RE: [avr-chat] Missed Optimisation ?

[Boyapati, Anitha]

>-----Original Message-----
>From: address@hidden [mailto:avr-
>address@hidden On Behalf Of Pertti
>Kellomäki
>Sent: Wednesday, March 02, 2011 1:08 PM
>To: address@hidden
>Subject: Re: [avr-chat] Missed Optimisation ?
>
>Like Colin said, the C standard defines the semantics of the language
>in terms of an abstract machine. Normally the compiler is allowed to
>use an "as if" approximation of the semantics and leave out things
>that would not change the state of the abstract machine.
>
>One way to look at volatile is that it is a way to tell the compiler
>that you know things that the compiler does not know, and therefore
>the compiler needs to do things "by the book", i.e. do all the things
>that the abstract machine semantics requires it to do.
>
>I don't have an AVR compiler handy, but it would be instructive to see
>what the compiler generates for "result = result*0" where result is
>volatile.

result =result * 0;

00000058  LDD R24,Y+1           Load indirect with displacement 
00000059  LDD R25,Y+2           Load indirect with displacement 
0000005A  LDD R26,Y+3           Load indirect with displacement 
0000005B  LDD R27,Y+4           Load indirect with displacement 
0000005C  STD Y+1,R1            Store indirect with displacement 
0000005D  STD Y+2,R1            Store indirect with displacement 
0000005E  STD Y+3,R1            Store indirect with displacement 
0000005F  STD Y+4,R1            Store indirect with displacement


Code generated with -O2 turned on. R1 is zero register. The compiler expects 
the value of 'result' to be 0 which I think is ok no matter what 'result' is 
during the assignment execution.

Likewise:

result =result * 1;

00000058  LDD R24,Y+1           Load indirect with displacement 
00000059  LDD R25,Y+2           Load indirect with displacement 
0000005A  LDD R26,Y+3           Load indirect with displacement 
0000005B  LDD R27,Y+4           Load indirect with displacement 
0000005C  STD Y+1,R24           Store indirect with displacement 
0000005D  STD Y+2,R25           Store indirect with displacement 
0000005E  STD Y+3,R26           Store indirect with displacement 
0000005F  STD Y+4,R27           Store indirect with displacement



Anitha