[PATCH] D152349: [InstCombine] Transform (icmp eq/ne rotate(X,A0),rotate(X,A1)) -> (icmp eq/ne rotate(X,A0-A1))

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
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Only do so if we don't create more instructions, so either both
rotates have one use or one of the rotates has one use and both `A0`
and `A1` are constant.
Proof: https://alive2.llvm.org/ce/z/tpAoCW


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152349

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/icmp-equality-rotate.ll


Index: llvm/test/Transforms/InstCombine/icmp-equality-rotate.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp-equality-rotate.ll
+++ llvm/test/Transforms/InstCombine/icmp-equality-rotate.ll
@@ -6,8 +6,8 @@
 declare void @use.i8(i8)
 define i1 @cmpeq_rorr_to_rorl(i8 %x, i8 %C) {
 ; CHECK-LABEL: @cmpeq_rorr_to_rorl(
-; CHECK-NEXT:    [[X_RORR:%.*]] = call i8 @llvm.fshr.i8(i8 [[X:%.*]], i8 [[X]], i8 [[C:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[X_RORR]], [[X]]
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 [[C:%.*]])
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[TMP1]], [[X]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %x_rorr = call i8 @llvm.fshr.i8(i8 %x, i8 %x, i8 %C)
@@ -31,7 +31,7 @@
 define i1 @cmpne_rorr_rorr(i8 %x, i8 %C0, i8 %C1) {
 ; CHECK-LABEL: @cmpne_rorr_rorr(
 ; CHECK-NEXT:    [[TMP1:%.*]] = sub i8 [[C0:%.*]], [[C1:%.*]]
-; CHECK-NEXT:    [[TMP2:%.*]] = call i8 @llvm.fshr.i8(i8 [[X:%.*]], i8 [[X]], i8 [[TMP1]])
+; CHECK-NEXT:    [[TMP2:%.*]] = call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 [[TMP1]])
 ; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[TMP2]], [[X]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5050,6 +5050,21 @@
     return new ICmpInst(CmpInst::getInversePredicate(Pred), Op1,
                         ConstantInt::getNullValue(Op1->getType()));
 
+  // Canonicalize:
+  // icmp eq/ne X, OneUse(rotate-right(X))
+  //    -> icmp eq/ne X, rotate-left(X)
+  // We generally try to convert rotate-right -> rotate-left, this just
+  // canonicalizes another case.
+  CmpInst::Predicate PredUnused = Pred;
+  if (match(&I, m_c_ICmp(PredUnused, m_Value(A),
+                         m_OneUse(m_Intrinsic<Intrinsic::fshr>(
+                             m_Deferred(A), m_Deferred(A), m_Value(B)))))) {
+    Module *Mod = I.getModule();
+    Function *Fshl =
+        Intrinsic::getDeclaration(Mod, Intrinsic::fshl, Op0->getType());
+    return new ICmpInst(Pred, A, Builder.CreateCall(Fshl, {A, A, B}));
+  }
+
   return nullptr;
 }
 


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