kvcache: Run tests both with and without PermutedV

The causal cache can store data differently depending on what is
best for the backend. We should run tests both ways.

kvcache/causal_test.go

 package kvcache
 
 import (
+	"fmt"
 	"math"
 	"slices"
 	"testing"
@@ -20,8 +21,17 @@ type testCase struct {
 	expectedMask  []float32
 }
 
+func runPermutedVariants(t *testing.T, fn func(t *testing.T, backend *testBackend)) {
+	t.Helper()
+	for _, permuted := range []bool{false, true} {
+		t.Run(fmt.Sprintf("PermutedV=%t", permuted), func(t *testing.T) {
+			fn(t, &testBackend{permutedV: permuted})
+		})
+	}
+}
+
 func TestStore(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewCausalCache(nil)
 		defer cache.Close()
 
@@ -51,10 +61,11 @@ func TestStore(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestSWA(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewSWACache(1, nil)
 		defer cache.Close()
 
@@ -94,10 +105,11 @@ func TestSWA(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestSWASeparateBatches(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewSWACache(1, nil)
 		defer cache.Close()
 
@@ -174,10 +186,11 @@ func TestSWASeparateBatches(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestSWAMem(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewSWAMemCache(1, 3, nil)
 		defer cache.Close()
 
@@ -217,19 +230,20 @@ func TestSWAMem(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestChunkedAttention(t *testing.T) {
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewChunkedAttentionCache(2, nil)
 		defer cache.Close()
 
-	var b testBackend
-	cache.Init(&b, ml.DTypeF16, 1, 16, 16)
+		cache.Init(backend, ml.DTypeF16, 1, 16, 16)
 
 		x := float32(math.Inf(-1))
 
 		testCache(
-		t, &b, cache,
+			t, backend, cache,
 			[]testCase{
 				{
 					name:          "FirstBatch",
@@ -275,10 +289,11 @@ func TestChunkedAttention(t *testing.T) {
 				},
 			},
 		)
+	})
 }
 
 func TestSequences(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewCausalCache(nil)
 		defer cache.Close()
 
@@ -308,10 +323,11 @@ func TestSequences(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestRemove(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewCausalCache(func(ctx ml.Context, layer int, key, shift ml.Tensor) (ml.Tensor, error) {
 			return key.Add(ctx, shift), nil
 		})
@@ -386,10 +402,11 @@ func TestRemove(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func TestCopy(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		cache := NewCausalCache(func(ctx ml.Context, layer int, key, shift ml.Tensor) (ml.Tensor, error) { return key, nil })
 		defer cache.Close()
 
@@ -426,6 +443,7 @@ func TestCopy(t *testing.T) {
 		}
 
 		testCache(t, backend, cache, tests)
+	})
 }
 
 func testCache(t *testing.T, backend ml.Backend, cache Cache, tests []testCase) {
@@ -463,7 +481,7 @@ func testCache(t *testing.T, backend ml.Backend, cache Cache, tests []testCase)
 }
 
 func TestCanResume(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		windowSize := int32(4)
 		cache := NewSWACache(windowSize, nil)
 		defer cache.Close()
@@ -534,10 +552,11 @@ func TestCanResume(t *testing.T) {
 		if !cache.CanResume(0, 5) {
 			t.Errorf("after shift: CanResume(0, 5) = false, want true (latest position)")
 		}
+	})
 }
 
 func TestCanResumeSWAMem(t *testing.T) {
-	backend := &testBackend{}
+	runPermutedVariants(t, func(t *testing.T, backend *testBackend) {
 		windowSize := int32(4)
 		memSize := int32(5)
 		cache := NewSWAMemCache(windowSize, memSize, nil)
@@ -598,10 +617,12 @@ func TestCanResumeSWAMem(t *testing.T) {
 		if !cache.CanResume(0, 7) {
 			t.Errorf("after shift: CanResume(0, 7) = false, want true (latest position)")
 		}
+	})
 }
 
 type testBackend struct {
 	ml.Backend
+	permutedV bool
 }
 
 func (b *testBackend) NewContext() ml.Context {
@@ -612,6 +633,10 @@ func (b *testBackend) NewContextSize(int) ml.Context {
 	return &testContext{}
 }
 
+func (b *testBackend) CacheConfig() ml.CacheConfig {
+	return ml.CacheConfig{PermutedV: b.permutedV}
+}
+
 type testContext struct {
 	ml.Context
 }
@@ -766,6 +791,102 @@ func (t *testTensor) View(ctx ml.Context, offset int, shape ...int) ml.Tensor {
 	return view
 }
 
+func (t *testTensor) Permute(ctx ml.Context, order ...int) ml.Tensor {
+	if len(t.shape) > 4 || len(order) > 4 {
+		panic("permute only supports up to 4 dimensions")
+	}
+
+	if len(order) != len(t.shape) && len(order) != 4 {
+		panic("invalid number of dimensions for permute")
+	}
+
+	// ggml_permute expects 4 axes, so fill in any missing dimensions.
+	orderFull := append(make([]int, 0, 4), order...)
+	for len(orderFull) < 4 {
+		orderFull = append(orderFull, len(orderFull))
+	}
+
+	seen := [4]bool{}
+
+	shape4 := [4]int{1, 1, 1, 1}
+	for i := 0; i < len(t.shape) && i < 4; i++ {
+		shape4[i] = t.shape[i]
+	}
+
+	newShape4 := [4]int{1, 1, 1, 1}
+	for axis := range 4 {
+		dst := orderFull[axis]
+		if dst < 0 || dst >= 4 {
+			panic("invalid axis for permute")
+		}
+		if seen[dst] {
+			panic("duplicate axis for permute")
+		}
+		seen[dst] = true
+		newShape4[dst] = shape4[axis]
+	}
+
+	total := len(t.data)
+	newData := make([]float32, total)
+
+	if total > 0 {
+		oldDims := shape4
+		newDims := newShape4
+
+		oldStride := [4]int{1, 1, 1, 1}
+		newStride := [4]int{1, 1, 1, 1}
+		for i := 1; i < 4; i++ {
+			oldStride[i] = oldStride[i-1] * oldDims[i-1]
+			newStride[i] = newStride[i-1] * newDims[i-1]
+		}
+
+		var coords [4]int
+		var newCoords [4]int
+
+		for idx := range total {
+			remainder := idx
+			for axis := range 4 {
+				dim := oldDims[axis]
+				if dim == 0 {
+					coords[axis] = 0
+					continue
+				}
+				coords[axis] = remainder % dim
+				remainder /= dim
+			}
+
+			for axis := range 4 {
+				newCoords[orderFull[axis]] = coords[axis]
+			}
+
+			newIndex := 0
+			for axis := range 4 {
+				if newDims[axis] == 0 {
+					continue
+				}
+				newIndex += newCoords[axis] * newStride[axis]
+			}
+
+			newData[newIndex] = t.data[idx]
+		}
+	}
+
+	numDims := 4
+	for numDims > 1 && newShape4[numDims-1] <= 1 {
+		numDims--
+	}
+
+	newShape := make([]int, numDims)
+	copy(newShape, newShape4[:numDims])
+
+	return &testTensor{
+		dtype:       t.dtype,
+		elementSize: t.elementSize,
+		data:        newData,
+		shape:       newShape,
+	}
+}
+
 func (t *testTensor) SetRows(ctx ml.Context, src ml.Tensor, idxs ml.Tensor) ml.Tensor {
 	dst := t
 	srcTensor := src.(*testTensor)