[InstCombine] Update the select operand when the cond is trunc …

…and has the `nuw` or `nsw` property. (llvm#105914)

This patch updates the select operand when the cond has the nuw or nsw
property. Considering the semantics of the nuw and nsw flag, if there is
no poison value in this expression, this code assumes that X can only be
0, 1 or -1.

close: llvm#96765
alive2: https://alive2.llvm.org/ce/z/3n3n2Q

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
+}