Re: [PATCH] removal of "static foo = 0"

Adam J. Richter
2000-11-27T06:26:37Z

Michael Meissner wrote:
>On Sat, Nov 25, 2000 at 11:55:11PM +0000, Tim Waugh wrote:
>> On Sat, Nov 25, 2000 at 10:53:00PM +0000, James A Sutherland wrote:
>>
>> > Which is silly. The variable is explicitly defined to be zero
>> > anyway, whether you put this in your code or not.
>>
>> Why doesn't the compiler just leave out explicit zeros from the
>> 'initial data' segment then? Seems like it ought to be tought to..
>
>Because sometimes it matters. For example, in kernel mode (and certainly for
>embedded programs that I'm more familiar with), the kernel does go through and
>zero out the so called BSS segment, so that normally uninitialized static
>variables will follow the rules as laid out under the C standards (both C89 and
>C99). I can imagine however, that the code that is executed before the BSS
>area is zeroed out needs to be extra careful in terms of statics that it
>references, and those must be hand initialized.

Since that code is already careful to hand initialize what
it needs and explicitly zeroes the BSS, that sounds like an argument
that it *is* safe to change gcc to move data that is intialized to
all zeroes into bss, either as a compiler option or even not
optionally.

I am not a gcc hacker, but it looks to me like one could
copy the code from output_constant and the functions that it
calls (in gcc-2.95.2/gcc/gcc/varasm.c) to walk the tree to figure
out if the data was all zeroes. I even started writing a routine
for assemble_variable to call to try to test just for the integer case
(basically just by cutting and pasting code). I include it here just
to illustrate. Note: this code doesn't even type check properly when
I try to compile it, so I know it's very wrong, but it's as good as
posting pseudo code to explain my thinking).

void
clear_zero_initialization(tree decl)
{
tree exp = DECL_INITIAL(decl);
enum tree_code code;

if (exp == NULL)
return;

code = TREE_CODE (TREE_TYPE (exp));
if (lang_expand_constant)
exp = (*lang_expand_constant) (exp);

while ((TREE_CODE (exp) == NOP_EXPR
&& (TREE_TYPE (exp) == TREE_TYPE (TREE_OPERAND (exp, 0))
|| AGGREGATE_TYPE_P (TREE_TYPE (exp))))
|| TREE_CODE (exp) == NON_LVALUE_EXPR)
exp = TREE_OPERAND (exp, 0);

if (code == INTEGER_TYPE && exp == const0_rtx)
DECL_INITIAL(decl) = NULL;
}

At the moment, I have started daydreaming about instead
writing an "elf squeezer" to do this and other space optimizations
by modifying objdump. However, I do think that such an improvement
to gcc would be at least a bit useful to the larger user base than
just those people who use binutils-based systems.

Adam J. Richter __ ______________ 4880 Stevens Creek Blvd, Suite 104
adam@yggdrasil.com \ / San Jose, California 95129-1034
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