Implementing native TLS on OS X in DMD

[Dan Olson via digitalmars-d-ldc]

Dan Olson <gorox at comcast.net> writes:

> Jacob Carlborg <doob at me.com> writes:
>
>> On 2016-01-08 17:40, Jacob Carlborg wrote:
>>
>> Adding the assembly for convenience
>>
>>> * I'm looking at the assembly output of LDC, it looks liked LDC aligns
>>> to the size of the type, i.e. "int" to 4 and "long" to 8 and so on, is
>>> that the case?
>>
>> Without initializer:
>>
>> .tbss __D4main1ai$tlv$init, 4, 3
>>
>> BTW, do you know that the above 3 is?
>
> 3 is alignment like .p2align (power of 2 alignment).
> 2^3 in this case (8-byte)
>
>>> * It looks like the only uses the above form of alignment if the symbol
>>> is placed in the __thread_bss section, i.e. doesn't have an initializer.
>>> Does that make sense? If it's has a initializer and is placed in the
>>> __thread_data section it will have the alignment of 3 or 4, depending of
>>> the size of the variable.
>>
>> With initializer:
>>
>> 	.section	__DATA,__thread_data,thread_local_regular
>> 	.align	3
>> __D4main1ai$tlv$init:
>> 	.long	4
>
> Same 8-byte alignment (OSX .align is synonym for .p2align).
>
> The tbss and tdata declarations match.

Just re-reading and it looks like alignments in your example are too big
for a 4-byte type, assuming var is an int.  .align only needs to be 2 here.

$ cat tls.c
__thread int x;
__thread int y = 42;

$ clang -S tls.c
$ cat tls.s
	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 10
	.section	__DATA,__thread_data,thread_local_regular
	.align	2                       ## @y
_y$tlv$init:
	.long	42                      ## 0x2a

	.section	__DATA,__thread_vars,thread_local_variables
	.globl	_y
_y:
	.quad	__tlv_bootstrap
	.quad	0
	.quad	_y$tlv$init

.tbss _x$tlv$init, 4, 2                 ## @x

	.globl	_x
_x:
	.quad	__tlv_bootstrap
	.quad	0
	.quad	_x$tlv$init


.subsections_via_symbols