On Wed, Nov 12, 2014 at 11:43:08PM +0800, Chen Gang wrote: > > (I assume the > > compiler could do things like replace an intended load from memory with > > a constant load or even no load at all) > > > > Excuse me, my English is not quite well, I can not understand what you > said above. (If necessary, please help provide more details for it).
I am concerned that writing regs[TREG_TP] is "undefined behavior" according to the C standard. This expression is equivalent to *(regs + TREG_TP). The expression (regs + TREG_TP) does not result in a pointer to any element of regs[], so dereferencing it is undefined behavior. (Source: C99 draft standard WG14/N1256, annex J.2, "[The behavior is undefined if t]he operand of the unary * operator has an invalid value") That is why the compiler showed the original diagnostic, but the same logic that made the loop's behavior undefined also makes the expression regs[TREG_TP] undefined whereever it appears. None of this is a specific problem with your proposed patch. Rather, it is a suggestion that the whole structure's design needs to be revisited in light of compilers beginning to notice that regs[TREG_TP] is undefined behavior and change their generated code as a result. Unfortunately it looks like this header is also a part of the userspace API, so it can't simply be changed just in case all in-kernel uses are changed. Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
