On Thu, Dec 14, 2017 at 11:00:36AM +0000, Richard Biener wrote: > On Fri, Nov 17, 2017 at 11:17 PM, Richard Sandiford > <richard.sandif...@linaro.org> wrote: > > This patch adds runtime alias checks for loops with variable strides, > > so that we can vectorise them even without a restrict qualifier. > > There are several parts to doing this: > > > > 1) For accesses like: > > > > x[i * n] += 1; > > > > we need to check whether n (and thus the DR_STEP) is nonzero. > > vect_analyze_data_ref_dependence records values that need to be > > checked in this way, then prune_runtime_alias_test_list records a > > bounds check on DR_STEP being outside the range [0, 0]. > > > > 2) For accesses like: > > > > x[i * n] = x[i * n + 1] + 1; > > > > we simply need to test whether abs (n) >= 2. > > prune_runtime_alias_test_list looks for cases like this and tries > > to guess whether it is better to use this kind of check or a check > > for non-overlapping ranges. (We could do an OR of the two conditions > > at runtime, but that isn't implemented yet.) > > > > 3) Checks for overlapping ranges need to cope with variable strides. > > At present the "length" of each segment in a range check is > > represented as an offset from the base that lies outside the > > touched range, in the same direction as DR_STEP. The length > > can therefore be negative and is sometimes conservative. > > > > With variable steps it's easier to reaon about if we split > > this into two: > > > > seg_len: > > distance travelled from the first iteration of interest > > to the last, e.g. DR_STEP * (VF - 1) > > > > access_size: > > the number of bytes accessed in each iteration > > > > with access_size always being a positive constant and seg_len > > possibly being variable. We can then combine alias checks > > for two accesses that are a constant number of bytes apart by > > adjusting the access size to account for the gap. This leaves > > the segment length unchanged, which allows the check to be combined > > with further accesses. > > > > When seg_len is positive, the runtime alias check has the form: > > > > base_a >= base_b + seg_len_b + access_size_b > > || base_b >= base_a + seg_len_a + access_size_a > > > > In many accesses the base will be aligned to the access size, which > > allows us to skip the addition: > > > > base_a > base_b + seg_len_b > > || base_b > base_a + seg_len_a > > > > A similar saving is possible with "negative" lengths. > > > > The patch therefore tracks the alignment in addition to seg_len > > and access_size. > > > > Tested on aarch64-linux-gnu (with and without SVE), x86_64-linux-gnu > > and powerpc64le-linux-gnu. OK to install? > > Ok.
The AArch64 tests are OK. Thanks, James > > PS. This is the last SVE patch for GCC 8! Congratulations! These patches have seen some outstanding work; both on your part and equally on the part of all the reviewers!
