Messages in this thread | | | Date | Sat, 2 May 2015 14:39:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86: Optimize variable_test_bit() |
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On Fri, May 01, 2015 at 01:49:52PM -0700, Linus Torvalds wrote: > On Fri, May 1, 2015 at 12:02 PM, Vladimir Makarov <vmakarov@redhat.com> wrote: > > > > GCC RA is a major reason to prohibit output operands for asm goto. > > Hmm.. Thinking some more about it, I think that what would actually > work really well at least for the kernel is: > > (a) allow *memory* operands (ie "=m") as outputs and having them be > meaningful even at any output labels (obviously with the caveat that > the asm instructions that write to memory would have to happen before > the branch ;) > > This covers the somewhat common case of having magic instructions that > result in conditions that can't be tested at a C level. Things like > "bit clear and test" on x86 (with or without the lock) .
Would not something like:
static inline bool __test_and_clear_bit(long nr, volatile unsigned long *addr) { bool oldbit;
asm volatile ("btr %2, %1" : "CF" (oldbit), "+m" (*addr) : "Ir" (nr));
return oldbit; }
Be the far better solution for this? Bug 59615 comment 7 states that they actually modeled the flags in the .md file, so the above should be possible to implement.
Now GCC can decide to use "sbb %0, %0" to convert CF into a register value or use "jnc" / "jc" for branches, depending on what __test_and_clear_bit() was used for.
We don't have to (ab)use asm goto for these things anymore; furthermore I think the above will naturally work with our __builtin_expect() hints, whereas the asm goto stuff has a hard time with that (afaik).
That's not to say output operants for asm goto would not still be useful for other things (like your EXTABLE example).
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