llvm dev - Feb 2011 - [LLVMdev] LLVM ExecutionEngine/JIT trampoline question

I have a question on the LLVM JIT

I did some brief memory reading one day and I found that a call to a
non-library function is resolved by the X86CompilationCallback, but the
X86CompilationCallback is reached through a trampoline. why can not  the
generated code jump to the X86CompilationCallback function directly ?

          0x2b0a6a4d103b: mov    $0x2b0a6a561010,%rax
          0x2b0a6a4d1045: callq  *%rax
// call foo
>
>
> ==>
>
> 0x2b0a6a561010: mov    $0xa743a0,%r10                   // some kind of
> trampoline
> 0x2b0a6a56101a: callq  *%r10
>
> ==>
>
> 0xa743a0 <X86CompilationCallback>:        push   %rbp
> 0xa743a1 <X86CompilationCallback+1>:    mov    %rsp,%rbp
> 0xa743a4 <X86CompilationCallback+4>:    push   %rdi
> 0xa743a5 <X86CompilationCallback+5>:    push   %rsi


-- 
Kind Regards

Xin Tong
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The address of the callee may be more than 2 GB away in memory, which
cannot be encoded as an immediate offset in the call instruction.  So,
the value is first materialized with a mov instruction which can
encode the immediate and then jumped to through a register.

Reid

On Tue, Feb 22, 2011 at 12:03 PM, Xin Tong Utoronto <x.tong at
utoronto.ca> wrote:
> I have a question on the LLVM JIT
>
> I did some brief memory reading one day and I found that a call to a
> non-library function is resolved by the X86CompilationCallback, but the
> X86CompilationCallback is reached through a trampoline. why can not  the
> generated code jump to the X86CompilationCallback function directly ?
>
>           0x2b0a6a4d103b: mov    $0x2b0a6a561010,%rax
>           0x2b0a6a4d1045: callq  *%rax
> // call foo
>>
>>
>> ==>
>>
>> 0x2b0a6a561010: mov    $0xa743a0,%r10                   // some kind of
>> trampoline
>> 0x2b0a6a56101a: callq  *%r10
>>
>> ==>
>>
>> 0xa743a0 <X86CompilationCallback>:        push   %rbp
>> 0xa743a1 <X86CompilationCallback+1>:    mov    %rsp,%rbp
>> 0xa743a4 <X86CompilationCallback+4>:    push   %rdi
>> 0xa743a5 <X86CompilationCallback+5>:    push   %rsi
>
> --
> Kind Regards
>
> Xin Tong
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

I understand that we need to push the address to a register then branch
using the register. But i am asking why there is a trampoline there such
that a call to foo is first branched to an snippet and the snippet branches
to the X86CompilationCallback. is this snippet necessary ?

Thanks

Xin


On Tue, Feb 22, 2011 at 12:39 PM, Reid Kleckner <reid.kleckner at
gmail.com>wrote:
> The address of the callee may be more than 2 GB away in memory, which
> cannot be encoded as an immediate offset in the call instruction.  So,
> the value is first materialized with a mov instruction which can
> encode the immediate and then jumped to through a register.
>
> Reid
>
> On Tue, Feb 22, 2011 at 12:03 PM, Xin Tong Utoronto <x.tong at
utoronto.ca>
> wrote:
> > I have a question on the LLVM JIT
> >
> > I did some brief memory reading one day and I found that a call to a
> > non-library function is resolved by the X86CompilationCallback, but
the
> > X86CompilationCallback is reached through a trampoline. why can not 
the
> > generated code jump to the X86CompilationCallback function directly ?
> >
> >           0x2b0a6a4d103b: mov    $0x2b0a6a561010,%rax
> >           0x2b0a6a4d1045: callq  *%rax
> > // call foo
> >>
> >>
> >> ==>
> >>
> >> 0x2b0a6a561010: mov    $0xa743a0,%r10                   // some
kind of
> >> trampoline
> >> 0x2b0a6a56101a: callq  *%r10
> >>
> >> ==>
> >>
> >> 0xa743a0 <X86CompilationCallback>:        push   %rbp
> >> 0xa743a1 <X86CompilationCallback+1>:    mov    %rsp,%rbp
> >> 0xa743a4 <X86CompilationCallback+4>:    push   %rdi
> >> 0xa743a5 <X86CompilationCallback+5>:    push   %rsi
> >
> > --
> > Kind Regards
> >
> > Xin Tong
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
>


-- 
Kind Regards

Xin Tong
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