[MVE] Don't try to unroll vectorised MVE loops

Due to the nature of the beat system in the MVE architecture, along with tail
predication and low-overhead loops, unrolling has less benefit compared to
normal loops. You can not, for example, hide the latency of a load with other
instructions as you can for scalar code. Preventing unrolling also makes the
code easier to read and reason about.

So if a loop contains vector code, don't enable the runtime unrolling. At least
for the time being.

Differential Revision: https://reviews.llvm.org/D65803

llvm-svn: 368530

2 files changed, 132 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
index fb5b08df5d3..9f5f40c8b5b 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -893,6 +893,11 @@ void ARMTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
         }
         return;
       }
+      // Don't unroll vectorised loop. MVE does not benefit from it as much as
+      // scalar code.
+      if (I.getType()->isVectorTy())
+        return;
+
       SmallVector<const Value*, 4> Operands(I.value_op_begin(),
                                             I.value_op_end());
       Cost += getUserCost(&I, Operands);
diff --git a/llvm/test/Transforms/LoopUnroll/ARM/mve-nounroll.ll b/llvm/test/Transforms/LoopUnroll/ARM/mve-nounroll.ll
new file mode 100644
index 00000000000..2067969a994
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/ARM/mve-nounroll.ll
@@ -0,0 +1,127 @@
+; RUN: opt -mtriple=thumbv8.1m.main -mattr=+mve.fp -loop-unroll -S < %s -o - | FileCheck %s
+
+; CHECK-LABEL: @loopfn
+; CHECK: vector.body:
+; CHECK:   br i1 %7, label %middle.block, label %vector.body, !llvm.loop !0
+; CHECK: middle.block:
+; CHECK:   br i1 %cmp.n, label %for.cond.cleanup, label %for.body.preheader13
+; CHECK: for.body:
+; CHECK:   br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body, !llvm.loop !2
+
+define void @loopfn(float* %s1, float* %s2, float* %d, i32 %n) {
+entry:
+  %cmp10 = icmp sgt i32 %n, 0
+  br i1 %cmp10, label %for.body.preheader, label %for.cond.cleanup
+
+for.body.preheader:                               ; preds = %entry
+  %min.iters.check = icmp ult i32 %n, 4
+  br i1 %min.iters.check, label %for.body.preheader13, label %vector.ph
+
+for.body.preheader13:                             ; preds = %middle.block, %for.body.preheader
+  %i.011.ph = phi i32 [ 0, %for.body.preheader ], [ %n.vec, %middle.block ]
+  br label %for.body
+
+vector.ph:                                        ; preds = %for.body.preheader
+  %n.vec = and i32 %n, -4
+  br label %vector.body
+
+vector.body:                                      ; preds = %vector.body, %vector.ph
+  %index = phi i32 [ 0, %vector.ph ], [ %index.next, %vector.body ]
+  %0 = getelementptr inbounds float, float* %s1, i32 %index
+  %1 = bitcast float* %0 to <4 x float>*
+  %wide.load = load <4 x float>, <4 x float>* %1, align 4
+  %2 = getelementptr inbounds float, float* %s2, i32 %index
+  %3 = bitcast float* %2 to <4 x float>*
+  %wide.load12 = load <4 x float>, <4 x float>* %3, align 4
+  %4 = fadd fast <4 x float> %wide.load12, %wide.load
+  %5 = getelementptr inbounds float, float* %d, i32 %index
+  %6 = bitcast float* %5 to <4 x float>*
+  store <4 x float> %4, <4 x float>* %6, align 4
+  %index.next = add i32 %index, 4
+  %7 = icmp eq i32 %index.next, %n.vec
+  br i1 %7, label %middle.block, label %vector.body, !llvm.loop !0
+
+middle.block:                                     ; preds = %vector.body
+  %cmp.n = icmp eq i32 %n.vec, %n
+  br i1 %cmp.n, label %for.cond.cleanup, label %for.body.preheader13
+
+for.cond.cleanup.loopexit:                        ; preds = %for.body
+  br label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond.cleanup.loopexit, %middle.block, %entry
+  ret void
+
+for.body:                                         ; preds = %for.body.preheader13, %for.body
+  %i.011 = phi i32 [ %add3, %for.body ], [ %i.011.ph, %for.body.preheader13 ]
+  %arrayidx = getelementptr inbounds float, float* %s1, i32 %i.011
+  %8 = load float, float* %arrayidx, align 4
+  %arrayidx1 = getelementptr inbounds float, float* %s2, i32 %i.011
+  %9 = load float, float* %arrayidx1, align 4
+  %add = fadd fast float %9, %8
+  %arrayidx2 = getelementptr inbounds float, float* %d, i32 %i.011
+  store float %add, float* %arrayidx2, align 4
+  %add3 = add nuw nsw i32 %i.011, 1
+  %exitcond = icmp eq i32 %add3, %n
+  br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body, !llvm.loop !2
+}
+
+
+
+; CHECK-LABEL: @nested
+; CHECK: for.outer:
+; CHECK:   br label %vector.body
+; CHECK: vector.body:
+; CHECK:   br i1 %8, label %for.latch, label %vector.body, !llvm.loop !0
+; CHECK: for.latch:
+; CHECK:   br i1 %exitcond34, label %for.cond.cleanup.loopexit, label %for.outer
+
+define void @nested(float* %s1, float* %s2, float* %d, i32 %n) {
+entry:
+  %cmp31 = icmp eq i32 %n, 0
+  br i1 %cmp31, label %for.cond.cleanup, label %for.outer.preheader
+
+for.outer.preheader:                 ; preds = %entry
+  %min.iters.check = icmp ult i32 %n, 4
+  %n.vec = and i32 %n, -4
+  %cmp.n = icmp eq i32 %n.vec, %n
+  br label %for.outer
+
+for.outer:                           ; preds = %for.outer.preheader, %for.cond1.for.cond.cleanup3_crit_edge.us
+  %j.032.us = phi i32 [ %inc.us, %for.latch ], [ 0, %for.outer.preheader ]
+  %mul.us = mul i32 %j.032.us, %n
+  br label %vector.body
+
+vector.body:                                      ; preds = %for.outer, %vector.body
+  %index = phi i32 [ %index.next, %vector.body ], [ 0, %for.outer ]
+  %0 = add i32 %index, %mul.us
+  %1 = getelementptr inbounds float, float* %s1, i32 %0
+  %2 = bitcast float* %1 to <4 x float>*
+  %wide.load = load <4 x float>, <4 x float>* %2, align 4
+  %3 = getelementptr inbounds float, float* %s2, i32 %0
+  %4 = bitcast float* %3 to <4 x float>*
+  %wide.load35 = load <4 x float>, <4 x float>* %4, align 4
+  %5 = fadd fast <4 x float> %wide.load35, %wide.load
+  %6 = getelementptr inbounds float, float* %d, i32 %0
+  %7 = bitcast float* %6 to <4 x float>*
+  store <4 x float> %5, <4 x float>* %7, align 4
+  %index.next = add i32 %index, 4
+  %8 = icmp eq i32 %index.next, %n.vec
+  br i1 %8, label %for.latch, label %vector.body, !llvm.loop !0
+
+for.latch:                           ; preds = %vector.body, %for.outer
+  %i.030.us.ph = phi i32 [ %n.vec, %vector.body ]
+  %inc.us = add nuw i32 %j.032.us, 1
+  %exitcond34 = icmp eq i32 %inc.us, %n
+  br i1 %exitcond34, label %for.cond.cleanup.loopexit, label %for.outer
+
+for.cond.cleanup.loopexit:
+  br label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond1.for.cond.cleanup3_crit_edge.us, %entry
+  ret void
+}
+
+!0 = distinct !{!0, !1}
+!1 = !{!"llvm.loop.isvectorized", i32 1}
+!2 = distinct !{!2, !3, !1}
+!3 = !{!"llvm.loop.unroll.runtime.disable"}