Revert "[AArch64] Improve codegen for shifted mask op"

This reverts commit b4e1466.

This causes a crash while building the Linux kernel. See the original
Phabricator review for a reduced C and LLVM IR reproducer.

Author: nathanchance

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -14442,23 +14442,15 @@ AArch64TargetLowering::isDesirableToCommuteWithShift(const SDNode *N,
   SDValue ShiftLHS = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
-  // If ShiftLHS is unsigned bit extraction: ((x >> C) & mask), then do not
-  // combine it with shift 'N' to let it be lowered to UBFX except:
-  // ((x >> C) & mask) << C.
+  // If ShiftLHS is unsigned bit extraction: ((x >> C) & mask), then do not combine
+  // it with shift 'N' to let it be lowered to UBFX.
   if (ShiftLHS.getOpcode() == ISD::AND && (VT == MVT::i32 || VT == MVT::i64) &&
       isa<ConstantSDNode>(ShiftLHS.getOperand(1))) {
     uint64_t TruncMask = ShiftLHS.getConstantOperandVal(1);
-    if (isMask_64(TruncMask)) {
-      SDValue AndLHS = ShiftLHS.getOperand(0);
-      if (AndLHS.getOpcode() == ISD::SRL) {
-        if (auto *SRLC = dyn_cast<ConstantSDNode>(AndLHS.getOperand(1))) {
-          if (N->getOpcode() == ISD::SHL)
-            if (auto *SHLC = dyn_cast<ConstantSDNode>(N->getOperand(1)))
-              return SRLC->getAPIntValue() == SHLC->getAPIntValue();
-          return false;
-        }
-      }
-    }
+    if (isMask_64(TruncMask) &&
+        ShiftLHS.getOperand(0).getOpcode() == ISD::SRL &&
+        isa<ConstantSDNode>(ShiftLHS.getOperand(0).getOperand(1)))
+      return false;
   }
   return true;
 }

llvm/test/CodeGen/AArch64/shift-logic.ll

@@ -151,27 +151,3 @@ define i32 @lshr_or_extra_use(i32 %x, i32 %y, i32* %p) nounwind {
   %sh1 = lshr i32 %r, 7
   ret i32 %sh1
 }
-
-define i64 @desirable_to_commute1(i64 %x) {
-; CHECK-LABEL: desirable_to_commute1:
-; CHECK:       // %bb.0:
-; CHECK-NEXT:    and x0, x0, #0x7fff8
-; CHECK-NEXT:    ret
-  %s1 = lshr i64 %x, 3
-  %a = and i64 %s1, 65535
-  %s2 = shl i64 %a, 3
-  ret i64 %s2
-}
-
-define i64 @desirable_to_commute2(i64* %p, i64 %i) {
-; CHECK-LABEL: desirable_to_commute2:
-; CHECK:       // %bb.0:
-; CHECK-NEXT:    and x8, x1, #0x1ff8
-; CHECK-NEXT:    ldr x0, [x0, x8]
-; CHECK-NEXT:    ret
-  %lshr = lshr i64 %i, 3
-  %and = and i64 %lshr, 1023
-  %pidx = getelementptr i64, i64* %p, i64 %and
-  %r = load i64, i64* %pidx
-  ret i64 %r
-}