[PATCH] D81151: [InstCombine] Simplify compare of Phi with constant inputs against a constant

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: spatel, fhahn, asbirlea, yrouban.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

We can simplify

  icmp <pred> phi(C1, C2, ...), CI

with

  phi(icmp(C1, CI), icmp(C2, CI), ...)

provided that all comparison of constants are constants themselves.


https://reviews.llvm.org/D81151

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/icmp-constant-phi.ll


Index: llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
+++ llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
@@ -2,8 +2,6 @@
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
-; TODO: Replace with boolean Phi.
-
 define i1 @test_eq(i1 %cond) {
 ; CHECK-LABEL: @test_eq(
 ; CHECK-NEXT:  entry:
@@ -13,11 +11,10 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE1:%.*]] = phi i1 [ true, [[IF_FALSE]] ], [ false, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp eq i32 [[PHI]], 456
-; CHECK-NEXT:    ret i1 [[COMPARE]]
+; CHECK-NEXT:    ret i1 [[COMPARE1]]
 ;
 entry:
   br i1 %cond, label %if.true, label %if.false
@@ -46,11 +43,10 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE1:%.*]] = phi i1 [ false, [[IF_FALSE]] ], [ true, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp ult i32 [[PHI]], 456
-; CHECK-NEXT:    ret i1 [[COMPARE]]
+; CHECK-NEXT:    ret i1 [[COMPARE1]]
 ;
 entry:
   br i1 %cond, label %if.true, label %if.false
@@ -110,11 +106,10 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE1:%.*]] = phi i1 [ false, [[IF_FALSE]] ], [ true, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp ne i32 [[PHI]], 456
-; CHECK-NEXT:    ret i1 [[COMPARE]]
+; CHECK-NEXT:    ret i1 [[COMPARE1]]
 ;
 entry:
   br i1 %cond, label %if.true, label %if.false
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1458,6 +1458,24 @@
     if (Instruction *Res = processUGT_ADDCST_ADD(Cmp, A, B, CI2, CI, *this))
       return Res;
 
+  // icmp(phi(C1, C2, ...), CI) -> phi(icmp(C1, CI), icmp(C2, CI), ...).
+  if (auto *Phi = dyn_cast<PHINode>(Op0))
+    if (Phi->hasOneUse() &&
+        all_of(Phi->operands(), [](Value *V) { return isa<ConstantInt>(V); })) {
+      Type *Ty = Cmp.getType();
+      Builder.SetInsertPoint(Phi);
+      PHINode *NewPhi =
+          Builder.CreatePHI(Ty, Phi->getNumOperands(), Cmp.getName());
+      const SimplifyQuery Q = SQ.getWithInstruction(&Cmp);
+      for (BasicBlock *Predecessor : predecessors(Phi->getParent())) {
+        auto *Input =
+            cast<ConstantInt>(Phi->getIncomingValueForBlock(Predecessor));
+        auto *BoolInput = SimplifyICmpInst(Pred, Input, CI, Q);
+        NewPhi->addIncoming(BoolInput, Predecessor);
+      }
+      return replaceInstUsesWith(Cmp, NewPhi);
+    }
+
   return nullptr;
 }
 


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