On Tue, Sep 17, 2013 at 10:20 AM, Richard Biener <richard.guent...@gmail.com> wrote: > On Mon, Sep 16, 2013 at 10:24 PM, Xinliang David Li <davi...@google.com> > wrote: >> On Mon, Sep 16, 2013 at 3:13 AM, Richard Biener >> <richard.guent...@gmail.com> wrote: >>> On Fri, Sep 13, 2013 at 5:16 PM, Xinliang David Li <davi...@google.com> >>> wrote: >>>> On Fri, Sep 13, 2013 at 1:30 AM, Richard Biener >>>> <richard.guent...@gmail.com> wrote: >>>>> On Thu, Sep 12, 2013 at 10:31 PM, Xinliang David Li <davi...@google.com> >>>>> wrote: >>>>>> Currently -ftree-vectorize turns on both loop and slp vectorizations, >>>>>> but there is no simple way to turn on loop vectorization alone. The >>>>>> logic for default O3 setting is also complicated. >>>>>> >>>>>> In this patch, two new options are introduced: >>>>>> >>>>>> 1) -ftree-loop-vectorize >>>>>> >>>>>> This option is used to turn on loop vectorization only. option >>>>>> -ftree-slp-vectorize also becomes a first class citizen, and no funny >>>>>> business of Init(2) is needed. With this change, -ftree-vectorize >>>>>> becomes a simple alias to -ftree-loop-vectorize + >>>>>> -ftree-slp-vectorize. >>>>>> >>>>>> For instance, to turn on only slp vectorize at O3, the old way is: >>>>>> >>>>>> -O3 -fno-tree-vectorize -ftree-slp-vectorize >>>>>> >>>>>> With the new change it becomes: >>>>>> >>>>>> -O3 -fno-loop-vectorize >>>>>> >>>>>> >>>>>> To turn on only loop vectorize at O2, the old way is >>>>>> >>>>>> -O2 -ftree-vectorize -fno-slp-vectorize >>>>>> >>>>>> The new way is >>>>>> >>>>>> -O2 -ftree-loop-vectorize >>>>>> >>>>>> >>>>>> >>>>>> 2) -ftree-vect-loop-peeling >>>>>> >>>>>> This option is used to turn on/off loop peeling for alignment. In the >>>>>> long run, this should be folded into the cheap cost model proposed by >>>>>> Richard. This option is also useful in scenarios where peeling can >>>>>> introduce runtime problems: >>>>>> http://gcc.gnu.org/ml/gcc/2005-12/msg00390.html which happens to be >>>>>> common in practice. >>>>>> >>>>>> >>>>>> >>>>>> Patch attached. Compiler boostrapped. Ok after testing? >>>>> >>>>> I'd like you to split 1) and 2), mainly because I agree on 1) but not on >>>>> 2). >>>> >>>> Ok. Can you also comment on 2) ? >>> >>> I think we want to decide how granular we want to control the vectorizer >>> and using which mechanism. My cost-model re-org makes >>> ftree-vect-loop-version a no-op (basically removes it), so 2) looks like >>> a step backwards in this context. >> >> Using cost model to do a coarse grain control/configuration is >> certainly something we want, but having a fine grain control is still >> useful. >> >>> >>> So, can you summarize what pieces (including versioning) of the vectorizer >>> you'd want to be able to disable separately? >> >> Loop peeling seems to be the main one. There is also a correctness >> issue related. For instance, the following code is common in practice, >> but loop peeling wrongly assumes initial base-alignment and generates >> aligned mov instruction after peeling, leading to SEGV. Peeling is >> not something we can blindly turned on -- even when it is on, there >> should be a way to turn it off explicitly: >> >> char a[10000]; >> >> void foo(int n) >> { >> int* b = (int*)(a+n); >> int i = 0; >> for (; i < 1000; ++i) >> b[i] = 1; >> } >> >> int main(int argn, char** argv) >> { >> foo(argn); >> } > > But that's just a bug that should be fixed (looking into it).
Bug in the testcase. b[i] asserts that b is aligned to 'int', so this invokes undefined behavior if peeling cannot reach an alignment of 16. Richard.
