Hello:
I have run into an annoying problem writing some kernel code in AS 4
update 4 environment. I was seeing stack overflows and found the problem.
Some not very long case/switch cases (15-25 statements) are compiled by
gcc compiler to use between 500 and 2000 bytes of stack space. This even
for using enums that run from 0 to N without holes (in which case gcc
optimizer is supposed to generate a table). Kbuild is used to build all
code, I do not use stack variables at all (except for 2 or 3 ints and
function args), and some of the code is even as simple as calling a
separate function for each case statement.
Re-writing the code to use a call function table for consecutive enums
solved the problem in that specific code -- nearly 0 stack usage. But
there is also code that uses kernel non-consecutive enums and does not
necessarily call functions for case entries (these are also most
problematic places where compiler "eats" up to 2 KB of stack). Writing
my own hash implementation and jump table instead of switch/case
statement seems an annoying ugly hack to me even though I can do it.
Is it a known gcc/kernel 4K stack limitation (i.e. switch/case is a
problem in 2.6 with 4K stack)? Is there an option to tell gcc "stack is
small, do not over-use it" ?
My concern is that there is core kernel code affected by this. I do not
think the problem is specific to AS 4, but will show up in Fedora as well.
I am sure there is no mistake, and the problem is switch/case construct
and not what is under case (this has been verified). BTW, using if/else
if/else if generates nearly identical code and does not solve the stack
usage problem.
Comments?
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Constantine Gavrilov
Kernel Developer
Qlusters Software Ltd
1 Azrieli Center, Tel-Aviv
Phone: +972-3-6081977
Fax: +972-3-6081841
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