gcc on the x86_64

Today I thought I’d isolate the portion of simh that crashes 4.X BSD, when SIMH is built with GCC. I managed to find that the “op_ldpctx” and “op_mtpr” procedures of vax_cpu1.c become unstable when built with -O2 flags.

So I extracted the two procedures so I could then built the remainder of SIMH with GCC’s O2 flags. Before going too far, I first went to verify that the results would be consistent with the i386/32bit binaries…

However the results were.. interesting. By default SIMH will build with -O0 flags for the VAX giving the following dhrystone benchmark in 4.3 UWisc BSD:

Dhrystone(1.1) time for 500000 passes = 24
This machine benchmarks at 20833 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 24
This machine benchmarks at 20833 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 24
This machine benchmarks at 20833 dhrystones/second

24 seconds, not so hot. Then the same thing with -O1 flags…

Dhrystone(1.1) time for 500000 passes = 18
This machine benchmarks at 27777 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 18
This machine benchmarks at 27777 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 18
This machine benchmarks at 27777 dhrystones/second

18 seconds! pretty good, thats a 33% increase! Now for my hybrid -O2/-O1

Dhrystone(1.1) time for 500000 passes = 17
This machine benchmarks at 29411 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 17
This machine benchmarks at 29411 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 17
This machine benchmarks at 29411 dhrystones/second

A 5% increase over the -O1 … Nothing too big, but still an INCREASE.

Now for the real fun, I re-ran these bulid factors with the x86_64 compiler.

The -O0/-O0 combination came in at 19 seconds!!

Dhrystone(1.1) time for 500000 passes = 19
This machine benchmarks at 26315 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 19
This machine benchmarks at 26315 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 19
This machine benchmarks at 26315 dhrystones/second

This is pretty amazing considering everything we do should make it faster… The next thing I tried was the -O1/-O1 combination and go the following:

Dhrystone(1.1) time for 500000 passes = 12
This machine benchmarks at 41666 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 13
This machine benchmarks at 38461 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 13
This machine benchmarks at 38461 dhrystones/second

A 12.5 second average! This blew the doors off of the -O2/-O1 on the i386! Naturally I was hoping for at least a 5% increase going to the -O2/-O1 flags on the x86_64, however I got this…

Dhrystone(1.1) time for 500000 passes = 14
This machine benchmarks at 35714 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 13
This machine benchmarks at 38461 dhrystones/second
Dhrystone(1.1) time for 500000 passes = 12
This machine benchmarks at 41666 dhrystones/second

And I ran it numerous times, but the bottom line is that the O2 flags actually produces a SLOWER SIMH on OS X 10.6.4 then -O1 flags… The whole exercise is a tad perplexing to me, to say the least, but unless you bench it, you’ll never know if the compiler is getting confused somewhere, and going to throw a loop. I should also point out that SIMH will not run with full -O2 flags as there is something in the code that breaks under optimization so this very well may not hold true 100% of the time, but at the same point things still have to be tested…

Author: neozeed

I live in SE Asia, doing generic work, enjoying my life & family

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