Hi Luis,
On 14/05/18 22:18, Luis Machado wrote:
Hi,
Here's an updated version of the patch (now reverted) that addresses the
previous bootstrap problem (signedness and long long/int conversion).
I've checked that it bootstraps properly on both aarch64-linux and x86_64-linux
and that tests look sane.
James, would you please give this one a try to see if you can still reproduce
PR85682? I couldn't reproduce it in multiple attempts.
The patch doesn't hit the regressions in PR85682 from what I can see.
I have a comment on the patch below.
Thanks,
Luis
On 01/22/2018 11:46 AM, Luis Machado wrote:
This patch adds a new option to control the minimum stride, for a memory
reference, after which the loop prefetch pass may issue software prefetch
hints for. There are two motivations:
* Make the pass less aggressive, only issuing prefetch hints for bigger strides
that are more likely to benefit from prefetching. I've noticed a case in cpu2017
where we were issuing thousands of hints, for example.
* For processors that have a hardware prefetcher, like Falkor, it allows the
loop prefetch pass to defer prefetching of smaller (less than the threshold)
strides to the hardware prefetcher instead. This prevents conflicts between
the software prefetcher and the hardware prefetcher.
I've noticed considerable reduction in the number of prefetch hints and
slightly positive performance numbers. This aligns GCC and LLVM in terms of
prefetch behavior for Falkor.
The default settings should guarantee no changes for existing targets. Those
are free to tweak the settings as necessary.
No regressions in the testsuite and bootstrapped ok on aarch64-linux.
Ok?
--- a/gcc/tree-ssa-loop-prefetch.c
+++ b/gcc/tree-ssa-loop-prefetch.c
@@ -992,6 +992,23 @@ prune_by_reuse (struct mem_ref_group *groups)
static bool
should_issue_prefetch_p (struct mem_ref *ref)
{
+ /* Some processors may have a hardware prefetcher that may conflict with
+ prefetch hints for a range of strides. Make sure we don't issue
+ prefetches for such cases if the stride is within this particular
+ range. */
+ if (cst_and_fits_in_hwi (ref->group->step)
+ && abs_hwi (int_cst_value (ref->group->step)) <
+ (HOST_WIDE_INT) PREFETCH_MINIMUM_STRIDE)
+ {
The '<' should go on the line below together with PREFETCH_MINIMUM_STRIDE.
Thanks,
Kyrill
