[lldb][AArch64] Handle different default vector length in SVE testing

This test previously ran on QEMU or A64FX both of which can/do have
512 bit SVE by default.

Graviton 3 has 256 bit SVE so the first part of the test failed.

To fix this, probe the supported vector lengths before starting
the test. The first check will use the default vector length and
the rest use either 256 or 128 bit.

Therefore this test will be skipped on a machine with only 128 bit SVE.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D154208

Author: DavidSpickett

lldb/test/API/commands/register/register/aarch64_sve_registers/rw_access_dynamic_resize/TestSVEThreadedDynamic.py

@@ -1,5 +1,10 @@
 """
 Test the AArch64 SVE registers dynamic resize with multiple threads.
+
+This test assumes a minimum supported vector length (VL) of 256 bits
+and will test 512 bits if possible. We refer to "vg" which is the
+register shown in lldb. This is in units of 64 bits. 256 bit VL is
+the same as a vg of 4.
 """
 
 import lldb
@@ -9,6 +14,39 @@
 
 
 class RegisterCommandsTestCase(TestBase):
+    def get_supported_vg(self):
+        # Changing VL trashes the register state, so we need to run the program
+        # just to test this. Then run it again for the test.
+        exe = self.getBuildArtifact("a.out")
+        self.runCmd("file " + exe, CURRENT_EXECUTABLE_SET)
+
+        main_thread_stop_line = line_number("main.c", "// Break in main thread")
+        lldbutil.run_break_set_by_file_and_line(self, "main.c", main_thread_stop_line)
+
+        self.runCmd("run", RUN_SUCCEEDED)
+
+        self.expect(
+            "thread info 1",
+            STOPPED_DUE_TO_BREAKPOINT,
+            substrs=["stop reason = breakpoint"],
+        )
+
+        # Write back the current vg to confirm read/write works at all.
+        current_vg = self.match("register read vg", ["(0x[0-9]+)"])
+        self.assertTrue(current_vg is not None)
+        self.expect("register write vg {}".format(current_vg.group()))
+
+        # Aka 128, 256 and 512 bit.
+        supported_vg = []
+        for vg in [2, 4, 8]:
+            # This could mask other errors but writing vg is tested elsewhere
+            # so we assume the hardware rejected the value.
+            self.runCmd("register write vg {}".format(vg), check=False)
+            if not self.res.GetError():
+                supported_vg.append(vg)
+
+        return supported_vg
+
     def check_sve_registers(self, vg_test_value):
         z_reg_size = vg_test_value * 8
         p_reg_size = int(z_reg_size / 8)
@@ -56,13 +94,18 @@ def check_sve_registers(self, vg_test_value):
     def test_sve_registers_dynamic_config(self):
         """Test AArch64 SVE registers multi-threaded dynamic resize."""
 
+        if not self.isAArch64SVE():
+            self.skipTest("SVE registers must be supported.")
+
         self.build()
+        supported_vg = self.get_supported_vg()
+
+        if not (2 in supported_vg and 4 in supported_vg):
+            self.skipTest("Not all required SVE vector lengths are supported.")
+
         exe = self.getBuildArtifact("a.out")
         self.runCmd("file " + exe, CURRENT_EXECUTABLE_SET)
 
-        if not self.isAArch64SVE():
-            self.skipTest("SVE registers must be supported.")
-
         main_thread_stop_line = line_number("main.c", "// Break in main thread")
         lldbutil.run_break_set_by_file_and_line(self, "main.c", main_thread_stop_line)
 
@@ -90,7 +133,10 @@ def test_sve_registers_dynamic_config(self):
             substrs=["stop reason = breakpoint"],
         )
 
-        self.check_sve_registers(8)
+        if 8 in supported_vg:
+            self.check_sve_registers(8)
+        else:
+            self.check_sve_registers(4)
 
         self.runCmd("process continue", RUN_SUCCEEDED)