Messages in this thread | | | Date | Thu, 04 Feb 2010 07:16:14 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT) |
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On 02/04/2010 07:10 AM, Borislav Petkov wrote: >> >> Arguably the "best" option is to have the alternative being a jump to an >> out-of-line stub which does the necessary parameter marshalling before >> calling a stub. This technique is already used in a few other places. > > Ok, here's a first alpha prototype and completely untested. The asm > output looks ok though. I've added separate 32-bit and 64-bit helpers in > order to dispense with the if-else tests. The hw-popcnt versions are the > opcodes for "popcnt %eax, %eax" and "popcnt %rax, %rax", respectively, > so %rAX has to be preloaded with the bitmask and the computed value > has to be retrieved from there afterwards. And yes, it looks not that > elegant so I'm open for suggestions. > > The good thing is, this should work on any toolchain since we don't rely > on the compiler to know about popcnt and we're protected by CPUID flag > so that the hw-popcnt version is used only on processors which support > it. > > Please take a good look and let me know what do you guys think. >
I think you misunderstood me. The idea was to use the alternatives mechanism to put the popcnt instruction inline instead of the call.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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