[PATCH] D133362: [InstCombine] Fold x + (x | -x) to x & (x - 1)

[Marc Auberer via Phabricator via llvm-commits]

marcauberer created this revision.
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Fixes #57531

This transformation may be particularly useful on x86-64, because x & (x - 1) can be performed by a single blsr instruction.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133362

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/add_or_sub.ll


Index: llvm/test/Transforms/InstCombine/add_or_sub.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/add_or_sub.ll
@@ -0,0 +1,26 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i32 @add_or_sub_i32_comb(i32 %x) {
+; CHECK-LABEL: @add_or_sub_i32_comb(
+; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[X:%.*]], -1
+; CHECK-NEXT:    [[ADD:%.*]] = and i32 [[TMP1]], [[X]]
+; CHECK-NEXT:    ret i32 [[ADD]]
+;
+  %sub = sub i32 0, %x
+  %or = or i32 %sub, %x
+  %add = add i32 %or, %x
+  ret i32 %add
+}
+
+define i8 @add_or_sub_i8_comb(i8 %x) {
+; CHECK-LABEL: @add_or_sub_i8_comb(
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[X:%.*]], -1
+; CHECK-NEXT:    [[ADD:%.*]] = and i8 [[TMP1]], [[X]]
+; CHECK-NEXT:    ret i8 [[ADD]]
+;
+  %sub = sub i8 0, %x
+  %or = or i8 %sub, %x
+  %add = add i8 %or, %x
+  ret i8 %add
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1421,6 +1421,15 @@
     replaceOperand(I, 1, B);
     return &I;
   }
+  
+  // (add (or (sub 0, A) A) A) --> (and (add A, -1) A)
+  // (add (or A (sub 0, A)) A) --> (and (add A, -1) A)
+  // (add A (or (sub 0, A) A)) --> (and (add A, -1) A)
+  // (add A (or A (sub 0, A))) --> (and (add A, -1) A)
+  if (match(&I, m_c_BinOp(m_Or(m_Sub(m_ZeroInt(), m_Value(A)), m_Value(A)), m_Value(A)))) {
+    Value *AddInst = Builder.CreateAdd(A, Constant::getAllOnesValue(A->getType()));
+    return BinaryOperator::CreateAnd(AddInst, A);
+  }
 
   // Canonicalize ((A & -A) - 1) --> ((A - 1) & ~A)
   // Forms all commutable operations, and simplifies ctpop -> cttz folds.


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