diff --git a/src/microsoft/compiler/dxil_module.h b/src/microsoft/compiler/dxil_module.h
index 701336ec4bc..13323280d03 100644
--- a/src/microsoft/compiler/dxil_module.h
+++ b/src/microsoft/compiler/dxil_module.h
@@ -197,7 +197,16 @@ struct dxil_module {
    struct dxil_signature_record inputs[DXIL_SHADER_MAX_IO_ROWS];
    struct dxil_signature_record outputs[DXIL_SHADER_MAX_IO_ROWS];
    struct dxil_signature_record patch_consts[DXIL_SHADER_MAX_IO_ROWS];
-   unsigned input_mappings[DXIL_SHADER_MAX_IO_ROWS];
+
+   /* This array is indexed using var->data.driver_location, which
+    * is not a direct match to IO rows, since a row is a vec4, and
+    * variables can occupy less than that, and several vars can
+    * be packed in a row. Hence the x4, but I doubt we can end up
+    * with more than 80x4 variables in practice. Maybe this array
+    * should be allocated dynamically based on on the maximum
+    * driver_location across all input vars.
+    */
+   unsigned input_mappings[DXIL_SHADER_MAX_IO_ROWS * 4];
 
    struct dxil_psv_signature_element psv_inputs[DXIL_SHADER_MAX_IO_ROWS];
    struct dxil_psv_signature_element psv_outputs[DXIL_SHADER_MAX_IO_ROWS];
diff --git a/src/microsoft/compiler/dxil_signature.c b/src/microsoft/compiler/dxil_signature.c
index 732ad42bf4e..b96e92c14fd 100644
--- a/src/microsoft/compiler/dxil_signature.c
+++ b/src/microsoft/compiler/dxil_signature.c
@@ -550,11 +550,7 @@ get_input_signature_group(struct dxil_module *mod, const struct dxil_mdnode **in
       mod->inputs[num_inputs].sysvalue = semantic.sysvalue_name;
       *row_iter = get_additional_semantic_info(s, var, &semantic, *row_iter, input_clip_size);
 
-      if (semantic.start_row >= 0) {
-         for (unsigned i = 0; i < semantic.rows; ++i)
-            mod->input_mappings[semantic.start_row + i] = num_inputs;
-      }
-
+      mod->input_mappings[var->data.driver_location] = num_inputs;
       mod->inputs[num_inputs].name = ralloc_strdup(mod->ralloc_ctx,
                                                    semantic.name);
       mod->inputs[num_inputs].num_elements = semantic.rows;