* optimizer question
@ 2002-09-20 12:15 Zoltan.2.Varga
2002-09-20 12:24 ` John Love-Jensen
2002-09-20 12:29 ` Jeff Law
0 siblings, 2 replies; 3+ messages in thread
From: Zoltan.2.Varga @ 2002-09-20 12:15 UTC (permalink / raw)
To: gcc-help
Hi,
When confronted with the following code:
void main()
{
int x = 2;
int y;
y = rand();
y = y / x;
printf("%d.\n", y);
}
Why does gcc generate a division instead of a shift ? Is this a shortcoming of the optimizer or some feature of C I don't
understand ? When I replace the 'x' in the division with '2', a shift is generated. I'm using gcc-3.2 -O3 on x86 linux.
thanks
Zoltan
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: optimizer question
2002-09-20 12:15 optimizer question Zoltan.2.Varga
@ 2002-09-20 12:24 ` John Love-Jensen
2002-09-20 12:29 ` Jeff Law
1 sibling, 0 replies; 3+ messages in thread
From: John Love-Jensen @ 2002-09-20 12:24 UTC (permalink / raw)
To: Zoltan.2.Varga, gcc-help
Hi Zoltan,
Change it to...
const int x = 2;
...and then the optimizer (-O / -O1, -O2, -O3, -Os) can do something
helpful.
--Eljay
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: optimizer question
2002-09-20 12:15 optimizer question Zoltan.2.Varga
2002-09-20 12:24 ` John Love-Jensen
@ 2002-09-20 12:29 ` Jeff Law
1 sibling, 0 replies; 3+ messages in thread
From: Jeff Law @ 2002-09-20 12:29 UTC (permalink / raw)
To: Zoltan.2.Varga; +Cc: gcc-help
In message <214ABAB6BF4DC24EAD9AB1C7461DA6A1045116@buebe002.europe.nokia.com>,
Zoltan.2.Varga@nokia.com writes:
>
> Hi,
>
> When confronted with the following code:
>
>void main()
>{
> int x = 2;
> int y;
>
> y = rand();
> y = y / x;
> printf("%d.\n", y);
>}
>
>Why does gcc generate a division instead of a shift ? Is this a shortcoming o
>f the optimizer or some feature of C I don't
>understand ? When I replace the 'x' in the division with '2', a shift is gene
>rated. I'm using gcc-3.2 -O3 on x86 linux.
Because conversion of division into shifts happens before constant propagation.
jeff
^ permalink raw reply [flat|nested] 3+ messages in thread
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2002-09-20 12:15 optimizer question Zoltan.2.Varga
2002-09-20 12:24 ` John Love-Jensen
2002-09-20 12:29 ` Jeff Law
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