[llvm-dev] inlining in exception handing region

[via llvm-dev]

I observed about +6% performance improvement in one of c++ tests in
spec2006 in AArch64 by avoiding inlining functions in exception handling
region. I guess this is not really rare because programers often make
small methods with throw statement like fCallee() in below sample c++
code.

Thanks,
Jun


> Hi Jun,
>
> It's a very common that C++ exception handling code blow up the inlining
> cost of the a function.
>
> It became more when compiler inlines calls in exception handling code.
>
> As exception handling code executes very rarely, you can apply some
> heuristic in inliner to avoid inlining
> for calls in exception region. By this it will open up more inlining
> opportunities.
>
> I'm not sure how much this will help, as these cases are very rare.
>
> How do you justify gains of this, do you have any data ?
>
> Regards,
> Ashutosh
>
> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of via
> llvm-dev
> Sent: Saturday, September 12, 2015 12:54 AM
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] inlining in exception handing region
>
> Hello,
>
> While I was investigating C++ benchmarks, I was aware of that some small
> functions with the throw statement were not inlined because the exception
> handling code increased the inline cost. In the c++ example below, the
> inline cost of fCallee is high mainly because of instructions for the
> throw statement, and note that the constructor of MyException is even
> inlined in fCallee.
>
> Assuming that the exception handling code is rarely executed, would it
> make sense to prevent CallSites in exception handling region (e.g., the
> constructor of MyException) from being inlined so that we can avoid code
> size blow-up in exception handling region and indirectly give more
> inlining opportunity to the small unwinding functions containing
> exception
> handling code?
>
> class MyBaseException {
>   int idx;
>   int limit;
>   const char* msg;
>   public :
>     MyBaseException(int i, int l, const char* m);
>     ~MyBaseException();
>     void handle(const char*m, int i, int l);
> };
>
> class MyException : MyBaseException
> {
>   public:
>     MyException(int i, int l, const char* m): MyBaseException(i, l, m) {
>       handle(m, i, l);
>     }
> };
>
> int *Agg;
>
> int fCallee(int idx, int limit) {
>   if (idx >= limit)
>     throw MyException(idx, limit, "error");
>   return Agg[idx];
> }
>
> int fCaller(int i, int l) {
>   return fCallee(i, l);
> }
>
> Thanks,
> Jun
>
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