 | Level: Introductory Matt Davis (matdavis@us.ibm.com ), Technical Consultant, IBM
06 Jun 2003 The path to enablement of IA32 code on Linux PPC travels through one of two compiler sets, the GNU Compiler Collection (GCC) and the Visual Age Compilers. This article reviews some tips for porting 32-bit software from Linux IA32 to Linux PowerPC with both of these compiler sets.
Introduction
The path to enablement of IA32 code on Linux on POWER travels through one of two
compiler sets. The GNU Compiler Collection (GCC) is the Free Software Foundation ’s
open source compiler set; this is the de facto standard for software compilation on Linux.
GCC ’ s strength rests in its standardization across several types of hardware, and therefore
the robust portability of code written to be compiled with GCC compilers. Additionally,
IBM has made available the VisualAge Compilers used traditionally on AIX to develop
and deploy applications for Linux on POWER. This document reviews some tips for
porting 32-bit software from Linux IA32 to Linux on POWER with both of these
compiler sets.
About POWER Architecture
Performance Optimized with Enhanced RISC (POWER) architecture is the result of more
than a decade of hardware engineering dating to the POWER1 chips released in 1990.
The POWER1 was IBM ’s first commercial RISC processor and debuted at 25 Mhz clock
speed.POWER4, the current IBM RISC processor offering operates at speeds between
1.1 and 1.45 Ghz. With the extensive parallelism and optimization in the hardware of the
current implementations, POWER can deliver more computation per cycle than other
chip architectures. Announced in 2001, POWER4 chips feature two 64 bit
microprocessors per CPU board and support SMP configurations up to 32 way. In
addition to the POWER4 hardware, Linux on POWER also runs on IBM eServer
hardware featuring POWER3 and RS64IV hardware. These architectures both feature 64
bit SMP capability and copper fabrication technology, but whereas the RS64IV
processors are still actively developed, POWER3 is considered the legacy predecessor to
POWER4. Each of the PowerPC derived chips offers industry leading performance and
provides a world class hardware platform for the deployment of Linux PowerPC .
About SLES 8 for POWER
SuSE Linux, available for more than 10 years, offers two levels of Linux distribution.
SuSE Linux 8.0 is intended for workstation environments, and SuSE Linux Enterprise
Server version 8 for enterprise systems. SLES 8 for POWER is the product of more than
three years of collaboration between SuSE, IBM, and the open source community.
Though SLES 8 boasts a number of accomplishments regarding 64 bit enablement of the
Linux operating system, the ability of Linux on POWER to run both 32 and 64 bit
applications simultaneously does not prohibit 32 bit applications from taking advantage
of this mature Linux distribution.Combined with IBM ’s VA Compilers for performance
critical applications, Linux on POWER environment offered by SLES 8 on IBM iSeries
and pSeries hardware is an unparalleled platform for developing and deploying Linux
solutions.For more information on SuSE Linux, see
http://www.suse.com/us/company/index.html.
About RHEL 3 for POWER
Red Hat Linux is widely recognized as an industry leader in both desktop and enterprise
class Linux markets. Compiler and threading implementations in RHEL 3 differ from
those in SLES 8.Native Posix Threads for Linux (NPTL), featured in RHEL 3, provide
better vertical scaling and general performance improvements over the traditional Linux
threading model.
Compiler Sets and Architecture Awareness
Unlike other Linux platforms, Linux PowerPC offers not only GCC, but also the IBM VA
Compiler set. GCC offers robust portability of code intended for compilation in Linux,
while the VA Compilers offer a substantial performance increase over GCC.
Both AIX and Linux on the POWER architecture are run as Big Endian (left to right
assignment of addresses to bytes in a data word). As most developers familiar with cross-
platform develop will attest, it is of utmost importance to consider endian friendly coding
habits when deploying across Little Endian and Big Endian platforms. Much has been
written on this topic, and for an excellent review of addressing endian concerns, please
see chapter 2 of the AIX 5L Porting Guide. Another practical and concise reference,
Migrating Tru64 Applications to AIX. These references may prove particularly useful
when migrating code with the VA Compilers.
Porting with the GCC Compiler Set
In general, porting code with GCC compilers should be straightforward. Architecture
may vary, and occasionally library version discrepancies may arise,but for the most part,
GCC is GCC, no matter which architecture it runs on. Architecture specific flags are
discouraged for compilation on Linux on POWER (for example, -m486, -powerpc), as GCC does
not have extensive processor maps for optimization routines on these architectures. Also,
without architecture specific flags, binary compatibility will be greater across different
models of POWER hardware.
On all architectures, libraries must be compiled with -fPIC; IA32 has this flag applied by
default with GCC. On POWER, this flag specifies that the generated code will be used in
a shared object.
Developers not familiar with the GPL should reference it carefully and make sure the
terms and conditions of the license are understood.
Porting with the VisualAge Compiler Set
Compiling code with the VA compiler set has shown a significant performance increase
on Linux for IBM eServer's featuring POWER hardware. These compilers, migrated from
the high performance compiler set used on AIX, have extensive optimization routines for
POWER hardware, and therefore should be used for any application that wants to take
full advantage of the POWER architecture ’s potential. However, software originally
intended to be compiled with GCC compilers may need more attention when recompiled
with VA compilers. Makefiles need to be edited to reflect the correct path to the VA
compilers, by default /opt/ibmcmp/. The correct optimization flags should also be set for
the architecture (for example, -03 -qarch=pwr4 -qtune=pwr4 for IBM POWER4 hardware; a
complete list of these flags can be found by executing the compiler binary with no
arguments).In addition to optimization modes for various POWER architecture
derivatives, the VisualAge compilers can be instructed to compile software in common
mode.This guarantees compatibility across all types of POWER architecture at the
expense of performance otherwise gained through architecture-specific optimization.
The VisualAge Compilers do not create object code directly. First, .w code is generated,
which is fed back to the compiler ’s backend optimizer. Once the most efficient
compilation method is determined, the compiler generates .o files, and then the final
binaries are generated.
A few tips to compiling GCC oriented code with the VisualAge compiler set are listed
below:
- C++comments: GCC allows C++style commenting to be used in C files by
default,but this is not the case with XLC, the VisualAge C compiler. Since it is
not economical to change all the comments in a group of source files to comply
with C style commenting, XLC provides a -q argument to allow these comments,
-q cpluscmt . When C code is compiled with this flag, both C and C++ style
commenting is interpreted.
- Environment variables are often the easiest way to configure build scripts. As
opposed to hand editing Configure scripts and Makefiles, it is recommend that
relevant environment variables such as $CC and $CFLAGS be set and the
Configure script be allowed to generate the Makefile without the need for hand
editing.
- Configure scripts also need to be aware of the proper platform type. This will be
either linux-powerpc-unknown-gnu or linux-powerpc64-unknown-gnu.This
should be set for compilation with either GCC or VA by appending the -target=
flag to the Configure script.
Although extensive documentation for the VisualAge compilers exists, an argument list is
provided at the console by running the compiler with no arguments, for example,
$COMPILER_PATH/bin/cc.
Java
In previous distributions of Linux on POWER, the Blackdown JDK was the only
available development kit. Blackdown, while renowned for offering a highly compatible
JDK, has not boasted the performance the IBM JDK has delivered to Java applications on
Linux Intel and AIX platforms.Yet, thanks to the joint effort of IBM,SuSE, and the
open source community, Java migration to Linux on POWER is now powered by the
IBM JDK version 1.4.1 running on either SLES 8 or RHEL 3. The IBM JDK offers the
formal compliance to Java Standards and enterprise class performance expected from
IBM development kits. Addition performance gains may be found by recompiling
requisite C libraries with the VA Compilers.
Developers not accustomed to developing with the IBM JDK should reference
http://ibm.com/developerworks/java. This may be helpful for understanding optimal
levels of garbage collection, JIT compiling, and other Java features.
Helpful Terms and Definitions
- POWER
- The 64-bit architecture RISC featured in many enterprise IBM servers.
- Linux for POWER
- The Linux operating system running on POWER hardware. It can be assumed in the
context of this document that this refers explicitly to IBM iSeries and pSeries servers.
- GNU Compiler Collection (GCC)
- The open source GNU Compiler Set, including gcc, the GNU C Compiler, g++, the GNU
C++compiler, and g77, the GNU Fortran77 compiler.
- SUSE Linux Enterprise Server 8 (SLES 8)
- SUSE Linux Enterprise Server, version 8. This is the most recent enterprise Linux
offering from SUSE Linux. In the context of this document, SLES 8 refers to SLES 8 for
POWER architecture.
- Red Hat Enterprise Linux 3 (RHEL 3)
- Red Hat Enterprise Linux, version 3. This is the most recent enterprise Linux offering
from Red Hat Linux. In the context of this document, RHEL 3 refers to RHEL 3 for
POWER architecture.
- VisualAge Compiler Set (VA Compiler)
- This compiler set, functionally identical to IBM ’ s high-performance compiler set for
AIX, is now available for Linux on POWER. The VA compiler set includes xlC, the VA
C compiler, xlC++, the VA C++compiler, and xlF, the VA Fortran compiler.
- IA32
- The 32 bit Intel architecture.
- GPL
- The GNU General Public License, under which the Linux kernel and much of the
software found in the open source community is licensed.More about the GPL can be
found at http://www.gnu.org.
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Summary
Linux on POWER offers an enterprise class Linux environment,complete with 64 bit
kernel, application environment, and toolchains. Unlike other Linux environments,
Linux on POWER offers a twin compiler set representing the best of both worlds:ease of
migration with open source code and high performance exploitation of the award winning
high performance POWER architecture. As a result of more than three years of
cooperation between IBM, SuSE, and the open source community, SLES 8 has emerged
as a genuine enterprise class distribution and a leader in its field. RHEL 3 is now
available as a Linux on POWER distribution from long-time Linux leader, Red Hat.
Application performance available on the POWER architecture is now augmented by
development tools never before offered on the Linux operating system.In summary,
Linux on POWER is the leading platform for the deployment of high performance Linux
applications.
Where to Find Compilers
The best source for GCC compilers for Linux on POWER is from a certified IBM Linux
vendor,such as SuSE Linux, as these vendors have taken efforts to build GCC and its
libraries to minimize any version discrepancies.
VisualAge compilers can be found at http://ibm.com/software/awdtools/vacpp/.
Resources
About the author | | | Matt Davis is a Linux technical consultant in the IBM eServer Solutions Enablement
team. As a member of the Linux on POWER project since its inception, he explored and
tested emerging technology for Linux on POWER and wrote several reports summarizing
his findings. These include Journaling File Systems for Linux PowerPC, Parallel Grid
Computing with Linux PowerPC, Open Source Alternatives to Commercial Software for
Linux PowerPC, and the Linux Solutions Catalog, to name a few. He came to IBM as an
intern during his tenure as a student at the University of Texas at Austin, from which he
earned two degrees with academic honors.
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