Messages in this thread | | | Date | Sat, 17 Jan 2009 16:10:50 -0800 | From | "H. Peter Anvin" <> | Subject | Re: "eliminate warn_on_slowpath()" change causes many gcc-3.2.3 warnings |
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Linus Torvalds wrote: > > On Sat, 17 Jan 2009, H. Peter Anvin wrote: >> At least on x86, the two ops should be the same cost? > > Not with the code Kyle had, which forces a memory load. > > But yes, with a constant address, it at least comes close. But with a > small explicit constant value, the compiler can often do even better. For > example, you can generate a 64-bit -1 in many ways, while a 64-bit random > address is much more work to generate. > > Of course, I don't know how much gcc takes advantage of this. Maybe it > always just generates a silly "movq" rather than being smarter about it > (eg "orl $-1,reg" can do it in four bytes, I think, because you can use a > single-byte constant). > > Of course, zero is even easier to generate, so NULL is the best constant > of all, but generally small integers are more amenable to optimization > than generic addresses. They're also generally easier to test for. >
When compiling with -O2 -mcmodel=kernel on gcc 4.3.2, you end up with the same 7-byte sequence:
4: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 7: R_X86_64_32S bluttan 10: 48 c7 c7 ff ff ff ff mov $0xffffffffffffffff,%rdi
With -Os -mcmodel=kernel, it's a bit better:
4: 48 c7 c7 00 00 00 00 mov $0x0,%rdi 7: R_X86_64_32S bluttan 10: 48 83 cf ff or $0xffffffffffffffff,%rdi
I would have expected it to have used leaq in the first case, but it's the same length (7 bytes) and probably has higher latencies.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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