Re: PATCH: Properly generate X32 IE sequence

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

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.

-- 
H.J.