Date: Sat, 28 Oct 2000 03:08:24 -0600 From: Chris Wasser <cwasser@v-wave.com> To: freebsd-questions@FreeBSD.ORG Subject: FreeBSD & GCC Optimizations & Other Suchness.. Message-ID: <20001028030824.A36767@area51.v-wave.com>
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Now I'm fully aware of the ramifications of doing this, however I'm one of those guys who says, "let's go for it" so here I am. My problem is I can't quite understand what the manpage is making out on this: http://gcc.gnu.org/onlinedocs/gcc_2.html#SEC34 is my reference. "Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If `-mpreferred-stack-boundary' is not specified, the default is 4 (16 bytes or 128 bits). The stack is required to be aligned on a 4 byte boundary. On Pentium and PentiumPro, double and long double values should be aligned to an 8 byte boundary (see `-malign-double') or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 suffers similar penalties if it is not 16 byte aligned." A little further up it states (in the -malign descriptions): "Align the start of functions to a 2 raised to num byte boundary. If `-malign-functions' is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486." If I understand this correctly, PII's should have the stack aligned to 8 bytes (2^3) and PIII's should be aligned to 16 bytes (2^4), the kernel compile is set to the default "preferred" alignment of 2. However, what about the -malign functions? PII/PIII according to the manpage state that -malign-double should be invoked for preformance reasons, but what about the actual values for the -malign directives: 2 = 386 4 = 486 What about 686+ ? (Such as PII/PIII/Athlon), using this information I came up with: -O3 -mpentiumpro -march=pentiumpro -malign-double -malign-loops=3 -malign-jumps=3 -malign-functions=3 -mpreferred-stack-boundary=3 Assuming that GCC takes the =3 as 2^3 = 8 Bytes for PII, or =4 as 2^4 for 16 bytes for PIII. Is this correct? (Both for the align directives and the stack-boundary) My basic question is for PII, PIII and Athlon optimization strings (and yes, I'm aware it may break code) what should I be using? I'm just sticking to these functions as I've had bad things happen to me by going too far in the optimizations (-fomit-frame-pointer, etc) and figured I'd try to get some input before I get too brave and strike out on my own :) Also, while I was searching the Net for information on this, I stumbled across a Linux kernel diff containing MMX and KNI patches as well as other Celeron/PII/PIII optimizations, is FreeBSD equipped with these yet? Such as: - Disable PII/PIII Serial Number at bootup + This makes the kernel disable the CPUID serial number that is + embedded on the new PIII CPUs at bootup. - Enable PII/PIII Extended Fast FPU save and restore support + This enables use of the new PII/PIII FXSAVE/FXRSTOR support. This + item is required to make use of the new PIII 128bit XMM registers. + It is safe to leave this enabled all the time. - Enable CPU Specific (MMX/MMX2) Optimizations + This enables use of the MMX registers and 128bit MMX2 registers + on CPUs that can support the new instructions (Pentium/AMD K6 or + newer). In order to support the Pentium III 128 bit XMM registers + you must enable both this and PII/PIII Extended Fast FPU save + support. It is safe to leave this enabled all the time. (Also, should be noted that the agp device [device agp] for the kernel config is not listed in LINT -- found it by browsing the mailinglists) I'm getting sidetracked here :) Any input on this would be greatly appriciated. If possible, please CC a reply to my email address (so I won't miss it in the flurry of messages that is the FreeBSD mailinglist(s) :)) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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