[ARM] Reduce loop unroll when low overhead branching is available

For processors with low overhead branching (LOB), runtime unrolling the
innermost loop is often detrimental to performance.
In these cases the loop remainder gets unrolled into a series of
compare-and-jump blocks, which in deeply nested loops get executed multiple
times, negating the benefits of LOB.
This is particularly noticable when the loop trip count of the innermost
loop varies withing the outer loop, such as in the case of triangular matrix
decompositions.

In these cases we will prefer to not unroll the innermost loop, with the
intention for it to be executed as a low overhead loop.

Author: VladiKrapp-Arm

llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp

@@ -2592,11 +2592,23 @@ void ARMTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
       return;
   }
 
+  bool Runtime = true;
+  if (ST->hasLOB()) {
+    if (SE.hasLoopInvariantBackedgeTakenCount(L)) {
+      const auto *BETC = SE.getBackedgeTakenCount(L);
+      auto *Outer = L->getOutermostLoop();
+      if ((L != Outer && Outer != L->getParentLoop()) ||
+          (L != Outer && BETC && !SE.isLoopInvariant(BETC, Outer))) {
+        Runtime = false;
+      }
+    }
+  }
+
   LLVM_DEBUG(dbgs() << "Cost of loop: " << Cost << "\n");
   LLVM_DEBUG(dbgs() << "Default Runtime Unroll Count: " << UnrollCount << "\n");
 
   UP.Partial = true;
-  UP.Runtime = true;
+  UP.Runtime = Runtime;
   UP.UnrollRemainder = true;
   UP.DefaultUnrollRuntimeCount = UnrollCount;
   UP.UnrollAndJam = true;

llvm/test/Transforms/LoopUnroll/ARM/lob-unroll.ll

@@ -1,17 +1,23 @@
+; RUN: opt -mcpu=cortex-m7 -mtriple=thumbv8.1m.main -passes=loop-unroll -S  %s -o - | FileCheck %s --check-prefix=NLOB
 ; RUN: opt -mcpu=cortex-m55 -mtriple=thumbv8.1m.main -passes=loop-unroll -S  %s -o - | FileCheck %s --check-prefix=LOB
 
 ; This test checks behaviour of loop unrolling on processors with low overhead branching available 
 
-; LOB-CHECK-LABEL: for.body{{.*}}.prol
-; LOB-COUNT-1:     fmul fast float 
-; LOB-CHECK-LABEL: for.body{{.*}}.prol.1
-; LOB-COUNT-1:     fmul fast float 
-; LOB-CHECK-LABEL: for.body{{.*}}.prol.2
-; LOB-COUNT-1:     fmul fast float 
-; LOB-CHECK-LABEL: for.body{{.*}}
-; LOB-COUNT-4:     fmul fast float 
+; NLOB-LABEL: for.body{{.*}}.prol:
+; NLOB-COUNT-1:     fmul fast float 
+; NLOB-LABEL: for.body{{.*}}.prol.1:
+; NLOB-COUNT-1:     fmul fast float 
+; NLOB-LABEL: for.body{{.*}}.prol.2:
+; NLOB-COUNT-1:     fmul fast float 
+; NLOB-LABEL: for.body{{.*}}:
+; NLOB-COUNT-4:     fmul fast float 
+; NLOB-NOT:     fmul fast float 
+
+; LOB-LABEL: for.body{{.*}}:
+; LOB:     fmul fast float 
 ; LOB-NOT:     fmul fast float 
 
+
 ; Function Attrs: nofree norecurse nosync nounwind memory(argmem: readwrite)
 define dso_local void @test(i32 noundef %n, ptr nocapture noundef %pA) local_unnamed_addr #0 {
 entry:
@@ -20,7 +26,7 @@ entry:
 
 for.cond.loopexit:                                ; preds = %for.cond6.for.cond.cleanup8_crit_edge.us, %for.body
   %exitcond49.not = icmp eq i32 %add, %n
-  br i1 %exitcond49.not, label %for.cond.cleanup, label %for.body
+  br i1 %exitcond49.not, label %for.cond.cleanup, label %for.body, !llvm.loop !0
 
 for.cond.cleanup:                                 ; preds = %for.cond.loopexit, %entry
   ret void
@@ -61,3 +67,6 @@ for.cond6.for.cond.cleanup8_crit_edge.us:         ; preds = %for.body9.us
   br i1 %exitcond48.not, label %for.cond.loopexit, label %for.cond6.preheader.us
 }
 
+!0 = distinct !{!0, !1, !2}
+!1 = !{!"llvm.loop.mustprogress"}
+!2 = !{!"llvm.loop.unroll.disable"}