optimization options not working

rohitgeek rohit23taneja@gmail.com
Wed Feb 24 05:23:00 GMT 2010




Ian Lance Taylor-3 wrote:
> 
> rohitgeek <rohit23taneja@gmail.com> writes:
> 
>> 1. Created the machine description files
>> 2. Build my target (successful)
>> 3. Compile 
>>        Now this step works correctly i.e. generate correct assembly if i
>> do
>> not put any optimization option.
>>    
>>      command : ----    ./target-gcc -S test.c     -> works
>>                               ./target-gcc -O1/2/3 -S test.c    -> no
>> assembly corresponding to the variable assignment and all other
>> operations
>> i.e. it generates only prologue and epilogue(if i am correct for later
>> part).
> 
> You didn't show us test.c.  You have to tell us what you did if you
> expect a good answer to your question.
> 
> Without seeing test.c, I would guess that the variables are not used,
> and were removed by the optimizers.
> 
> 
>> Give me some insight into working of peephole optimizations, how do i
>> start
>> doing that.
> 
> Peephole optimization is a specific type of optimization.  See
> define_peephole and define_peephole2 in the internals documentation.
> See many examples in the existing backends.
> 
> 
> Ian
> 
> 




I will try to be descriptive this time .

This is test.c

 main()
  {
  int a,b,c;
   a= 5;
   b=10;
   d=30;
  c=  a*b + 30 ;
}

now , suppose i build gcc for an ARM processor.
After build, compile stage ---

with no optimization options as mentioned above, this is what i get in
assembly.

        .file   "test1234.c"
        .text
        .align  2
        .global main
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        mov     ip, sp
        stmfd   sp!, {fp, ip, lr, pc}
        sub     fp, ip, #4
        sub     sp, sp, #16
        mov     r3, #5
        str     r3, [fp, #-28]
        mov     r3, #10
        str     r3, [fp, #-24]
        mov     r3, #30
        str     r3, [fp, #-16]
        ldr     r2, [fp, #-28]
        ldr     r3, [fp, #-24]
        mul     r1, r3, r2
        mov     r3, r1
        mov     r2, r3, asl #2
        mov     r3, r2, asl #5
        rsb     r3, r2, r3
        add     r3, r3, r1
        mov     r2, r3, asl #2
        add     r3, r3, r2
        mov     r3, r3, asl #4
        add     r3, r3, #299008
        add     r3, r3, #992
        str     r3, [fp, #-20]
        sub     sp, fp, #12
        ldmfd   sp, {fp, sp, pc}
        .size   main, .-main


 with optimization option turned on (mentioned in command line) 
  
assembly :- 

        .file   "test1234.c"
        .text
        .align  2
        .global main
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        bx      lr
        .size   main, .-main


Did i missed something during building time?

Also bringing into notice the point of single instruction generation instead
of 2 instructions (multiply and accumulate), what part of gcc will do it and
how to proceed regarding that.


Thanks
   
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