Date: Mon, 10 Jan 2005 11:07:52 -0500 From: "William H. Magill" <magill@mcgillsociety.org> To: David Gilbert <dgilbert@dclg.ca> Cc: freebsd-alpha@freebsd.org Subject: Re: processor type. Message-ID: <C81C63C4-6321-11D9-B8BD-000393768D2C@mcgillsociety.org> In-Reply-To: <16866.32790.398095.651691@canoe.dclg.ca> References: <16866.32790.398095.651691@canoe.dclg.ca>
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On 10 Jan, 2005, at 08:16, David Gilbert wrote:
> I see in the compiler lines crawling by that gcc is asked to optimize
> for 'EV5' while being compatible with 'EV4'. My Alpha is an EV4 ---
> I'm wondering if I would see better performance with a different flag
> there, but the gcc manual doesn't even acknowledge the existance of
> the options that are in use, let alone the available options.
I'm not a programmer type, but it was pretty well known that the GCC
compiler generated from pretty poor, to downright bad code on the
Alphas, no matter which one, when compared to the DEC C compiler
(known as ccc on the Linux tools CD).
However, I understand that the GCC compiler picked up (some?many?all)
the Alpha optimization enhancements offered by Compaq shortly before
the Intel/HP deal.
And I do know from long TRU64 experience that optimizing for a
particular
EVx chip can make a big difference.
The man page for the Dec C Compiler under 5.1A:
Compaq C V6.3-028 on Compaq Tru64 UNIX V5.1 (Rev. 732)
Compiler Driver V6.3-026 (sys) cc Driver
states:
(Note: There are two relevant options for ccc -arch and -tune.)
-arch option
Specifies which version of the Alpha architecture to generate
instructions for. All Alpha processors implement a core set of
instruc-
tions and, in some cases, the following extensions: BWX
(byte/word-
manipulation extension), MVI (multimedia extension), FIX (square
root
and floating-point convert extension), and CIX (count extension).
(The
Alpha Architecture Reference Manual describes the extensions in
detail.)
The option specified by the -arch option determines which
instructions
the compiler can generate:
generic
Generate instructions that are appropriate for all Alpha
proces-
sors. This option is the default.
host
Generate instructions for the processor that the compiler is
run-
ning on (for example, EV6 instructions on an EV6 processor).
ev4,ev5
Generate instructions for the EV4 processor (21064, 21064A,
21066,
and 21068 chips) and EV5 processor (some 21164 chips). (Note
that
chip number 21164 is used for both EV5 and EV56 processors.)
Applications compiled with this option will not incur any
emulation
overhead on any Alpha processor.
ev56
Generate instructions for EV56 processors (some 21164 chips).
This option permits the compiler to generate any EV4
instruction,
plus any instructions contained in the BWX extension.
Applications compiled with this option may incur emulation
overhead
on EV4 and EV5 processors.
ev6 Generate instructions for EV6 processors (21264 chips).
This option permits the compiler to generate any EV6
instruction,
plus any instructions contained in the following extensions:
BWX,
MVI, and FIX.
Applications compiled with this option may incur emulation
overhead
on EV4, EV5, EV56, and PCA56 processors.
ev67
Generate instructions for EV67 processors (21264A chips).
This option is the same as the ev6 option except that it also
per-
mits the compiler to generate any instructions contained in
the CIX
extension.
If your application uses CIX instructions, it may incur
emulation
overhead on all processors that are older than EV67.
pca56
Generate instructions for PCA56 processors (21164PC chips).
This option permits the compiler to generate any EV4
instruction,
plus any instructions contained in the BWX and MVI extensions.
Applications compiled with this option may incur emulation
overhead
on EV4, EV5, and EV56 processors.
A program compiled with any of the options will run on any Alpha
pro-
cessor. Beginning with DIGITAL UNIX V4.0 and continuing with
subse-
quent versions, the operating system kernel includes an
instruction
emulator. This capability allows any Alpha chip to execute and
produce
correct results from Alpha instructions--even if the some of the
instructions are not implemented on the chip. Applications using
emu-
lated instructions will run correctly, but may incur significant
emula-
tion overhead at run time.
The psrinfo -v command can be used to determine which type of
processor
is installed on any given Alpha system.
Note the following differences between the -arch evx and -tune evx
options (where x designates a specific processor):
+ -arch evx implies -tune evx, but -tune evx does not imply
-arch
evx.
+ -arch evx can generate unguarded evx-specific instructions.
If
you run that application on a pre-evx processor, those
instruc-
tions may get emulated (and emulated instructions can be up
to
1000 times slower than actual instructions).
+ -tune evx can generate evx-specific instructions, but those
are
always amask-guarded. That expands the code size but avoids
instruction emulation.
+ If you want the best performance possible on an evx
processor and
are not concerned about performance on earlier processors,
the
best choice would be -arch evx (which implies -tune evx).
+ If you want good performance on an evx processor but also
want the
application to run reasonably fast on earlier processors,
the best
choice would probably be -tune evx.
===============
-tune option
Instructs the optimizer to tune the application for a specific
version
of the Alpha hardware. This will not prevent the application from
run-
ning correctly on other versions of Alpha but it may run more
slowly
than generically-tuned code on those versions.
The option argument can be one of the following, which selects
instruc-
tion tuning appropriate for the listed processor(s):
generic
All Alpha processors. This is the default.
host
The processor on which the code is compiled.
ev4 The 21064, 21064A, and 21068 processors.
ev5,ev56
The 21164 processor. (Both EV5 and EV56 are numbered 21164.)
ev6 The 21264 processor.
ev67
The 21264A processor.
See also the -arch option for an explanation of the differences
between
-tune and -arch.
T.T.F.N.
William H. Magill
# Beige G3 [Rev A motherboard - 300 MHz 768 Meg] OS X 10.2.8
# Flat-panel iMac (2.1) [800MHz - Super Drive - 768 Meg] OS X 10.3.7
# PWS433a [Alpha 21164 Rev 7.2 (EV56)- 64 Meg] Tru64 5.1a
# XP1000 [Alpha 21264-3 (EV6) - 256 meg] FreeBSD 5.3
# XP1000 [Alpha 21264-A (EV 6.7) - 384 meg] FreeBSD 5.3
magill@mcgillsociety.org
magill@acm.org
magill@mac.com
whmagill@gmail.com
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