[llvm] 43c6fb2 - [InstCombine] Update the `select` operand when the `cond` is `trunc` and has the `nuw` or `nsw` property. (#105914)

[via llvm-commits]

Author: c8ef
Date: 2024-08-24T19:56:59+08:00
New Revision: 43c6fb29a64b9443367bf4085a11ca68f7cd4492

URL: https://github.com/llvm/llvm-project/commit/43c6fb29a64b9443367bf4085a11ca68f7cd4492
DIFF: https://github.com/llvm/llvm-project/commit/43c6fb29a64b9443367bf4085a11ca68f7cd4492.diff

LOG: [InstCombine] Update the `select` operand when the `cond` is `trunc` and has the `nuw` or `nsw` property.  (#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: #96765
alive2: https://alive2.llvm.org/ce/z/3n3n2Q

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

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 18ffc209f259e0..fcd11126073bf1 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/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;
 }

diff  --git a/llvm/test/Transforms/InstCombine/fold-select-trunc.ll b/llvm/test/Transforms/InstCombine/fold-select-trunc.ll
new file mode 100644
index 00000000000000..5567d7d5e1fca9
--- /dev/null
+++ b/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
+}