[InstCombine] Update the select operand when the cond is trunc and has the nuw or nsw property.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -4201,5 +4201,24 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  // select (trunc nuw X to i1), X, Y --> select (trunc nuw X to i1), 1, Y
+  // select (trunc nuw X to i1), Y, X --> select (trunc nuw X to i1), Y, 0
+  // select (trunc nsw X to i1), X, Y --> select (trunc nsw X to i1), -1, Y
+  // select (trunc nsw X to i1), Y, X --> select (trunc nsw X to i1), Y, 0
+  Value *Trunc;
+  if (match(CondVal, m_NUWTrunc(m_Value(Trunc)))) {
+    if (TrueVal == Trunc)
+      return replaceOperand(SI, 1, ConstantInt::get(TrueVal->getType(), 1));
+    if (FalseVal == Trunc)
+      return replaceOperand(SI, 2, ConstantInt::get(FalseVal->getType(), 0));
+  }
+  if (match(CondVal, m_NSWTrunc(m_Value(Trunc)))) {
+    if (TrueVal == Trunc)
+      return replaceOperand(SI, 1,
+                            Constant::getAllOnesValue(TrueVal->getType()));
+    if (FalseVal == Trunc)
+      return replaceOperand(SI, 2, ConstantInt::get(FalseVal->getType(), 0));
+  }
+
   return nullptr;
 }

llvm/test/Transforms/InstCombine/fold-select-trunc.ll

@@ -0,0 +1,68 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i8 @fold_select_trunc_nuw_true(i8 %x, i8 %y) {
+; CHECK-LABEL: @fold_select_trunc_nuw_true(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw i8 [[X:%.*]] to i1
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TRUNC]], i8 1, i8 [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %trunc = trunc nuw i8 %x to i1
+  %ret = select i1 %trunc, i8 %x, i8 %y
+  ret i8 %ret
+}
+
+define i8 @fold_select_trunc_nuw_false(i8 %x, i8 %y) {
+; CHECK-LABEL: @fold_select_trunc_nuw_false(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw i8 [[X:%.*]] to i1
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TRUNC]], i8 [[Y:%.*]], i8 0
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %trunc = trunc nuw i8 %x to i1
+  %ret = select i1 %trunc, i8 %y, i8 %x
+  ret i8 %ret
+}
+
+define i128 @fold_select_trunc_nsw_true(i128 %x, i128 %y) {
+; CHECK-LABEL: @fold_select_trunc_nsw_true(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nsw i128 [[X:%.*]] to i1
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TRUNC]], i128 -1, i128 [[Y:%.*]]
+; CHECK-NEXT:    ret i128 [[RET]]
+;
+  %trunc = trunc nsw i128 %x to i1
+  %ret = select i1 %trunc, i128 %x, i128 %y
+  ret i128 %ret
+}
+
+define i8 @fold_select_trunc_nsw_false(i8 %x, i8 %y) {
+; CHECK-LABEL: @fold_select_trunc_nsw_false(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nsw i8 [[X:%.*]] to i1
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TRUNC]], i8 [[Y:%.*]], i8 0
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %trunc = trunc nsw i8 %x to i1
+  %ret = select i1 %trunc, i8 %y, i8 %x
+  ret i8 %ret
+}
+
+define i8 @fold_select_trunc_negative(i8 %x, i8 %y) {
+; CHECK-LABEL: @fold_select_trunc_negative(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i8 [[X:%.*]] to i1
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TRUNC]], i8 [[X]], i8 [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[RET]]
+;
+  %trunc = trunc i8 %x to i1
+  %ret = select i1 %trunc, i8 %x, i8 %y
+  ret i8 %ret
+}
+
+define <2 x i8> @fold_select_trunc_vector(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @fold_select_trunc_vector(
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc nuw <2 x i8> [[X:%.*]] to <2 x i1>
+; CHECK-NEXT:    [[RET:%.*]] = select <2 x i1> [[TRUNC]], <2 x i8> <i8 1, i8 1>, <2 x i8> [[Y:%.*]]
+; CHECK-NEXT:    ret <2 x i8> [[RET]]
+;
+  %trunc = trunc nuw <2 x i8> %x to <2 x i1>
+  %ret = select <2 x i1> %trunc, <2 x i8> %x, <2 x i8> %y
+  ret <2 x i8> %ret
+}