RFR: 8225779: Remove unused CollectedHeap::cell_header_size()

[Per Liden]

Hi,

On 6/14/19 11:26 AM, Erik Österlund wrote:
> Hi Per,
> 
> I don't think it is a good idea to completely revert the changes made to 
> MacroAssembler::needs_explicit_null_check(). This was not just done for 
> interfacing. The code we had before was actually wrong.
> 
> We called that function with both addresses (from signal handlers) and 
> offsets (from the compiler), and tried to look at the passed in value 
> whether it quacks like an offset or an address based on its value. If it 
> looked like an address it would first be normalized to look like an 
> offset. But it was possible that a much-larger-than-page offset is 
> passed in that actually looks like an address according to the address 
> quacking code, which turns the supposedly address looking value into an 
> offset normalized form that looks like it wouldn't need explicit null 
> checks because in that form the offset is within one page, while the 
> original offset passed in totally would need an explicit null check 
> really, because it totally wasn't within that one page offset we allowed.
> 
> To make this more sound, the function was split into two - one used for 
> addresses passed in by signal handlers, and one used for offsets used by 
> the compiler. This completely removed the value based check to determine 
> if the argument looks like an address or offset. I would definitely like 
> to keep that structure going forward, because otherwise that bug will be 
> re-introduced.

I see. I'll revise the patch.

Thanks!
Per

> 
> Otherwise, I do encourage removing the now unnecessary cell header size 
> abstraction.
> 
> /Erik
> 
> On 2019-06-14 08:29, Per Liden wrote:
>> CollectedHeap::cell_header_size() is no longer used and can be 
>> removed. This patch is a hg backout of [1] where it was introduced, 
>> plus the two followup bug fixes [2] & [3].
>>
>> This is a clean backout, with the exception of light editing needed in 
>> two places, because:
>>
>> 1) The function Universe::narrow_oop_base() has since been renamed to 
>> CompressedOops::base().
>> 2) The include guard CPU_ARM_VM_MACROASSEMBLER_ARM_HPP has since been 
>> renamed to CPU_ARM_MACROASSEMBLER_ARM_HPP.
>>
>> Testing: Tier1-3 on all Oracle platforms
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8225779
>> Webrev: http://cr.openjdk.java.net/~pliden/8225779/webrev.0
>>
>> /Per
>>
>> [1] http://hg.openjdk.java.net/jdk/jdk/rev/4ad404da0088
>> [2] http://hg.openjdk.java.net/jdk/jdk/rev/f642ede2eb07
>> [3] http://hg.openjdk.java.net/jdk/jdk/rev/18bd95c0e463