[esp32] Drop printf wrap on IDF 6.0+ (picolibc no longer needs it) (#16189)

esphome/components/esp32/__init__.py

@@ -1740,7 +1740,17 @@ async def to_code(config):
 
         # Wrap FILE*-based printf functions to eliminate newlib's _vfprintf_r
         # (~11 KB). See printf_stubs.cpp for implementation.
-        if conf[CONF_ADVANCED][CONF_ENABLE_FULL_PRINTF]:
+        #
+        # The wrap is only beneficial against newlib. Picolibc's tinystdio
+        # implements vsnprintf by building a string-output FILE and calling
+        # vfprintf, so vfprintf is unconditionally linked in by any caller
+        # of snprintf/vsnprintf — effectively every build — and the wrap
+        # saves nothing while costing ~170 B of shim. IDF 5.x defaults to
+        # newlib on every variant; IDF 6.0+ switches to picolibc on every
+        # variant.
+        if conf[CONF_ADVANCED][CONF_ENABLE_FULL_PRINTF] or idf_version() >= cv.Version(
+            6, 0, 0
+        ):
             cg.add_define("USE_FULL_PRINTF")
         else:
             for symbol in ("vprintf", "printf", "fprintf", "vfprintf"):

esphome/components/esp32/printf_stubs.cpp

@@ -1,91 +1,48 @@
 /*
- * Linker wrap stubs for FILE*-based printf functions.
+ * Linker wrap stubs for FILE*-based printf functions (newlib only).
  *
  * ESP-IDF SDK components (gpio driver, ringbuf, log_write) reference
- * fprintf(), printf(), vprintf(), and vfprintf() which pull in the full
+ * fprintf(), printf(), vprintf(), and vfprintf(), which on newlib pull
- * printf implementation (~11 KB on newlib's _vfprintf_r, ~2.8 KB on
+ * in _vfprintf_r (~11 KB) — a separate implementation from the one used
- * picolibc's vfprintf). This is a separate implementation from the one
+ * by snprintf/vsnprintf that handles FILE* stream I/O with buffering.
- * used by snprintf/vsnprintf that handles FILE* stream I/O with buffering
- * and locking.
  *
  * ESPHome replaces the ESP-IDF log handler via esp_log_set_vprintf_(),
  * so the SDK's vprintf() path is dead code at runtime. The fprintf()
  * and printf() calls in SDK components are only in debug/assert paths
  * (gpio_dump_io_configuration, ringbuf diagnostics) that are either
  * GC'd or never called. Crash backtraces and panic output are
- * unaffected; they use esp_rom_printf() which is a ROM function
+ * unaffected; they use esp_rom_printf() which is a ROM function and
- * and does not go through libc.
+ * does not go through libc.
  *
- * On picolibc (default for IDF >= 5 on RISC-V, IDF >= 6 everywhere) we
+ * This wrap is newlib-only. On picolibc, vsnprintf is implemented as
- * route output through a stack-allocated cookie FILE that forwards each
+ * vfprintf into a string-output FILE, so vfprintf is unconditionally
- * byte to the real target stream via fputc(). Picolibc's tinystdio
+ * linked in by any caller of snprintf/vsnprintf and the wrap can never
- * vfprintf walks the FILE::put callback one character at a time, so this
+ * elide it — it just adds shim cost. Codegen forces USE_FULL_PRINTF
- * costs ~32 bytes of stack for the cookie struct vs. a 512-byte format
+ * on picolibc builds (IDF 6.0+ on all variants) so this file compiles
- * buffer. The buffered path overflows the loopTask stack on IDF 6.
+ * to nothing there; the #error below catches a desynchronised gate.
  *
- * On newlib (IDF <= 5 on Xtensa) we keep the original snprintf-then-fwrite
+ * Saves ~11 KB of flash on newlib.
- * path because that loopTask stack budget has plenty of headroom for the
- * 512-byte buffer; the picolibc-only crash above does not affect it.
  *
- * Saves ~11 KB of flash on newlib, ~2.8 KB on picolibc.
+ * To disable this wrap on newlib, set enable_full_printf: true in the
- *
+ * esp32 advanced config section.
- * To disable these wraps, set enable_full_printf: true in the esp32
- * advanced config section.
  */
 
 #if defined(USE_ESP_IDF) && !defined(USE_FULL_PRINTF)
+
+#ifdef __PICOLIBC__
+#error "printf wrap is net-negative on picolibc; codegen should set USE_FULL_PRINTF"
+#endif
+
 #include <cstdarg>
 #include <cstdio>
 
-#ifndef __PICOLIBC__
 #include "esp_system.h"
-#endif
 
 namespace esphome::esp32 {}
 
 // NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
 extern "C" {
 
-#ifdef __PICOLIBC__
-
-#include <cstddef>
-#include <type_traits>
-
-extern int __real_vfprintf(FILE *stream, const char *fmt, va_list ap);
-
-namespace {
-
-struct CookieFile {
-  FILE base;
-  FILE *target;
-};
-
-// cookie_put() recovers CookieFile* from FILE* via reinterpret_cast, which is
-// only well-defined when FILE is the first member at offset 0 and CookieFile
-// is standard-layout.
-static_assert(offsetof(CookieFile, base) == 0, "FILE must be the first member of CookieFile");
-static_assert(std::is_standard_layout<CookieFile>::value, "CookieFile must be standard-layout");
-
-int cookie_put(char c, FILE *stream) {
-  auto *cookie = reinterpret_cast<CookieFile *>(stream);
-  return fputc(static_cast<unsigned char>(c), cookie->target);
-}
-
-const FILE COOKIE_FILE_TEMPLATE = FDEV_SETUP_STREAM(cookie_put, nullptr, nullptr, _FDEV_SETUP_WRITE);
-
-}  // namespace
-
-int __wrap_vfprintf(FILE *stream, const char *fmt, va_list ap) {
-  CookieFile cookie;
-  cookie.base = COOKIE_FILE_TEMPLATE;
-  cookie.target = stream;
-  return __real_vfprintf(&cookie.base, fmt, ap);
-}
-
-int __wrap_vprintf(const char *fmt, va_list ap) { return __wrap_vfprintf(stdout, fmt, ap); }
-
-#else  // !__PICOLIBC__
-
 static constexpr size_t PRINTF_BUFFER_SIZE = 512;
 
 // These stubs are essentially dead code at runtime — ESPHome replaces the
@@ -117,8 +74,6 @@ int __wrap_vfprintf(FILE *stream, const char *fmt, va_list ap) {
   return write_printf_buffer(stream, buf, vsnprintf(buf, sizeof(buf), fmt, ap));
 }
 
-#endif  // __PICOLIBC__
-
 int __wrap_printf(const char *fmt, ...) {
   va_list ap;
   va_start(ap, fmt);