Help in GIMPLE to RTL transformation

Amruta G. Gokhale amruta@cse.iitb.ac.in
Sat Mar 18 08:55:00 GMT 2006


Hi Ramana,

Thanks for your prompt reply.

Yes, I am looking at the code produced using i386.md file. And yes, there 
is a "divmod" pattern in i386.md file which, as you said, has led to 
generation of this single INSN.

But it would be great if you can let me know, which function or file in 
GCC source code is involved in producing this single INSN. I have been 
trying to trace out GCC source code for this, but with little success.

Regards,
Amruta


On Sat, 18 Mar 2006, Ramana Radhakrishnan wrote:

> Hi Amruta,
>
> I presume you are looking at the i386 backend. It can't be combine.
> Combine comes into the picture much much later than the initial
> expansion rounds. There are a set of named patterns that need to be
> supported in the backend and if the named patterns can be found, gcc
> expands using the named patterns.
>
> Look at the expander pattern for divmodsi4 .That creates this particular
> pattern and can be used for cases like the x86 where a single
> instruction can create both the quotient and the remainder.
>
>> From info gccint . (Read M as the machine mode :)  )
>
>
> `divmodM4'
>     Signed division that produces both a quotient and a remainder.
>     Operand 1 is divided by operand 2 to produce a quotient stored in
>     operand 0 and a remainder stored in operand 3.
>
>     For machines with an instruction that produces both a quotient and
>     a remainder, provide a pattern for `divmodM4' but do not provide
>     patterns for `divM3' and `modM3'.  This allows optimization in the
>     relatively common case when both the quotient and remainder are
>     computed.
>
>     If an instruction that just produces a quotient or just a remainder
>     exists and is more efficient than the instruction that produces
>     both, write the output routine of `divmodM4' to call
>     `find_reg_note' and look for a `REG_UNUSED' note on the quotient
>     or remainder and generate the appropriate instruction.
>
> HTH
>
> cheers
> Ramana
>
> On Sat, 2006-03-18 at 12:22 +0530, Amruta G. Gokhale wrote:
>> Hi,
>>
>> I am looking at some part of GCC source code which does the translation
>> from GIMPLE to RTL.
>> I created an example, having two consecutive instructions, one with "div"
>> and other with "mod".
>>
>> Example:
>> int a, b, c, d;
>> a = b / c;
>> d = b % c;
>>
>> The first-cut RTL code corresponding to these two instructions is a
>> PARALLEL INSN, with two SET INSNs, one with "div" and other with "mod".
>> (insn 22 21 23 1 (parallel [
>>              (set (reg:SI 70)
>>                  (div:SI (reg:SI 72)
>>                      (mem/i:SI (plus:SI (reg/f:SI 54 virtual-stack-vars)
>>                              (const_int -8 [0xfffffff8])) [0 c+0 S4 A32])))
>>              (set (reg:SI 71)
>>                  (mod:SI (reg:SI 72)
>>                      (mem/i:SI (plus:SI (reg/f:SI 54 virtual-stack-vars)
>>                              (const_int -8 [0xfffffff8])) [0 c+0 S4 A32])))
>> )
>>
>> I am unable to understand how a single INSN gets generated for these two
>> instructions. The GIMPLE code has two different tree nodes for the two
>> instructions, namely "trunc_div_expr" and "trunc_mod_expr". So there
>> should be two different INSNs. My guess is, some pass, like "combine",
>> must be combining two INSNs into one.
>> I looked at functions in file "combine.c", but no function in that file is
>> called!
>>
>> Can anybody suggest something? I am really stuck at this point And can't
>> proceed any further.
>>
>> Regards,
>>
>> Amruta Gokhale
>> Graduate Student
>> IIT Bombay,
>> Mumbai.
>
>



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