On 11/12/2013, at 5:17 am, Ramana Radhakrishnan <ramana....@googlemail.com> wrote:
> On Mon, Jul 1, 2013 at 5:31 PM, Paulo Matos <pma...@broadcom.com> wrote: >> Hi, >> >> Near the start of schedule_block, find_modifiable_mems is called if >> DONT_BREAK_DEPENDENCIES is not enabled for this scheduling pass. It seems on >> c6x backend currently uses this. >> However, it's quite strange that this is not a requirement for all backends >> since find_modifiable_mems, moves all my dependencies in SD_LIST_HARD_BACK >> to SD_LIST_SPEC_BACK even though I don't have DO_SPECULATION enabled. >> >> Since dependencies are accessed later on from try_ready (for example), I >> would have thought that it would be always good not to call >> find_modifiable_mems, given that it seems to 'literally' break dependencies. >> >> Is the behaviour of find_modifiable_mems a bug or somehow expected? "Breaking" a dependency in scheduler involves modification of instructions that would allow scheduler to move one instruction past the other. The most common case of breaking a dependency is "r2 = r1 + 4; r3 = [r2];" which can be transformed into "r3 = [r1 + 4]; r2 = r1 + 4;". Breaking a dependency is not ignoring it, speculatively or otherwise; it is an equivalent code transformation to allow scheduler more freedom to fill up CPU cycles. > > > It's funny how I've been trying to track down a glitch and ended up > asking the same question today. Additionally if I use > TARGET_SCHED_SET_SCHED_FLAGS on a port that doesn't use the selective > scheduler, this does nothing. Does anyone know why is this the default > for ports where we don't turn on selective scheduling and might need a > hook to turn this off ? SCHED_FLAGS is used to enable or disable various parts of GCC scheduler. On an architecture that supports speculative scheduling with recovery (IA64) it can turn this feature on or off. The documentation for various features of sched-rgn, sched-ebb and sel-sched is not the best and one will likely get weird artefacts by trying out non-default settings. I believe that only IA64 backend supports selective scheduling reliably. I've other ports trying out selective scheduling, but I don't know whether those efforts got positive results. -- Maxim Kuvyrkov www.kugelworks.com
