[LLVMdev] Stack protector performance

[Kostya Serebryany]

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 at gmail.com> 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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> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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