RFR: 8276563: Undefined Behaviour in class Assembler [v3]

[Andrew Haley]

On Sat, 13 Nov 2021 07:43:12 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Re-establish the FloatRegister::successor() hack.
>
> src/hotspot/cpu/aarch64/register_aarch64.hpp line 255:
> 
>> 253:   enum {
>> 254:     number_of_registers = 16,
>> 255:     number_of_declared_registers = 16,
> 
> Is there a semantic difference between number of registers and declared registers for float and P registers? Otherwise, having two constants for the same thing makes the code less clear and maybe more error prone.

Perhaps, but I'm trying to do something really general. There is _sometimes_ a difference between the number of physical registers and the number of named registers, and that's what I'm trying to capture here.

> src/hotspot/share/asm/register.hpp line 62:
> 
>> 60: #else // USE_POINTERS_TO_REGISTER_IMPL_ARRAY
>> 61: 
>> 62: #define REGISTER_IMPL_DECLARATION(type, impl_type)                      \
> 
> nit: align backslashes?

You've got a point. I'll have another look.

> src/hotspot/share/asm/register.hpp line 64:
> 
>> 62: #define REGISTER_IMPL_DECLARATION(type, impl_type)                      \
>> 63: inline const type as_ ## type(int encoding) {                           \
>> 64:   assert(encoding <= impl_type::number_of_declared_registers, "invalid register"); \
> 
> assert for >= -1 too?

OK.

> src/hotspot/share/asm/register.hpp line 87:
> 
>> 85: // OS-specific way.
>> 86: #ifdef __GNUC__
>> 87: #define INTERNAL_VISIBILITY  __attribute__ ((visibility ("internal")))
> 
> I try to understand this, is this to allow other object files to see these symbols while preventing them from being exported from the libjvm?

Preventing them from being _redefined_ by other shared libraries. Without this, every time you load the address of `all_Registers` you load from the Global Offset Table, rather than using an immediate operand.

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PR: https://git.openjdk.java.net/jdk/pull/6280