Ian Lance Taylor-3 wrote:
>
> rohitgeek <rohit23taneja@xxxxxxxxx> 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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