py/builtinimport: Remove weak links.

In order to keep "import umodule" working, the existing mechanism is
replaced with a simple fallback to drop the "u".

This makes importing of built-ins no longer touch the filesystem, which
makes a typical built-in import take ~0.15ms rather than 3-5ms.

(Weak links were added in c14a81662c1df812c0c6b4299f97966302f16477)

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

9 files changed, 5 insertions, 59 deletions

diff --git a/docs/develop/porting.rst b/docs/develop/porting.rst
index fab8a751b..63919b97a 100644
--- a/docs/develop/porting.rst
+++ b/docs/develop/porting.rst
@@ -151,9 +151,6 @@ The following is an example of an ``mpconfigport.h`` file:
    #define MICROPY_ERROR_REPORTING                 (MICROPY_ERROR_REPORTING_TERSE)
    #define MICROPY_FLOAT_IMPL                      (MICROPY_FLOAT_IMPL_FLOAT)
 
-   // Enable u-modules to be imported with their standard name, like sys.
-   #define MICROPY_MODULE_WEAK_LINKS               (1)
-
    // Fine control over Python builtins, classes, modules, etc.
    #define MICROPY_PY_ASYNC_AWAIT                  (0)
    #define MICROPY_PY_BUILTINS_SET                 (0)
diff --git a/ports/cc3200/mpconfigport.h b/ports/cc3200/mpconfigport.h
index 6bdaa9b55..aa78005d6 100644
--- a/ports/cc3200/mpconfigport.h
+++ b/ports/cc3200/mpconfigport.h
@@ -71,7 +71,6 @@
 #define MICROPY_FATFS_SYNC_T                        SemaphoreHandle_t
 
 #define MICROPY_STREAMS_NON_BLOCK                   (1)
-#define MICROPY_MODULE_WEAK_LINKS                   (1)
 #define MICROPY_CAN_OVERRIDE_BUILTINS               (1)
 #define MICROPY_USE_INTERNAL_ERRNO                  (1)
 #define MICROPY_VFS                                 (1)
diff --git a/ports/nrf/mpconfigport.h b/ports/nrf/mpconfigport.h
index 23f7e560d..3d482f4fe 100644
--- a/ports/nrf/mpconfigport.h
+++ b/ports/nrf/mpconfigport.h
@@ -137,7 +137,6 @@
 #define MICROPY_PY_UOS              (0)
 
 #define MICROPY_STREAMS_NON_BLOCK   (1)
-#define MICROPY_MODULE_WEAK_LINKS   (1)
 #define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_USE_INTERNAL_ERRNO  (1)
 #if MICROPY_HW_USB_CDC_1200BPS_TOUCH
diff --git a/ports/samd/mpconfigport.h b/ports/samd/mpconfigport.h
index 7392b0305..a0a58a2fd 100644
--- a/ports/samd/mpconfigport.h
+++ b/ports/samd/mpconfigport.h
@@ -59,7 +59,6 @@
 #define MICROPY_PY_BUILTINS_HELP_MODULES    (1)
 #define MICROPY_ENABLE_SCHEDULER            (1)
 #define MICROPY_SCHEDULER_STATIC_NODES      (1)
-#define MICROPY_MODULE_WEAK_LINKS           (1)
 #define MICROPY_HW_ENABLE_USBDEV            (1)
 #define MICROPY_HW_USB_CDC_1200BPS_TOUCH    (1)
 
diff --git a/ports/windows/mpconfigport.h b/ports/windows/mpconfigport.h
index fd3a76408..5e56923d9 100644
--- a/ports/windows/mpconfigport.h
+++ b/ports/windows/mpconfigport.h
@@ -72,7 +72,6 @@
 #endif
 #define MICROPY_STREAMS_POSIX_API   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (0)
-#define MICROPY_MODULE_WEAK_LINKS   (1)
 #define MICROPY_MODULE_OVERRIDE_MAIN_IMPORT (1)
 #define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #ifndef MICROPY_ENABLE_SCHEDULER
diff --git a/ports/zephyr/mpconfigport.h b/ports/zephyr/mpconfigport.h
index d3ef4375c..30ccccb62 100644
--- a/ports/zephyr/mpconfigport.h
+++ b/ports/zephyr/mpconfigport.h
@@ -69,7 +69,6 @@
 #define MICROPY_PY_MACHINE_SPI_MSB (SPI_TRANSFER_MSB)
 #define MICROPY_PY_MACHINE_SPI_LSB (SPI_TRANSFER_LSB)
 #define MICROPY_PY_MACHINE_PIN_MAKE_NEW mp_pin_make_new
-#define MICROPY_MODULE_WEAK_LINKS   (1)
 #define MICROPY_PY_STRUCT           (0)
 #ifdef CONFIG_NETWORKING
 // If we have networking, we likely want errno comfort
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 8827be612..299877de6 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -45,15 +45,6 @@
 #define DEBUG_printf(...) (void)0
 #endif
 
-#if MICROPY_MODULE_WEAK_LINKS
-STATIC qstr make_weak_link_name(vstr_t *buffer, qstr name) {
-    vstr_reset(buffer);
-    vstr_add_char(buffer, 'u');
-    vstr_add_str(buffer, qstr_str(name));
-    return qstr_from_strn(buffer->buf, buffer->len);
-}
-#endif
-
 #if MICROPY_ENABLE_EXTERNAL_IMPORT
 
 // Must be a string of one byte.
@@ -391,13 +382,8 @@ STATIC mp_obj_t process_import_at_level(qstr full_mod_name, qstr level_mod_name,
     if (outer_module_obj == MP_OBJ_NULL) {
         DEBUG_printf("Searching for top-level module\n");
 
-        // An exact match of a built-in will always bypass the filesystem.
-        // Note that CPython-compatible built-ins are named e.g. utime, so this
-        // means that an exact match is only for `import utime`, so `import
-        // time` will search the filesystem and failing that hit the weak
-        // link handling below. Whereas micropython-specific built-ins like
-        // `micropython`, `pyb`, `network`, etc will match exactly and cannot
-        // be overridden by the filesystem.
+        // An import of a non-extensible built-in will always bypass the
+        // filesystem. e.g. `import micropython` or `import pyb`.
         module_obj = mp_module_get_builtin(level_mod_name);
         if (module_obj != MP_OBJ_NULL) {
             return module_obj;
@@ -417,20 +403,9 @@ STATIC mp_obj_t process_import_at_level(qstr full_mod_name, qstr level_mod_name,
         // relative to all the locations in sys.path.
         stat = stat_top_level(level_mod_name, &path);
 
-        #if MICROPY_MODULE_WEAK_LINKS
-        if (stat == MP_IMPORT_STAT_NO_EXIST) {
-            // No match on the filesystem. (And not a built-in either).
-            // If "foo" was requested, then try "ufoo" as a built-in. This
-            // allows `import time` to use built-in `utime`, unless `time`
-            // exists on the filesystem. This feature was formerly known
-            // as "weak links".
-            qstr umodule_name = make_weak_link_name(&path, level_mod_name);
-            module_obj = mp_module_get_builtin(umodule_name);
-            if (module_obj != MP_OBJ_NULL) {
-                return module_obj;
-            }
-        }
-        #endif
+        // TODO: If stat failed, now try extensible built-in modules.
+
+        // TODO: If importing `ufoo`, try `foo`.
     } else {
         DEBUG_printf("Searching for sub-module\n");
 
@@ -667,21 +642,6 @@ mp_obj_t mp_builtin___import___default(size_t n_args, const mp_obj_t *args) {
         return module_obj;
     }
 
-    #if MICROPY_MODULE_WEAK_LINKS
-    // Check if the u-prefixed name is a built-in.
-    VSTR_FIXED(umodule_path, MICROPY_ALLOC_PATH_MAX);
-    qstr umodule_name_qstr = make_weak_link_name(&umodule_path, module_name_qstr);
-    module_obj = mp_module_get_builtin(umodule_name_qstr);
-    if (module_obj != MP_OBJ_NULL) {
-        return module_obj;
-    }
-    #elif MICROPY_PY_SYS
-    // Special handling to make `import sys` work even if weak links aren't enabled.
-    if (module_name_qstr == MP_QSTR_sys) {
-        return MP_OBJ_FROM_PTR(&mp_module_sys);
-    }
-    #endif
-
     // Couldn't find the module, so fail
     #if MICROPY_ERROR_REPORTING <= MICROPY_ERROR_REPORTING_TERSE
     mp_raise_msg(&mp_type_ImportError, MP_ERROR_TEXT("module not found"));
diff --git a/py/mpconfig.h b/py/mpconfig.h
index afef744ab..9f4d88ec0 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -871,11 +871,6 @@ typedef double mp_float_t;
 #define MICROPY_MODULE_GETATTR (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_CORE_FEATURES)
 #endif
 
-// Whether module weak links are supported
-#ifndef MICROPY_MODULE_WEAK_LINKS
-#define MICROPY_MODULE_WEAK_LINKS (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_EXTRA_FEATURES)
-#endif
-
 // Whether to enable importing foo.py with __name__ set to '__main__'
 // Used by the unix port for the -m flag.
 #ifndef MICROPY_MODULE_OVERRIDE_MAIN_IMPORT
diff --git a/py/objmodule.c b/py/objmodule.c
index 7326fbe2d..e63fb18a1 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -169,7 +169,6 @@ STATIC const mp_rom_map_elem_t mp_builtin_module_table[] = {
     // builtin modules declared with MP_REGISTER_MODULE()
     MICROPY_REGISTERED_MODULES
 };
-
 MP_DEFINE_CONST_MAP(mp_builtin_module_map, mp_builtin_module_table);
 
 // Attempts to find (and initialise) a builtin, otherwise returns