On Thu, 19 Sep 2002, Richard Henderson wrote:
> On Thu, Sep 19, 2002 at 02:51:44PM -0400, Brian Gerst wrote:
> > > The parameter area belongs to the callee, and it may *always* be modified.
> > The parameters can not be modified if they are declared const though,
> > that's my point.
> Yes they can.
> extern void bar(int x, int y, int z);
> void foo(const int a, const int b, const int c)
> bar(a+1, b+1, c+1);
> subl $12, %esp
> movl 20(%esp), %eax
> incl %eax
> movl %eax, 20(%esp)
> movl 16(%esp), %eax
> incl %eax
> incl 24(%esp)
> movl %eax, 16(%esp)
> addl $12, %esp
> jmp bar
> (Not sure why gcc doesn't use incl on all three memories, nor
> should it allocate that stack frame...)
Well it's not modifying those values. It's putting the
constant value into a register and modifying the value
in the register before calling a function that takes int.
Note that the parameter passed to the function, a, b, and c,
are local copies. gcc can whack those anyway it wants. In
fact, it does strange things above which may not be valid.
It subtracts an offset from esp for local variables ($12).
There aren't any local variables!. Therefore, it has to
access the passed parameters at their pushed offset + 12.
Then, after it's through mucking with them, it collapses
the local stack area (levels the stack), then jumps
to the called function. It will use the early 'call'
return-value to return to the caller.
It's really bad code because it could have done:
Note that, in every case, the constant value was pushed onto the
stack and this function called. That copy of the constant value
can be trashed anyway the callee wants. It's his copy.
I thought you were going to do something like:
Script started on Thu Sep 19 15:22:05 2002
# cat zzz.c
int foo(const int a, const int b, const int c)
a += b;
a += c;
# gcc -c -o zzz zzz.c
zzz.c: In function `foo':
zzz.c:6: warning: assignment of read-only location
zzz.c:7: warning: assignment of read-only location
Script done on Thu Sep 19 15:22:23 2002
Which makes gcc barf when you attempt to modify the
const value. This allows you to check if the code is
doing the wrong thing.
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This archive was generated by hypermail 2b29 : Mon Sep 23 2002 - 22:00:27 EST