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Displaying posts with tag: gcc (reset)
Feedback directed optimization with GCC and Perf


Gcc 5.0 has added support for FDO which uses perf to generate profile. There is documentation for this in gcc manual, to quote:

-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following optimizations which are generally profitable only with profile feedback available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, -ftracer, -ftree-vectorize,
-finline-functions, -fipa-cp, -fipa-cp-clone, -fpredictive-commoning, -funswitch-loops, -fgcse-after-reload, and -ftree-loop-distribute-patterns.
path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO …

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MySQL 5.6 on POWER (patch available)

The following sentence is brought to you by IBM Legal. The postings on this site are my own and don’t necessarily represent IBM’s positions, strategies or opinions.

Okay, now that is out of the way….

If you’re the kind of person who follows the MySQL bugs database closely or subscribes to the MySQL Internals mailing list, you may have worked out that I’ve spent a small amount of time poking at MySQL on modern POWER systems.

Unlike Intel CPUs, POWER CPUs require explicit memory barriers to synchronize memory state between different CPUs. This means that when you’re implementing synchronization primitives, you have one extra thing to get …

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Caring about stack usage

It may not be surprising that there’s been a few projects over the years that I’ve worked on where we’ve had to care about stack usage (to varying degrees).

For threaded userspace applications (e.g. MySQL, Drizzle) you get a certain amount of stack per thread – and you really don’t want to bust that. For a great many years now, there’s been both a configuration parameter in MySQL to set how much stack each thread (connection) gets as well as various checks in the source code to ensure there’s enough free stack to do a particular operation (IIRC open_table is the most hairy one of this in MySQL).

For the Linux Kernel, stack usage is a relatively (in)famous problem… although by now just about every real problem has been fixed and merely mentioning it is probably just the influence of the odd grey beard hairs I’m pretending not to notice.

In a current project I’m working on, it’s also something we have …

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Limiting functions to 32k stack in Drizzle (and scoped_ptr)

I wonder if this comes under “Code Style” or not…

Anyway, Monty and I finished getting Drizzle ready for adding “-Wframe-larger-than=32768″ as a standard compiler flag. This means that no function within the Drizzle source tree can use greater than 32kb stack – it’s a compiler warning – and with -Werror, it means that it’s a build error.

GCC is not perfect at detecting stack usage, but it’s pretty good.

Why have we done this?

Well, there is a little bit of recursion in the server… and we can craft queries to blow a small stack (not so good). On MacOS X, the default thread stack size is only 512kb. This gives not many frames if 32kb stack is a even remotely common.

I found some interesting places to throw a lot of things on the stack too – that would be rather far down on a …

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Profile guided optimization with gcc

Yesterday I wrote how certain build optimizations can have performance differences – and I decided to step a bit deeper into a quite interesting field – profile guided binary optimization. There’re quite a few interesting projects out there, like LLVM (I hear it is used extensively in iphone?) – which analyze run-time profile of compiled code and can do just in time adjustments of binary code. Apparently, you don’t need that fancy technology, and can use plain old gcc.

The whole plan is:

  1. Compile all code with -fprofile-generate in {C|CXX|LD}FLAGS
  2. Run the binary
  3. Run your application/benchmark against that binary
  4. Recompile all code with -fprofile-use (above steps will place lots of .gcda files in source tree)
  5. PROFIT!!! (note the omission of “???” step)

How much …

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Fun with the 387

Filed  GCC bug 39228:

#include <stdio.h>
#include <math.h>
int main()
{
        double a= 10.0;
        double b= 1e+308;
        printf("%d %d %dn", isinf(a*b), __builtin_isinf(a*b), __isinf(a*b));
        return 0;
}

mtaylor@drizzle-dev:~$ gcc -o test test.c
mtaylor@drizzle-dev:~$ ./test
0 0 1
mtaylor@drizzle-dev:~$ gcc -o test test.c -std=c99
mtaylor@drizzle-dev:~$ ./test
1 0 1
mtaylor@drizzle-dev:~$ gcc -o test test.c   -mfpmath=sse -march=pentium4
mtaylor@drizzle-dev:~$ ./test
1 1 1
mtaylor@drizzle-dev:~$ g++ -o test test.c
mtaylor@drizzle-dev:~$ ./test
1 0 1

Originally I found the simple isinf() case to be different on x86 than x86-64, ppc32 and sparc (32 and 64).

After more research, I found that x86-64 uses the sse …

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floating point is not fun
#include <stdio.h>
#include <math.h>

int main()
{
        double a= 10.0;
        double b= 1e+308;
        printf("%dn",isinf(a * b));
        return 0;
}

Prints 1 on: 64bit intel, 32bit PowerPC, 32bit SPARC, 64bit Sparc. But prints zero on 32bit intel.

Oh, but if you build that with g++ instead of gcc on 32bit Intel, you get 1.

workbench-5.1.1-alpha on Fedora 9

So, you want to compile Workbench for Linux, on Fedora 9. You need to install the following packages:


autoconf automake libtool libzip-devel libxml2-devel libsigc++20-devel libglade2-devel gtkmm24-devel mesa-libGLU-devel mysql-libs mysql mysql-devel uuid-devel lua-devel glitz-devel glitz-glx-devel pixman-devel pcre-devel libgnome-devel gtk+-devel pango-devel cairo

I feel I’m being too liberal with dependencies, but I’m not about to strip it, I just want to get it working first :)

You need to have ctemplate and ctemplate-devel installed from updates-testing-newkey (relevant koji build log).

By default, configure.in in Workbench looks for “google-ctemplate”, as opposed to just “ctemplate” as Fedora calls it. You can fix this (easy), or “cheat” - in /usr/local/include you can do sudo ln -s …

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Showing entries 1 to 8