[PATCH] D151758: [Reassociate] Group select of constants together with constants

[Mikhail Gudim via Phabricator via llvm-commits]

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If we assign the same rank to select of constants as to constants, we enable InstCombine to absorb constant into the select.


https://reviews.llvm.org/D151758

Files:
  llvm/lib/Transforms/Scalar/Reassociate.cpp
  llvm/test/Transforms/Reassociate/select-of-constants.ll


Index: llvm/test/Transforms/Reassociate/select-of-constants.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/Reassociate/select-of-constants.ll
@@ -0,0 +1,19 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=reassociate < %s | FileCheck %s
+
+; Test that reassociation groups select of constants together with constants.
+
+define dso_local signext i32 @select_of_constants(i32 noundef signext %a, i1 %c)  {
+; CHECK-LABEL: @select_of_constants(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SELECT_:%.*]] = select i1 [[C:%.*]], i32 8, i32 2
+; CHECK-NEXT:    [[ADD4:%.*]] = add i32 [[SELECT_]], 4
+; CHECK-NEXT:    [[ADD_:%.*]] = add i32 [[ADD4]], [[A:%.*]]
+; CHECK-NEXT:    ret i32 [[ADD_]]
+;
+entry:
+  %select_ = select i1 %c, i32 8, i32 2
+  %add4 = add i32 %a, 4
+  %add_ = add i32 %add4, %select_
+  ret i32 %add_
+}
Index: llvm/lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/Reassociate.cpp
+++ llvm/lib/Transforms/Scalar/Reassociate.cpp
@@ -195,6 +195,13 @@
   }
 }
 
+static bool isSelectOfConstants(Instruction &I) {
+  Value *Cond;
+  Constant *TrueC, *FalseC;
+  return match(&I,
+               m_Select(m_Value(Cond), m_Constant(TrueC), m_Constant(FalseC)));
+}
+
 unsigned ReassociatePass::getRank(Value *V) {
   Instruction *I = dyn_cast<Instruction>(V);
   if (!I) {
@@ -202,6 +209,13 @@
     return 0;  // Otherwise it's a global or constant, rank 0.
   }
 
+  // TODO: what should we do if we have several selects of constants?
+  // What if conditions of selects are different?
+  if (isSelectOfConstants(*I))
+    // We want to group select of constants with constants, so that
+    // InstCombine can absorb addition of constant into the select.
+    return 0;
+
   if (unsigned Rank = ValueRankMap[I])
     return Rank;    // Rank already known?
 


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