[PATCH] D157307: [ValueTracking] Add support for `fshl`/`fshr` in `isKnownToBeAPowerOfTwo`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
goldstein.w.n added reviewers: nikic, Allen.
Herald added subscribers: StephenFan, hiraditya.
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If the funnel shifts are rotates (op0 == op1) then the number of 1s/0s
don't change so we can just look through op0/op1.

Proofs: https://alive2.llvm.org/ce/z/Pja5yu


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157307

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-power-of-two.ll


Index: llvm/test/Analysis/ValueTracking/known-power-of-two.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-power-of-two.ll
+++ llvm/test/Analysis/ValueTracking/known-power-of-two.ll
@@ -214,7 +214,8 @@
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[Z:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
@@ -245,7 +246,7 @@
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
 ; CHECK-NEXT:    [[AND:%.*]] = and i16 [[XX]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i16 [[AND]], [[XX]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i16 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xp2 = shl i16 1, %x
@@ -278,7 +279,8 @@
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[Z:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshr.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
@@ -309,7 +311,7 @@
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshr.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
 ; CHECK-NEXT:    [[AND:%.*]] = and i16 [[XX]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i16 [[AND]], [[XX]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i16 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xp2 = shl i16 1, %x
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2144,6 +2144,12 @@
       case Intrinsic::bitreverse:
       case Intrinsic::bswap:
         return isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
+      case Intrinsic::fshr:
+      case Intrinsic::fshl:
+        // If Op0 == Op1, this is a rotate. is_pow2(rotate(x, y)) == is_pow2(x)
+        if (II->getArgOperand(0) == II->getArgOperand(1))
+          return isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
+        break;
       default:
         break;
       }


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