Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints

[David Howells]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> > So, using the "+" constraint modifier for memory, like "+m" is bogus.
> > We must simply specify "=m" which handles the case correctly.
> 
> No. This is wrong.

Agreed.

> "=m" means that the new value of the memory location is *set*.
> 
> Which means that gcc will potentially optimize away any previous stores to 
> that memory location.
> 
> And yes, it happens, and yes, we've seen it.

I had a lot of "fun" with this when mucking around with the asm-optimised R/W
semaphores.

David
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