Re: "eliminate warn_on_slowpath()" change causes many gcc-3.2.3warnings

From: Linus Torvalds
Date: Sat Jan 17 2009 - 19:02:54 EST

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.

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