Re: EH & Optimization
"Staffan Langin" wrote:
[...] It seems that the compiler (VS 2005) doesn't emit a
SEH prolog/epilog when constructing/destructing objects
with an empty exception specification (throw()). That's
fine and the behavior I expect/want. However, when
constructing/destructing an array of the same object-type
the compiler seems to emit a SEH prolog/epilog for each
array. The overhead is quite significant, about 60
instructions for each object, especially when the
constructor/destructor in a release build is inlined and
performs no operation ie should compile to a no-op. So my
question: How do I configure the compiler to not emit a
SEH prolog/epilog when constructing/destructing
array-members with an empty exception specification?
First of all, I should warn you that exception specification
may lead to something different from what you expect. There
is popular misunderstanding that exception specification may
improve performance. Quite often it is not true. Moreover,
exception specification may eventually deteriorate
performance because:
a) compiler must ensure that the function indeed behaves
according to its exception specification, i.e. instead of
optimizing out irrelevant code now compiler must generate
something like this:
try
{
void foo() throw() // won't throw
}
catch(...)
{
std::unexpected();
}
b) exception specification interferes with optimizer and may
prevent many useful optimizations;
c) exception specification almost certainly prevents
inlining;
Considering the above you should exercise due caution when
using exception specifications.
The second question is: Do these 60 instructions per object
really make any significant difference at run time? Did you
measure run time performance for the straightforward code
(no exception specification) versus "improved" code?
Does your class have any parent classes or class members? If
yes, the what their ctors/dtors contain?
Could you provide concise example demonstrating the problem?
Alex