Messages in this thread | | | Date | Sun, 27 Oct 2013 12:35:21 -0700 | Subject | Re: [GIT PULL] locking fix | From | Linus Torvalds <> |
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On Sun, Oct 27, 2013 at 12:23 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > The *ONLY* thing it is testing for is "how much can the compiler > optimize this", and as such the *ONLY* thing it tests for is compiler > differences.
Side note: testing "can the compiler optimize this expression at compile time" is actually sometimes an interesting question, so it can be a valid thing to test.
But people should understand that the question is literally about THAT (ie visibility into compiler optimization) rather than about the value itself.
So generally, the only thing that a __builtin_constant_p() test can be used for is in *conjunction* with having an actual test for an actual value, and then having special-case logic that pertains to that value.
So for example, *this* is a valid test:
if (__builtin_constant_p(ww_ctx) && ww_ctx == NULL) { ... special compile-time shortcut for the NULL value .. } else { ... generic code that *also* handles the NULL value .. }
and it's useful for triggering a compile-time optimized code-sequence that is only true for NULL. But because __builtin_constant_p() is about "how well can the compiler optimize this", that "else" statement had better be able to handle the generic case too.
And yes, there are a few places where we do expect a certain minimal set of optimizations. So in some cases we *might* have the rule that the only valid use of NULL in a case like the above is when the pointer passed in is passed in as a constant. And then we might say "we rely on the compiler always returning true for __builtin_constant_p(NULL)", and then we might say "so the "generic" version of the code is guaranteed to never see NULL".
But notice how *different* that
__builtin_constant_p(ww_ctx) && ww_ctx == NULL
test is from
__builtin_constant_p(ww_ctx == NULL)
and really, the two tests are *fundamentally* really really different. The first one can make sense. While the second one is pure and utter garbage.
Linus
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