @logical_and_noundef_b(i1 %a, i1 noundef %b) {
 ; CHECK-LABEL: @logical_and_noundef_b(
 ; CHECK-NEXT:    [[RES:%.*]] = select i1 [[A:%.*]], i1 [[B:%.*]], i1 false