RFR: 8319822: Use a linear-time algorithm for assert_different_registers() [v12]

[Thomas Stuefe]

On Mon, 3 Jun 2024 15:23:32 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> So I'm getting this bizarre failure on arm32. I'm guessing it's actually a compiler bug, but I suppose it might be some dusty corner of C++ to do with template arg substitution. Any thoughts?
>> 
>> 
>> In file included from /home/runner/work/jdk/jdk/src/hotspot/share/utilities/globalDefinitions.hpp:29,
>>                  from /home/runner/work/jdk/jdk/src/hotspot/share/nmt/memflags.hpp:28,
>>                  from /home/runner/work/jdk/jdk/src/hotspot/share/memory/allocation.hpp:29,
>>                  from ad_arm.hpp:30,
>>                  from ad_arm.cpp:28:
>> /home/runner/work/jdk/jdk/src/hotspot/share/asm/register.hpp: In instantiation of ‘class AbstractRegSet<RegisterImpl*>’:
>> /home/runner/work/jdk/jdk/src/hotspot/share/asm/register.hpp:272:30:   required from ‘constexpr bool different_registers(R, Rx ...) [with R = RegisterImpl*; Rx = {RegisterImpl*, RegisterImpl*}]’
>> /home/runner/work/jdk/jdk/src/hotspot/share/asm/register.hpp:278:27:   required from ‘void assert_different_registers(R, Rx ...) [with R = RegisterImpl*; Rx = {RegisterImpl*, RegisterImpl*}]’
>> /home/runner/work/jdk/jdk/src/hotspot/cpu/arm/arm.ad:8984:52:   required from here
>> /home/runner/work/jdk/jdk/src/hotspot/share/asm/register.hpp:96:26: error: ‘number_of_registers’ is not a member of ‘RegisterImpl*’
>>    96 |   STATIC_ASSERT(RegImpl::number_of_registers <= 64);
>>       |                          ^~~~~~~~~~~~~~~~~~~
>> /home/runner/work/jdk/jdk/src/hotspot/share/utilities/debug.hpp:287:44: note: in definition of macro ‘STATIC_ASSERT’
>>   287 | #define STATIC_ASSERT(Cond) static_assert((Cond), #Cond)
>>       |                                            ^~~~
>
> I just changed the static assert to a regular assert, in the hope that it might work better on arm32.

Urgh. 

Seems on arm32 (and on zero, but I guess that does not matter) Register is a typedef to a pointer:

https://github.com/openjdk/jdk/blob/9686e804a2b058955ff88149c54a0a7896c0a2eb/src/hotspot/cpu/arm/register_arm.hpp#L136

and RegImpl is a pointer type, so it cannot have members.

On all other platforms, Register is a class (and that works since it overrides operator->() )

I don't see a quick fix other than to make the assert dependent on !arm !zero. Or, leave it for a followup fix.

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BTW, note that on Arm32, we can have 64 float registers:

https://github.com/openjdk/jdk/blob/9686e804a2b058955ff88149c54a0a7896c0a2eb/src/hotspot/cpu/arm/register_arm.hpp#L178

So, I would make the underlying type 64-bit for 32-bit builds too.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16617#discussion_r1624768687