[arm64] A shift of 0 is not allowed in ubfx.

R=bmeurer@chromium.org, v8-arm-ports@googlegroups.com
BUG=v8:5951

Review-Url: https://codereview.chromium.org/2685943003
Cr-Commit-Position: refs/heads/master@{#43090}

src/compiler/arm64/instruction-selector-arm64.cc

@@ -957,17 +957,19 @@ void InstructionSelector::VisitWord64And(Node* node) {
         // Any shift value can match; int64 shifts use `value % 64`.
         uint32_t lsb = static_cast<uint32_t>(mleft.right().Value() & 0x3f);
 
-        // Ubfx cannot extract bits past the register size, however since
-        // shifting the original value would have introduced some zeros we can
-        // still use ubfx with a smaller mask and the remaining bits will be
-        // zeros.
-        if (lsb + mask_width > 64) mask_width = 64 - lsb;
-
-        Emit(kArm64Ubfx, g.DefineAsRegister(node),
-             g.UseRegister(mleft.left().node()),
-             g.UseImmediateOrTemp(mleft.right().node(), lsb),
-             g.TempImmediate(static_cast<int32_t>(mask_width)));
-        return;
+        if (lsb != 0) {
+          // Ubfx cannot extract bits past the register size, however since
+          // shifting the original value would have introduced some zeros we can
+          // still use ubfx with a smaller mask and the remaining bits will be
+          // zeros.
+          if (lsb + mask_width > 64) mask_width = 64 - lsb;
+
+          Emit(kArm64Ubfx, g.DefineAsRegister(node),
+               g.UseRegister(mleft.left().node()),
+               g.UseImmediateOrTemp(mleft.right().node(), lsb),
+               g.TempImmediate(static_cast<int32_t>(mask_width)));
+          return;
+        }
       }
       // Other cases fall through to the normal And operation.
     }

test/cctest/compiler/test-run-machops.cc

@@ -6730,6 +6730,14 @@ TEST(Regression5923) {
   }
 }
 
+TEST(Regression5951) {
+  BufferedRawMachineAssemblerTester<int64_t> m(MachineType::Int64());
+  m.Return(m.Word64And(m.Word64Shr(m.Parameter(0), m.Int64Constant(0)),
+                       m.Int64Constant(0xffffffffffffffffl)));
+  int64_t input = 1234;
+  CHECK_EQ(input, m.Call(input));
+}
+
 #endif  // V8_TARGET_ARCH_64_BIT
 
 }  // namespace compiler