[LLVMdev] Testing canaries

Duncan Sands baldrick at free.fr
Sat Jan 26 08:47:03 PST 2013

Hi Izabela,

On 21/01/13 18:37, Izabela Maffra wrote:
> Dear LLVMers,
>      I am trying to measure the performance overhead (if any) of the canaries
> that clang inserts in the code. I would like to do this automatically, using the
> LLVM test infra-structure. However, I am not sure if that is possible. Could
> someone tell me which flags in the TEST.nightly.Makefile script, (or any other
> script) I must change to have this done? Usually I insert canaries with the
> following command line:
> $> clang -S -fstack-protector hello.c
> But I would like to know which part of LLVM calls the pass that inserts the
> canaries. It seems it is llc, as the pass that inserts the canaries is in
> /lib/CodeGen/StackProtector.cpp. Yet, I cannot see 'fstack-protector' in the llc
> --help list, although I can see 'stack-protector-buffer-size' in llc --help.

the -fstack-protector option is processed by the clang front-end.  If you grep
for stack-protector in the clang source, you will discover that the action is
happening in clang/lib/Driver/Tools.cpp, here:

   if (StackProtectorLevel) {

The argument -stack-protector is for the code generators, and you should find
that llc has the same argument.  However if you look for it it doesn't seem to

$ llc -help | grep stack-protector
   -stack-protector-buffer-size=<uint> - Lower bound for a buffer to be 
considered for stack protection

What is going on?  Some options aren't considered generally useful and are
hidden.  If you use -help-hidden you get to see all options:

$ llc -help-hidden | grep stack-protector
     =stack-protector                      -   Insert stack protectors
     =stack-protector                      -   Insert stack protectors
   -stack-protector-buffer-size=<uint>     - Lower bound for a buffer to be 
considered for stack protection

There it is!  It's not clear to me why -stack-protector is hidden away like
that.  Hidden options work just the same as visible options: just pass
-stack-protector to llc.

Ciao, Duncan.

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