http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50457
--- Comment #16 from Oleg Endo <olegendo at gcc dot gnu.org> 2012-11-12 20:57:12 UTC --- Another thing that might be useful for dealing with atomics on SH1* and SH2* targets is to make the compiler generate the rewind sequences in interrupt handler functions. On SH3* and SH4* targets, the interrupt handling jump table is usually designed in such a way, that it can contain (small) setup code before a user specified interrupt handling function is invoked. However, on SH1* and SH2* targets, the processor directly jumps to the user specified interrupt handling function via the interrupt jump table. It would be possible to have two such jump tables, where code in the first table handles atomic sequence rewinding and then jumps to the second (actual) interrupt handling function specified by the user. This is not necessary, if the compiler can insert the atomic sequence rewind code snippet (according to the currently selected atomic model) at the beginning of an interrupt handler function. Moreover, the compiler can omit the atomic rewind sequence if... * an ISR doesn't call any other (non-inlined) functions * an ISR doesn't contain any atomic sequences itself For example, a very simple timer ISR that just increments the high word of the system's free running timer doesn't need to handle any atomics. Another example would be an SCI/SCIF ISR that receives/sends data to/from a buffer. In this case, the ISR probably has two code execution paths. While the receive/send operation is still in progress the code path is probably very short and updates only a few internal variables, which doesn't need any atomic handling. When the receive/send operation has completed, the completion condition has to be propagated to user code and most likely requires calling other functions and thus handling atomic sequences. Ideally, the compiler would insert atomic handling code in the ISR only to those paths, which actually need it. This would reduce interrupt handling time. I'd propose to add a new function attribute 'handle_atomics' that can be used in combination with the 'interrupt_handler' function attribute for this. SH3* and SH4* targets could also benefit from this.
