From: Ian Lance Taylor <[EMAIL PROTECTED]> Date: 25 Oct 2007 21:50:16 -0700
> We could have a global variable "lock_is_held". The > code could look like this: > > if (lock_is_held) > ++still_holding; > > If the earlier code should work in a multi-threaded environment, then > this code should work. I think it should be disallowed for this code to write to memory if lock_is_held is falso too. When you conditionalize moves amongst registers, this is %100 local execution state. But once you start conditionlizing memory references, in particular things that write, you are asking for serious trouble. I realize the desire to use conditional compututations on memory on platforms that can do it, but in these circumstances it is simply unreasonable to do so. At best the compiler can, if it wants, conditionalize the store. This is not a game or some fun theoretical discussion about language semantics. People will be harmed and lose lots of their own personal time debugging these kinds of things if GCC generates code like this. It's unreasonable, regardless of what the standards say. Sometimes the standards are wrong or fail to guide the implementation in these grey areas, and GCC should do what's best for the users in these cases. And I believe that this means to not do conditional computations on memory even though it might be more efficient in some situations.
