ARM: Make use of d16-d31 when available.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 280 matching lines @@
             "enable use of VFP2 instructions if available")
 DEFINE_bool(enable_armv7, true,
             "enable use of ARMv7 instructions if available (ARM only)")
 DEFINE_bool(enable_sudiv, true,
             "enable use of SDIV and UDIV instructions if available (ARM only)")
 DEFINE_bool(enable_movw_movt, false,
             "enable loading 32-bit constant by means of movw/movt "
             "instruction pairs (ARM only)")
 DEFINE_bool(enable_unaligned_accesses, true,
             "enable unaligned accesses for ARMv7 (ARM only)")
+DEFINE_bool(enable_32dregs, false,

[Rodolph Perfetta, 2012/12/17 20:03:25]

I would set this to true by default and update CpuFeatures::IsSupported to be
overridden by the flag when it is false (in the same way as VFP3).

currently the flag only seems to affect the simulator:
 * on the command line if you set the flag to true but the hardware doesn't
support it, it will be ignored.
 * when the flag is false but the hardware provides 32 D-regs then the feature
is enabled by runtime probing.

If you default to true and update IsSupported then you can disabled the feature
even if the CPU supports it.

[hans, 2012/12/18 11:09:08]

I was worried that this would get included in the snapshot if enabled by
default, but after talking to Ulan we concluded that this doesn't happen because
CpuFeatures::Probe() returns early in the case that the serializer is enabled.

Also added a compile-time flag that can be used to enable this feature both for
the snapshot and target.

+            "enable use of d16-d31 registers on ARM - this requires VFP3")
 DEFINE_bool(enable_fpu, true,
             "enable use of MIPS FPU instructions if available (MIPS only)")
 
 // bootstrapper.cc
 DEFINE_string(expose_natives_as, NULL, "expose natives in global object")
 DEFINE_string(expose_debug_as, NULL, "expose debug in global object")
 DEFINE_bool(expose_gc, false, "expose gc extension")
 DEFINE_bool(expose_externalize_string, false,
             "expose externalize string extension")
 DEFINE_int(stack_trace_limit, 10, "number of stack frames to capture")
@@ skipping 406 matching lines @@
 #undef DEFINE_bool
 #undef DEFINE_int
 #undef DEFINE_string
 #undef DEFINE_implication
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS