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Re: Stack protector performance

by Kostya Serebryany :: Rate this Message:

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What optimization level are you using?

-O0 is not interesting, and at -O1 the optimizer nukes all the code

In your example, the stack variable and the stack accesses are optimized away:

% ./build/Release+Asserts/bin/clang -O1 -S -emit-llvm -o - stack.c

define void @canary() nounwind uwtable readnone {

entry:

ret void

}

define i32 @main() nounwind uwtable readnone {

for.end:

ret i32 0

}

You need to prepare a more optimizer-resistant benchmark.

--kcc

On Fri, Mar 9, 2012 at 2:52 AM, Job Noorman <jobnoorman@...> wrote:

I have a question about the performance of the implementation of the stack
protector in LLVM.

Consider the following C program:
=====
void canary()
{
char buf[20];
buf[0]++;
}

int main()
{
int i;
for (i = 0; i < 1000000000; ++i)
canary();
return 0;
}
=====

This should definately run slower when stack protection is enabled, right?

I have measured the runtime of this program on two different systems compiled
with GCC and LLVM. Here are the results (percentages are the difference with
the unprotected version of the program):

| Desktop | Laptop |
-----+---------+--------+
GCC | +13% | +277% |
LLVM | -3%(!) | +330% |

(These measurements are the median values of 10 runs.)

So the obvious question is: can anybody explain how it is possible that using
the stack protector causes the program to run 3% faster on my desktop?

I have tried profiling the program using valgrind (cachegrind & callgrind) but
the results show absolutely no reason at all for these measurements.

I have attached an archive with the source code and compiled binaries.

Here are the specs of the two systems:
* Desktop
- Ubuntu 11.10
- Linux 3.0.0-16-generic-pae
- Intel(R) Core(TM)2 Duo CPU E4500 @ 2.20GHz (2048K cache)
* Laptop
- Ubuntu 11.10
- Linux 3.0.0-16-generic
- Intel(R) Atom(TM) CPU N450 @ 1.66GHz (512K cache)

Kind regards,
Job

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