[tests] Tweak ffi/unaligned_test to reflect decision made in #45009

Skip float and double access tests on ARM because they will
always trigger alignment traps. These traps might or might not
be hidden from the user depending on the specific OS configuration,
e.g. many Linux distrubutions choose to perform fixup in the kernel,
but Android disables that. Similarly there is a difference in QEMU
behavior depending on QEMU version.

Change-Id: Idca9553486e0f479ec1a32c25131d3d1bd4ef74d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338140
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>
Commit-Queue: Daco Harkes <[email protected]>
Auto-Submit: Slava Egorov <[email protected]>

Author: mraleph

tests/ffi/unaligned_test.dart

@@ -12,14 +12,36 @@ import 'dart:io';
 import 'package:expect/expect.dart';
 import 'package:ffi/ffi.dart';
 
+final bool isUnalignedFloatingPointAccessSupported = switch (Abi.current()) {
+  // ARMv7-A in [Unaligned Access][1] specifies that VSTR and VLDR will trigger
+  // alignment trap if address is not word aligned irrespective of SCTLR.A
+  // state. Some operating systems (e.g. Linux via [`/proc/cpu/alignment`][2])
+  // can be configured to catch alignment trap, perform unaligned read in
+  // kernel and resume the execution. Android on the other hand configures
+  // `/proc/cpu/alignment` to always forward alignment trap to the application
+  // as a SIGBUS. Finally, different version of QEMU implement different
+  // behavior for unaligned accesses: older versions ignored ARMv7-A
+  // requirements while newer versions will correctly trigger alignment trap.
+  //
+  // We have decided in https://dartbug.com/45009 that we are not going to
+  // support unaligned accesses in FFI in a special way (e.g. it is on user
+  // to be aware of potential problems with unaligned accesses). Consequently
+  // we simply ignore double and float unaligned tests in configurations
+  // where they cause alignment traps (irrespective of whether OS will fixup
+  // and hide the trap from the user or not).
+  //
+  // [1]: https://developer.arm.com/documentation/ddi0406/c/Application-Level-Architecture/Application-Level-Memory-Model/Alignment-support/Unaligned-data-access?lang=en
+  // [2]: https://docs.kernel.org/arch/arm/mem_alignment.html
+  Abi.androidArm || Abi.linuxArm || Abi.iosArm  => false,
+  _ => true,
+};
+
 void main() {
   print("hello");
   testUnalignedInt16(); //# 01: ok
   testUnalignedInt32(); //# 02: ok
   testUnalignedInt64(); //# 03: ok
-  if (!Platform.isAndroid || sizeOf<Pointer>() == 8) {
-    // TODO(http://dartbug.com/45009): Support unaligned reads/writes on
-    // Android arm32.
+  if (isUnalignedFloatingPointAccessSupported) {
     testUnalignedFloat(); //# 04: ok
     testUnalignedDouble(); //# 05: ok
   }