On Sun, Mar 11, 2012 at 6:11 PM, H.J. Lu <hjl.to...@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...@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?
I think we should just scrap all these complications and go with the
idea of clearing MASK_TLS_DIRECT_SEG_REFS.
Uros.
