Haicheng Wu via llvm-dev
2016-Apr-28 15:07 UTC
[llvm-dev] [AArch64/Cyclone] ZCZeroing Feature
Hi, I am trying to port FeatureZCZeroing from Cyclone to Kryo. Using immediate #0 to zero out W and X registers works great in Kryo. But using #0 to zero out float registers sometimes causes extra register spills or move instructions on either Cyclone or Kryo. Take the following C function as an example double foo(int n) { double r=-10000; for (int i=0;i<n;i++) { x = sin(i); r = max(r,x); } return r; } If compiled towards Cyclone, the loop body has one spill and two reloads as below .LBB0_1: // %for.body // =>This Inner Loop Header: Depth=1 str q0, [sp] // 16-byte Folded Spill ldr q0, [sp] // 16-byte Folded Reload bl sin fmaxnm d8, d8, d0 ldr q0, [sp] // 16-byte Folded Reload fadd d0, d0, d9 add w20, w20, #1 // =1 cmp w20, w19 b.lt .LBB0_1 If FeatureZCZeroing is disabled (together with FeatureZCRegMove) on Cyclone, the translated assembly does not have these load/store instructions: .LBB0_1: // %for.body // =>This Inner Loop Header: Depth=1 mov v0.16b, v8.16b bl sin fmaxnm d9, d9, d0 fadd d8, d8, d10 add w20, w20, #1 // =1 cmp w20, w19 b.lt .LBB0_1 PR27454 has an attached .ll test case. It would be nice if this problem could be solved so that Kryo and Cyclone could use the united method to zero out float registers. Best, Haicheng -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160428/124236da/attachment-0001.html>