[InstCombine] ror/rol(X, RotAmt) == C --> X == rol/ror(C, RotAmt) (PR51567)

[xbolva00]

Summary

----------------------------------------
define i1 @src(i32 %0) {
%1:
  %2 = fshl i32 %0, i32 %0, i32 25
  %3 = icmp eq i32 %2, 5
  ret i1 %3
}
=>
define i1 @tgt(i32 %0) {
%1:
  %2 = icmp eq i32 %0, 640
  ret i1 %2
}
Transformation seems to be correct!

https://alive2.llvm.org/ce/z/GdY8Jm

Solves PR51567

[xbolva00]

llvm/test/Transforms/InstCombine/icmp-rotate.ll

149

Enough tests?

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

3238

Calling this "RotC" or "RotAmtC" makes it more obvious that we are matching a constant.

llvm/test/Transforms/InstCombine/icmp-rotate.ll

114

The over-shifting isn't relevant - please reduce to a valid amount (for example 28 -> 4) in this test and others.
But don't use "4" with a bitwidth of 8 either because that makes the test ambiguous about whether it is rotating in the wrong direction.

149

We're missing a fshl with ne. Also at least one extra-use test should be here.
Please pre-commit baseline tests.

[xbolva00]

Addressed review comments

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

if (IsZero || C == BitWidth)
                           IsZero ? Constant::getNullValue(Ty)
                                  : Constant::getAllOnesValue(Ty));
 
     break;
   }
 
   case Intrinsic::fshl:
   case Intrinsic::fshr:
+    if (II->getArgOperand(0) == II->getArgOperand(1)) {
       // (rot X, ?) == 0/-1 --> X == 0/-1
       // TODO: This transform is safe to re-use undef elts in a vector, but
       //       the constant value passed in by the caller doesn't allow that.
       if (C.isNullValue() || C.isAllOnesValue())
-      if (II->getArgOperand(0) == II->getArgOperand(1))
         return new ICmpInst(Pred, II->getArgOperand(0), Cmp.getOperand(1));
+
+      const APInt *RotAmtC;
+      // ror(X, RotAmtC) == C --> X == rol(C, RotAmtC)
+      // rol(X, RotAmtC) == C --> X == ror(C, RotAmtC)
+      if (match(II->getArgOperand(2), m_APInt(RotAmtC)))
+        return new ICmpInst(Pred, II->getArgOperand(0),
+                            II->getIntrinsicID() == Intrinsic::fshl
+                                ? ConstantInt::get(Ty, C.rotr(*RotAmtC))
+                                : ConstantInt::get(Ty, C.rotl(*RotAmtC)));
+    }
     break;
 
   case Intrinsic::uadd_sat: {
     // uadd.sat(a, b) == 0  ->  (a | b) == 0
     if (C.isNullValue()) {
       Value *Or = Builder.CreateOr(II->getArgOperand(0), II->getArgOperand(1));
       return new ICmpInst(Pred, Or, Constant::getNullValue(Ty));
     }

llvm/test/Transforms/InstCombine/icmp-rotate.ll

;
   %f2 = tail call <2 x i5> @llvm.fshr.v2i5(<2 x i5> %y, <2 x i5> %y, <2 x i5> %z)
   %r = icmp eq <2 x i5> %f, %f2
   ret <2 x i1> %r
 }
 
 
 define i1 @rol_eq_cst(i8 %x) {
 ; CHECK-LABEL: @rol_eq_cst(
-; CHECK-NEXT:    [[F:%.*]] = tail call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 3)
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[F]], 2
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[X:%.*]], 64
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = tail call i8 @llvm.fshl.i8(i8 %x, i8 %x, i8 3)
   %r = icmp eq i8 %f, 2
   ret i1 %r
 }
 
 define i1 @rol_ne_cst(i8 %x) {
 ; CHECK-LABEL: @rol_ne_cst(
-; CHECK-NEXT:    [[F:%.*]] = tail call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 3)
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[F]], 2
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[X:%.*]], 64
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = tail call i8 @llvm.fshl.i8(i8 %x, i8 %x, i8 3)
   %r = icmp ne i8 %f, 2
   ret i1 %r
 }
 
 define i1 @rol_eq_cst_use(i8 %x) {
 ; CHECK-LABEL: @rol_eq_cst_use(
 ; CHECK-NEXT:    [[F:%.*]] = tail call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 3)
 ; CHECK-NEXT:    call void @use(i8 [[F]])
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[F]], 2
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[X]], 64
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = tail call i8 @llvm.fshl.i8(i8 %x, i8 %x, i8 3)
   call void @use(i8 %f)
   %r = icmp eq i8 %f, 2
   ret i1 %r
 }
 
 define i1 @ror_eq_cst(i8 %x) {
 ; CHECK-LABEL: @ror_eq_cst(
-; CHECK-NEXT:    [[F:%.*]] = call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 6)
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[F]], 3
+; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[X:%.*]], 12
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = tail call i8 @llvm.fshr.i8(i8 %x, i8 %x, i8 2)
   %r = icmp eq i8 %f, 3
   ret i1 %r
 }
 
 define i1 @ror_ne_cst(i8 %x) {
 ; CHECK-LABEL: @ror_ne_cst(
-; CHECK-NEXT:    [[F:%.*]] = call i8 @llvm.fshl.i8(i8 [[X:%.*]], i8 [[X]], i8 6)
-; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[F]], 3
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[X:%.*]], 12
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %f = tail call i8 @llvm.fshr.i8(i8 %x, i8 %x, i8 2)
   %r = icmp ne i8 %f, 3
   ret i1 %r
 }
 
 define <2 x i1> @rol_eq_cst_vec(<2 x i5> %x) {
 ; CHECK-LABEL: @rol_eq_cst_vec(
-; CHECK-NEXT:    [[F:%.*]] = tail call <2 x i5> @llvm.fshl.v2i5(<2 x i5> [[X:%.*]], <2 x i5> [[X]], <2 x i5> <i5 3, i5 3>)
-; CHECK-NEXT:    [[R:%.*]] = icmp eq <2 x i5> [[F]], <i5 2, i5 2>
+; CHECK-NEXT:    [[R:%.*]] = icmp eq <2 x i5> [[X:%.*]], <i5 8, i5 8>
 ; CHECK-NEXT:    ret <2 x i1> [[R]]
 ;
   %f = tail call <2 x i5> @llvm.fshl.v2i5(<2 x i5> %x, <2 x i5> %x, <2 x i5> <i5 3, i5 3>)
   %r = icmp eq <2 x i5> %f, <i5 2, i5 2>
   ret <2 x i1> %r
 }
 
 define <2 x i1> @rol_eq_cst_undef(<2 x i5> %x) {