metal : implement rope_back operator (#24725)

Reuse existing rope kernels with a function constant to toggle forward/backward
rotation, avoiding duplicate kernel code.

Assisted-by: pi:llama.cpp/Qwen3.6-27B

ggml/src/ggml-metal/ggml-metal-device.cpp

@@ -1703,7 +1703,9 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_norm(ggml_metal_
 }
 
 ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_rope(ggml_metal_library_t lib, const ggml_tensor * op) {
-    assert(op->op == GGML_OP_ROPE);
+    assert(op->op == GGML_OP_ROPE || op->op == GGML_OP_ROPE_BACK);
+
+    const bool is_back = op->op == GGML_OP_ROPE_BACK;
 
     char base[256];
     char name[256];
@@ -1727,13 +1729,14 @@ ggml_metal_pipeline_with_params ggml_metal_library_get_pipeline_rope(ggml_metal_
         snprintf(base, 256, "kernel_rope_norm_%s", ggml_type_name(op->src[0]->type));
     }
 
-    snprintf(name, 256, "%s_imrope=%d", base, is_imrope ? 1 : 0);
+    snprintf(name, 256, "%s_imrope=%d_is_back=%d", base, is_imrope ? 1 : 0, is_back ? 1 : 0);
 
     ggml_metal_pipeline_with_params res = ggml_metal_library_get_pipeline(lib, name);
     if (!res.pipeline) {
         ggml_metal_cv_t cv = ggml_metal_cv_init();
 
         ggml_metal_cv_set_bool(cv, is_imrope, FC_ROPE + 0);
+        ggml_metal_cv_set_bool(cv, is_back,   FC_ROPE + 1);
 
         res = ggml_metal_library_compile_pipeline(lib, base, name, cv);
 

ggml/src/ggml-metal/ggml-metal-device.m

@@ -1184,6 +1184,7 @@ bool ggml_metal_device_supports_op(ggml_metal_device_t dev, const struct ggml_te
         case GGML_OP_RMS_NORM:
             return has_simdgroup_reduction && (ggml_is_contiguous_rows(op->src[0]));
         case GGML_OP_ROPE:
+        case GGML_OP_ROPE_BACK:
             return true;
         case GGML_OP_IM2COL:
             return ggml_is_contiguous(op->src[1]) && op->src[1]->type == GGML_TYPE_F32 && (op->type == GGML_TYPE_F16 || op->type == GGML_TYPE_F32);

ggml/src/ggml-metal/ggml-metal-ops.cpp

@@ -375,6 +375,7 @@ static int ggml_metal_op_encode_impl(ggml_metal_op_t ctx, int idx) {
                 n_fuse = ggml_metal_op_norm(ctx, idx);
             } break;
         case GGML_OP_ROPE:
+        case GGML_OP_ROPE_BACK:
             {
                 n_fuse = ggml_metal_op_rope(ctx, idx);
             } break;

ggml/src/ggml-metal/ggml-metal.metal

@@ -4358,6 +4358,7 @@ template [[host_name("kernel_mul_mv_bf16_bf16_short")]] kernel mul_mv_t_t_short_
 #endif
 
 constant bool FC_rope_is_imrope [[function_constant(FC_ROPE + 0)]];
+constant bool FC_rope_is_back   [[function_constant(FC_ROPE + 1)]];
 
 static float rope_yarn_ramp(const float low, const float high, const int i0) {
     const float y = (i0 / 2 - low) / max(0.001f, high - low);
@@ -4381,6 +4382,9 @@ static void rope_yarn(
     }
     *cos_theta = cos(theta) * mscale;
     *sin_theta = sin(theta) * mscale;
+    if (FC_rope_is_back) {
+        *sin_theta *= -1.0f;
+    }
 }
 
 // Apparently solving `n_rot = 2pi * x * base^((2 * max_pos_emb) / n_dims)` for x, we get