[ValueTracking] Add support for `bswap` and `bitreverse` in `isKnownToBeAPowerOfTwo`

[goldstein.w.n]

Summary

Both of these intrinsics don't change the number of 1s/0s so we can
look directly through them.

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

[nikic]

LGTM

[goldstein.w.n]

Rebase

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

case Instruction::Call: {
     if (auto *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       case Intrinsic::umax:
       case Intrinsic::smax:
       case Intrinsic::umin:
       case Intrinsic::smin:
         return isKnownToBeAPowerOfTwo(II->getArgOperand(1), OrZero, Depth, Q) &&
                isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
+      // bswap/bitreverse just move around bits, but don't change any 1s/0s
+      // thus dont change pow2/non-pow2 status.
+      case Intrinsic::bitreverse:
+      case Intrinsic::bswap:
+        return isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
       default:
         break;
       }
     }
     return false;
   }
   default:
     return false;

llvm/test/Analysis/ValueTracking/known-power-of-two.ll

;
   ret i1 %r
 }
 
 define i16 @bswap_is_pow2_or_zero(i16 %x, i16 %y) {
 ; CHECK-LABEL: define i16 @bswap_is_pow2_or_zero
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.bswap.i16(i16 [[XP2]])
-; 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
   %xx = call i16 @llvm.bswap.i16(i16 %xp2)
 
   %r = urem i16 %y, %xx
   ret i16 %r
 }

 }
 
 define i1 @bswap_is_pow2(i16 %x, i16 %y) {
 ; CHECK-LABEL: define i1 @bswap_is_pow2
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.bswap.i16(i16 [[XP2]])
 ; 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
   %xx = call i16 @llvm.bswap.i16(i16 %xp2)
 
   %and = and i16 %y, %xx
   %r = icmp eq i16 %and, %xx
   ret i1 %r
;
   ret i1 %r
 }
 
 define i16 @bitreverse_is_pow2_or_zero(i16 %x, i16 %y) {
 ; CHECK-LABEL: define i16 @bitreverse_is_pow2_or_zero
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.bitreverse.i16(i16 [[XP2]])
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
   %xx = call i16 @llvm.bitreverse.i16(i16 %xp2)
 
   %r = urem i16 %y, %xx
   ret i16 %r
 }

 }
 
 define i1 @bitreverse_is_pow2(i16 %x, i16 %y) {
 ; CHECK-LABEL: define i1 @bitreverse_is_pow2
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.bitreverse.i16(i16 [[XP2]])
 ; 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
   %xx = call i16 @llvm.bitreverse.i16(i16 %xp2)
 
   %and = and i16 %y, %xx
   %r = icmp eq i16 %and, %xx
   ret i1 %r