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Re: gcc compile-time performance
- From: Mark Mitchell <mark at codesourcery dot com>
- To: Robert Dewar <dewar at gnat dot com>, "jss at ast dot cam dot ac dot uk" <jss at ast dot cam dot ac dot uk>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Sat, 18 May 2002 13:36:36 -0700
- Subject: Re: gcc compile-time performance
- References: <20020518195940.E540EF2CB4@nile.gnat.com>
--On Saturday, May 18, 2002 03:59:40 PM -0400 Robert Dewar <email@example.com>
> <<This is a common misconception. The problem is that sometimes (often)
> the headers are the *vast* majority of the code in a single translation
> unit. (Often, more than 95%). In order to be conformant, you must
> not only lex and parse all of that code -- you must perform significant
> semantic analysis.
> Well of course that's true. It's true in Ada too.
Not in nearly the same way. Remember that people can -- and do -- encode
Turing programs in C++ headers, and the compiler must execute the program
as it reads the header. The compiler must be an interpreter. That
can be very expensive.
> the back end for almost all programs. Precompiled headers (in Ada
> precompiling specs) will typically only be able to help the front end
Not true; in C++ function bodies are present in the headers and the
precompiled header may store the already optimized form of the code.
> comments (perhaps half that without comments). The tree file (which is
> what would correspond to a precompiled header -- it contains a full
> symbol table and fully decorated semantic tree) is about 5 megabytes.
In C++, for real programs, I've seen tree images as large as 5 gigabytes.
> I do agree that if you don't have a really fast front end, then
> precompiled headers can be a big win, I am just not convinced that it is
> necessarily a win for a front end written for maximum speed.
As I've said, there are ways to get significant speedups out of G++; it's
not tuned for maximum speed. On the other hand, I'm intimately familiar
with several C++ compilers and in all of them the front ends require
very significant amount of computation and all -- except G++ -- use
precompiled headers as a way to cut that down substantially.
I think you're just not familiar with the kinds of things people are doing
with C++. POOMA is a good example; you may in the end get code that is
a total of only a few kilobytes: a bunch of loops doing little scientific
kernels. But, you parse and analyze and inline -- all in the front end --
literally hundreds of thousands of functions.
Mark Mitchell firstname.lastname@example.org
CodeSourcery, LLC http://www.codesourcery.com