Index: llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -2188,12 +2188,39 @@
     }
   }
 
+  // On v6m cores, there are very few registers available. We can easily end up
+  // spilling and reloading more registers in an unrolled loop. Look at the
+  // number of LCSSA phis as a rough measure of how many registers will need to
+  // be live out of the loop, reducing the default unroll count if more than 1
+  // value is needed.  In the long run, all of this should be being learnt by a
+  // machine.
+  unsigned UnrollCount = 4;
+  if (ST->isThumb1Only()) {
+    unsigned ExitingValues = 0;
+    SmallVector<BasicBlock *, 4> ExitBlocks;
+    L->getExitBlocks(ExitBlocks);
+    for (auto *Exit : ExitBlocks) {
+      // Count the number of LCSSA phis. Exclude values coming from GEP's as
+      // only the last is expected to be needed for address operands.
+      unsigned LiveOuts = count_if(Exit->phis(), [](auto &PH) {
+        return PH.getNumOperands() != 1 ||
+               !isa<GetElementPtrInst>(PH.getOperand(0));
+      });
+      ExitingValues = ExitingValues < LiveOuts ? LiveOuts : ExitingValues;
+    }
+    if (ExitingValues)
+      UnrollCount /= ExitingValues;
+    if (UnrollCount <= 1)
+      return;
+  }
+
   LLVM_DEBUG(dbgs() << "Cost of loop: " << Cost << "\n");
+  LLVM_DEBUG(dbgs() << "Default Runtime Unroll Count: " << UnrollCount << "\n");
 
   UP.Partial = true;
   UP.Runtime = true;
   UP.UnrollRemainder = true;
-  UP.DefaultUnrollRuntimeCount = 4;
+  UP.DefaultUnrollRuntimeCount = UnrollCount;
   UP.UnrollAndJam = true;
   UP.UnrollAndJamInnerLoopThreshold = 60;
 
Index: llvm/test/Transforms/LoopUnroll/ARM/v6munroll.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopUnroll/ARM/v6munroll.ll
@@ -0,0 +1,143 @@
+; RUN: opt -mtriple=arm-none-none-eabi -mcpu=cortex-m23 -loop-unroll -unroll-runtime-multi-exit -S %s -o - | FileCheck %s
+
+; This loop has too many live outs, and should not be unrolled under v6m.
+; CHECK-LABEL: multiple_liveouts
+; CHECK: for.body
+; CHECK: br i1 %cmp.not, label %for.cond.cleanup.loopexit, label %for.body
+define void @multiple_liveouts(i32* %x, i32* %y, i32* %d, i32 %n) {
+entry:
+  %0 = load i32, i32* %d, align 4
+  %arrayidx1 = getelementptr inbounds i32, i32* %d, i32 1
+  %1 = load i32, i32* %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds i32, i32* %d, i32 2
+  %2 = load i32, i32* %arrayidx2, align 4
+  %arrayidx3 = getelementptr inbounds i32, i32* %d, i32 3
+  %3 = load i32, i32* %arrayidx3, align 4
+  %cmp.not58 = icmp eq i32 %n, 0
+  br i1 %cmp.not58, label %for.cond.cleanup, label %for.body.preheader
+
+for.body.preheader:                               ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.body
+  %x.addr.065 = phi i32* [ %incdec.ptr, %for.body ], [ %x, %for.body.preheader ]
+  %y.addr.064 = phi i32* [ %incdec.ptr25, %for.body ], [ %y, %for.body.preheader ]
+  %res00.063 = phi i32 [ %add, %for.body ], [ %0, %for.body.preheader ]
+  %rhs_cols_idx.062 = phi i32 [ %dec, %for.body ], [ %n, %for.body.preheader ]
+  %res11.061 = phi i32 [ %add24, %for.body ], [ %3, %for.body.preheader ]
+  %res10.060 = phi i32 [ %add20, %for.body ], [ %2, %for.body.preheader ]
+  %res01.059 = phi i32 [ %add14, %for.body ], [ %1, %for.body.preheader ]
+  %4 = load i32, i32* %x.addr.065, align 4
+  %arrayidx5 = getelementptr inbounds i32, i32* %x.addr.065, i32 %n
+  %5 = load i32, i32* %arrayidx5, align 4
+  %6 = load i32, i32* %y.addr.064, align 4
+  %conv9 = and i32 %6, 255
+  %conv10 = and i32 %4, 255
+  %mul = mul nuw nsw i32 %conv9, %conv10
+  %add = add nsw i32 %mul, %res00.063
+  %conv12 = and i32 %5, 255
+  %mul13 = mul nuw nsw i32 %conv9, %conv12
+  %add14 = add nsw i32 %mul13, %res01.059
+  %arrayidx15 = getelementptr inbounds i32, i32* %y.addr.064, i32 %n
+  %7 = load i32, i32* %arrayidx15, align 4
+  %conv17 = and i32 %7, 255
+  %mul19 = mul nuw nsw i32 %conv17, %conv10
+  %add20 = add nsw i32 %mul19, %res10.060
+  %mul23 = mul nuw nsw i32 %conv17, %conv12
+  %add24 = add nsw i32 %mul23, %res11.061
+  %incdec.ptr = getelementptr inbounds i32, i32* %x.addr.065, i32 1
+  %incdec.ptr25 = getelementptr inbounds i32, i32* %y.addr.064, i32 1
+  %dec = add nsw i32 %rhs_cols_idx.062, -1
+  %cmp.not = icmp eq i32 %dec, 0
+  br i1 %cmp.not, label %for.cond.cleanup.loopexit, label %for.body
+
+for.cond.cleanup.loopexit:                        ; preds = %for.body
+  %add.lcssa = phi i32 [ %add, %for.body ]
+  %add14.lcssa = phi i32 [ %add14, %for.body ]
+  %add20.lcssa = phi i32 [ %add20, %for.body ]
+  %add24.lcssa = phi i32 [ %add24, %for.body ]
+  br label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond.cleanup.loopexit, %entry
+  %res01.0 = phi i32 [ %1, %entry ], [ %add14.lcssa, %for.cond.cleanup.loopexit ]
+  %res10.0 = phi i32 [ %2, %entry ], [ %add20.lcssa, %for.cond.cleanup.loopexit ]
+  %res11.0 = phi i32 [ %3, %entry ], [ %add24.lcssa, %for.cond.cleanup.loopexit ]
+  %res00.0 = phi i32 [ %0, %entry ], [ %add.lcssa, %for.cond.cleanup.loopexit ]
+  store i32 %res00.0, i32* %d, align 4
+  store i32 %res01.0, i32* %arrayidx1, align 4
+  store i32 %res10.0, i32* %arrayidx2, align 4
+  store i32 %res11.0, i32* %arrayidx3, align 4
+  ret void
+}
+
+; This loop has both multiple exit blocks and multiple liveouts
+; CHECK-LABEL: multiple_liveouts_doubleexit
+; CHECK: for.body
+; CHECK: br i1 %cmp.not, label %cleanup22.loopexit2, label %for.body
+define void @multiple_liveouts_doubleexit(i32 %n, i32* %x, i32* %y, i32* %z) {
+entry:
+  %cmp.not55 = icmp eq i32 %n, 0
+  br i1 %cmp.not55, label %cleanup22, label %for.body.preheader
+
+for.body.preheader:                               ; preds = %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.body.preheader, %for.inc
+  %x.addr.062 = phi i32* [ %incdec.ptr, %for.inc ], [ %x, %for.body.preheader ]
+  %y.addr.061 = phi i32* [ %incdec.ptr19, %for.inc ], [ %y, %for.body.preheader ]
+  %rhs_cols_idx.060 = phi i32 [ %dec, %for.inc ], [ %n, %for.body.preheader ]
+  %res11.059 = phi i32 [ %add18, %for.inc ], [ 0, %for.body.preheader ]
+  %res10.058 = phi i32 [ %add16, %for.inc ], [ 0, %for.body.preheader ]
+  %res01.057 = phi i32 [ %add8, %for.inc ], [ 0, %for.body.preheader ]
+  %res00.056 = phi i32 [ %add, %for.inc ], [ 0, %for.body.preheader ]
+  %0 = load i32, i32* %x.addr.062, align 4
+  %1 = load i32, i32* %y.addr.061, align 4
+  %conv5 = and i32 %1, 255
+  %conv6 = and i32 %0, 255
+  %mul = mul nuw nsw i32 %conv5, %conv6
+  %add = add nuw nsw i32 %mul, %res00.056
+  %add8 = add nuw nsw i32 %conv5, %res01.057
+  %cmp9 = icmp ugt i32 %add8, 100
+  br i1 %cmp9, label %cleanup22.loopexit, label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %arrayidx11 = getelementptr inbounds i32, i32* %y.addr.061, i32 %n
+  %2 = load i32, i32* %arrayidx11, align 4
+  %conv13 = and i32 %2, 255
+  %mul15 = mul nuw nsw i32 %conv13, %conv6
+  %add16 = add nuw nsw i32 %mul15, %res10.058
+  %add18 = add nuw nsw i32 %conv13, %res11.059
+  %incdec.ptr = getelementptr inbounds i32, i32* %x.addr.062, i32 1
+  %incdec.ptr19 = getelementptr inbounds i32, i32* %y.addr.061, i32 1
+  %dec = add nsw i32 %rhs_cols_idx.060, -1
+  %cmp.not = icmp eq i32 %dec, 0
+  br i1 %cmp.not, label %cleanup22.loopexit2, label %for.body
+
+cleanup22.loopexit:                               ; preds = %for.body
+  %add.lcssa = phi i32 [ %add, %for.body ]
+  %add8.lcssa = phi i32 [ %add8, %for.body ]
+  %res10.0.lcssa.ph = phi i32 [ %res10.058, %for.body ]
+  %res11.0.lcssa.ph = phi i32 [ %res11.059, %for.body ]
+  br label %cleanup22
+
+cleanup22.loopexit2:                               ; preds = %for.inc
+  %add.lcssa2 = phi i32 [ %add, %for.inc ]
+  %add8.lcssa2 = phi i32 [ %add8, %for.inc ]
+  %res10.0.lcssa.ph2 = phi i32 [ %add16, %for.inc ]
+  %res11.0.lcssa.ph2 = phi i32 [ %add18, %for.inc ]
+  br label %cleanup22
+
+cleanup22:                                        ; preds = %cleanup22.loopexit, %entry
+  %res10.0.lcssa = phi i32 [ 0, %entry ], [ %res10.0.lcssa.ph, %cleanup22.loopexit ], [ %res10.0.lcssa.ph2, %cleanup22.loopexit2 ]
+  %res11.0.lcssa = phi i32 [ 0, %entry ], [ %res11.0.lcssa.ph, %cleanup22.loopexit ], [ %res11.0.lcssa.ph2, %cleanup22.loopexit2 ]
+  %res00.1 = phi i32 [ 0, %entry ], [ %add.lcssa, %cleanup22.loopexit ], [ %add.lcssa2, %cleanup22.loopexit2 ]
+  %res01.1 = phi i32 [ 0, %entry ], [ %add8.lcssa, %cleanup22.loopexit ], [ %add8.lcssa2, %cleanup22.loopexit2 ]
+  store i32 %res00.1, i32* %z, align 4
+  %arrayidx24 = getelementptr inbounds i32, i32* %z, i32 1
+  store i32 %res01.1, i32* %arrayidx24, align 4
+  %arrayidx25 = getelementptr inbounds i32, i32* %z, i32 2
+  store i32 %res10.0.lcssa, i32* %arrayidx25, align 4
+  %arrayidx26 = getelementptr inbounds i32, i32* %z, i32 3
+  store i32 %res11.0.lcssa, i32* %arrayidx26, align 4
+  ret void
+}