Re: PATCH: Properly generate X32 IE sequence

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Mar 10, 2012 at 10:49 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sat, Mar 10, 2012 at 5:09 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>> On Fri, Mar 9, 2012 at 11:26 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Mon, Mar 5, 2012 at 9:25 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>>>> On Mon, Mar 5, 2012 at 6:03 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>
>>>>>>> X86-64 linker optimizes TLS_MODEL_INITIAL_EXEC to TLS_MODEL_LOCAL_EXEC
>>>>>>> by checking
>>>>>>>
>>>>>>> ? ? ? ?movq foo@gottpoff(%rip), %reg
>>>>>>>
>>>>>>> and
>>>>>>>
>>>>>>> ? ? ? ?addq foo@gottpoff(%rip), %reg
>>>>>>>
>>>>>>> It uses the REX prefix to avoid the last byte of the previous
>>>>>>> instruction. ?With 32bit Pmode, we may not have the REX prefix and
>>>>>>> the last byte of the previous instruction may be an offset, which
>>>>>>> may look like a REX prefix. ?IE->LE optimization will generate corrupted
>>>>>>> binary. ?This patch makes sure we always output an REX pfrefix for
>>>>>>> UNSPEC_GOTNTPOFF. ?OK for trunk?
>>>>>>
>>>>>> Actually, linker has:
>>>>>>
>>>>>> ? ?case R_X86_64_GOTTPOFF:
>>>>>> ? ? ?/* Check transition from IE access model:
>>>>>> ? ? ? ? ? ? ? ?mov foo@gottpoff(%rip), %reg
>>>>>> ? ? ? ? ? ? ? ?add foo@gottpoff(%rip), %reg
>>>>>> ? ? ? */
>>>>>>
>>>>>> ? ? ?/* Check REX prefix first. ?*/
>>>>>> ? ? ?if (offset >= 3 && (offset + 4) <= sec->size)
>>>>>> ? ? ? ?{
>>>>>> ? ? ? ? ?val = bfd_get_8 (abfd, contents + offset - 3);
>>>>>> ? ? ? ? ?if (val != 0x48 && val != 0x4c)
>>>>>> ? ? ? ? ? ?{
>>>>>> ? ? ? ? ? ? ?/* X32 may have 0x44 REX prefix or no REX prefix. ?*/
>>>>>> ? ? ? ? ? ? ?if (ABI_64_P (abfd))
>>>>>> ? ? ? ? ? ? ? ?return FALSE;
>>>>>> ? ? ? ? ? ?}
>>>>>> ? ? ? ?}
>>>>>> ? ? ?else
>>>>>> ? ? ? ?{
>>>>>> ? ? ? ? ?/* X32 may not have any REX prefix. ?*/
>>>>>> ? ? ? ? ?if (ABI_64_P (abfd))
>>>>>> ? ? ? ? ? ?return FALSE;
>>>>>> ? ? ? ? ?if (offset < 2 || (offset + 3) > sec->size)
>>>>>> ? ? ? ? ? ?return FALSE;
>>>>>> ? ? ? ?}
>>>>>>
>>>>>> So, it should handle the case without REX just OK. If it doesn't, then
>>>>>> this is a bug in binutils.
>>>>>>
>>>>>
>>>>> The last byte of the displacement in the previous instruction
>>>>> may happen to look like a REX byte. In that case, linker
>>>>> will overwrite the last byte of the previous instruction and
>>>>> generate the wrong instruction sequence.
>>>>>
>>>>> I need to update linker to enforce the REX byte check.
>>>>
>>>> One important observation: if we want to follow the x86_64 TLS spec
>>>> strictly, we have to use existing DImode patterns only. This also
>>>> means that we should NOT convert other TLS patterns to Pmode, since
>>>> they explicitly state movq and addq. If this is not the case, then we
>>>> need new TLS specification for X32.
>>>
>>> Here is a patch to properly generate X32 IE sequence.
>>>
>>> This is the summary of differences between x86-64 TLS and x32 TLS:
>>>
>>> ? ? ? ? ? ? ? ? ? ? x86-64 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? x32
>>> GD
>>> ? ?byte 0x66; leaq foo@tlsgd(%rip),%rdi; ? ? ? ? leaq foo@tlsgd(%rip),%rdi;
>>> ? ?.word 0x6666; rex64; call __tls_get_addr@plt ?.word 0x6666; rex64;
>>> call __tls_get_addr@plt
>>>
>>> GD->IE optimization
>>> ? movq %fs:0,%rax; addq x@gottpoff(%rip),%rax ? ?movl %fs:0,%eax;
>>> addq x@gottpoff(%rip),%rax
>>>
>>> GD->LE optimization
>>> ? movq %fs:0,%rax; leaq x@tpoff(%rax),%rax ? ? ? movl %fs:0,%eax;
>>> leaq x@tpoff(%rax),%rax
>>>
>>> LD
>>> ?leaq foo@tlsld(%rip),%rdi; ? ? ? ? ? ? ? ? ? ? ?leaq foo@tlsld(%rip),%rdi;
>>> ?call __tls_get_addr@plt ? ? ? ? ? ? ? ? ? ? ? ? call __tls_get_addr@plt
>>>
>>> LD->LE optimization
>>> ?.word 0x6666; .byte 0x66; movq %fs:0, %rax ? ? ?nopl 0x0(%rax); movl
>>> %fs:0, %eax
>>>
>>> IE
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? addq x@gottpoff(%rip),%reg64 ? ? ? ? ? ? ? ? ? addl x@gottpoff(%rip),%reg32
>>>
>>> ? or
>>> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Not supported if
>>> Pmode == SImode
>>> ? movq x@gottpoff(%rip),%reg64; ? ? ? ? ? ? ? ? ?movq x@gottpoff(%rip),%reg64;
>>> ? movq %fs:(%reg64),%reg32 ? ? ? ? ? ? ? ? ? ? ? movl %fs:(%reg64), %reg32
>>>
>>> IE->LE optimization
>>>
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? addq x@gottpoff(%rip),%reg64 ? ? ? ? ? ? ? ? ? addl x@gottpoff(%rip),%reg32
>>>
>>> ? to
>>>
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? addq foo@tpoff, %reg64 ? ? ? ? ? ? ? ? ? ? ? ? addl foo@tpoff, %reg32
>>>
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? leaq foo@tpoff(%reg64), %reg64 ? ? ? ? ? ? ? ? leal foo@tpoff(%reg32), %reg32
>>>
>>> ? or
>>>
>>> ? movq x@gottpoff(%rip),%reg64 ? ? ? ? ? ? ? ? ? movq x@gottpoff(%rip),%reg64;
>>> ? movl %fs:(%reg64),%reg32 ? ? ? ? ? ? ? ? ? ? ? movl %fs:(%reg64), %reg32
>>>
>>> ? to
>>>
>>> ? movq foo@tpoff, %reg64 ? ? ? ? ? ? ? ? ? ? ? ? movq foo@tpoff, %reg64
>>> ? movl %fs:(%reeg64),%reg32 ? ? ? ? ? ? ? ? ? ? ?movl %fs:(%reg64), %reg32
>>>
>>> LE
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? leaq x@tpoff(%reg64),%reg32 ? ? ? ? ? ? ? ? ? ?leal x@tpoff(%reg32),%reg32
>>>
>>> ? or
>>>
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? addq $x@tpoff,%reg64 ? ? ? ? ? ? ? ? ? ? ? ? ? addl $x@tpoff,%reg32
>>>
>>> ? or
>>>
>>> ? movq %fs:0,%reg64; ? ? ? ? ? ? ? ? ? ? ? ? ? ? movl %fs:0,%reg32;
>>> ? movl x@tpoff(%reg64),%reg32 ? ? ? ? ? ? ? ? ? ?movl x@tpoff(%reg32),%reg32
>>>
>>> ? or
>>>
>>> ? movl %fs:x@tpoff,%reg32 ? ? ? ? ? ? ? ? ? ? ? ?movl %fs:x@tpoff,%reg32
>>>
>>>
>>> X32 TLS implementation is straight forward, except for IE:
>>>
>>> 1. Since address override works only on the (reg32) part in fs:(reg32),
>>> we can't use it as memory operand. ?This patch changes ix86_decompose_address
>>> to disallow ?fs:(reg) if Pmode != word_mode.
>>> 2. When Pmode == SImode, there may be no REX prefix for ADD. ?Avoid
>>> any instructions between MOV and ADD, which may interfere linker
>>> IE->LE optimization, since the last byte of the previous instruction
>>> before ADD may look like a REX prefix. ?This patch adds tls_initial_exec_x32
>>> to make sure that we always have
>>>
>>> movl %fs:0, %reg32
>>> addl xgottpoff(%rip), %reg32
>>>
>>> so that the last byte of the previous instruction before ADD will
>>> never be a REX byte. ?Tested on Linux/x32.
>>>
>>> 2012-03-09 ?H.J. Lu ?<hongjiu.lu@intel.com>
>>>
>>> ? ? ? ?* config/i386/i386.c (ix86_decompose_address): Disallow fs:(reg)
>>> ? ? ? ?if Pmode != word_mode.
>>> ? ? ? ?(legitimize_tls_address): Call gen_tls_initial_exec_x32 if
>>> ? ? ? ?Pmode == SImode for x32.
>>>
>>> ? ? ? ?* config/i386/i386.md (UNSPEC_TLS_IE_X32): New.
>>> ? ? ? ?(tls_initial_exec_x32): Likewise.
>>
>> Nice solution!
>>
>> OK for mainline.
>
> Done.
>
>> BTW: Did you investigate the issue with memory aliasing?
>>
>
> It isn't a problem since it is wrapped in UNSPEC_TLS_IE_X32
> which loads address of the TLS symbol.
>
> Thanks.
>

Since we must use reg64 in %fs:(%reg) memory operand like

movq x@gottpoff(%rip),%reg64;
mov %fs:(%reg64),%reg

this patch optimizes x32 TLS IE load and store by wrapping
%reg64 inside of UNSPEC when Pmode == SImode.  OK for
trunk?

Thanks.

-- 
H.J.
---
2012-03-11  H.J. Lu  <hongjiu.lu@intel.com>

	* config/i386/i386.md (*tls_initial_exec_x32_load): New.
	(*tls_initial_exec_x32_store): Likewise.

Attachment: gcc-x32-tls-2.patch
Description: Text document