Re: Help in GIMPLE to RTL transformation

[Ramana Radhakrishnan <ramana dot radhakrishnan at codito dot com>]

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.