GCC conversion warnings from internal header files

[dalcinl]

Bug report

Compiling Cython-generated code with GCC -Wconversion produces a bunch of warnings within inline function defined in internal CPython header files.

/usr/include/python3.12/internal/pycore_code.h: In function ‘write_varint’:
/usr/include/python3.12/internal/pycore_code.h:362:12: warning: conversion from ‘unsigned int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion]
  362 |     *ptr = val;
      |            ^~~
/usr/include/python3.12/internal/pycore_code.h: In function ‘write_signed_varint’:
/usr/include/python3.12/internal/pycore_code.h:375:30: warning: conversion to ‘unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
  375 |     return write_varint(ptr, val);
      |                              ^~~
/usr/include/python3.12/internal/pycore_code.h: In function ‘write_location_entry_start’:
/usr/include/python3.12/internal/pycore_code.h:382:12: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion]
  382 |     *ptr = 128 | (code << 3) | (length - 1);
      |            ^~~
/usr/include/python3.12/internal/pycore_code.h: In function ‘adaptive_counter_bits’:
/usr/include/python3.12/internal/pycore_code.h:423:45: warning: conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} may change value [-Wconversion]
  423 |     return (value << ADAPTIVE_BACKOFF_BITS) |
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
  424 |         (backoff & ((1<<ADAPTIVE_BACKOFF_BITS)-1));
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/python3.12/internal/pycore_code.h: In function ‘adaptive_counter_backoff’:
/usr/include/python3.12/internal/pycore_code.h:446:26: warning: conversion to ‘unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
  446 |     unsigned int value = (1 << backoff) - 1;
      |                          ^
/usr/include/python3.12/internal/pycore_code.h:447:34: warning: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Wsign-conversion]
  447 |     return adaptive_counter_bits(value, backoff);
      |                                  ^~~~~
/usr/include/python3.12/internal/pycore_code.h:447:41: warning: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Wsign-conversion]
  447 |     return adaptive_counter_bits(value, backoff);

A tentative (untested) patch would be the following:

diff --git a/Include/internal/pycore_code.h b/Include/internal/pycore_code.h
index d1829eb324..a7b22968ba 100644
--- a/Include/internal/pycore_code.h
+++ b/Include/internal/pycore_code.h
@@ -427,9 +427,9 @@ write_location_entry_start(uint8_t *ptr, int code, int length)
 
 
 static inline uint16_t
-adaptive_counter_bits(int value, int backoff) {
-    return (value << ADAPTIVE_BACKOFF_BITS) |
-        (backoff & ((1<<ADAPTIVE_BACKOFF_BITS)-1));
+adaptive_counter_bits(unsigned int value, unsigned int backoff) {
+    return (uint16_t) ((value << ADAPTIVE_BACKOFF_BITS) |
+                       (backoff & ((1U<<ADAPTIVE_BACKOFF_BITS)-1U)));
 }
 
 static inline uint16_t
@@ -446,12 +446,12 @@ adaptive_counter_cooldown(void) {
 
 static inline uint16_t
 adaptive_counter_backoff(uint16_t counter) {
-    unsigned int backoff = counter & ((1<<ADAPTIVE_BACKOFF_BITS)-1);
+    unsigned int backoff = counter & ((1U<<ADAPTIVE_BACKOFF_BITS)-1U);
     backoff++;
     if (backoff > MAX_BACKOFF_VALUE) {
         backoff = MAX_BACKOFF_VALUE;
     }
-    unsigned int value = (1 << backoff) - 1;
+    unsigned int value = (1U << backoff) - 1U;
     return adaptive_counter_bits(value, backoff);
 }

If this approach of adding an explicit (uint16_t) cast and adding U suffices to integer literals where needed is OK with you, I can provide a full and tested patch or pull request.

Your environment

• CPython versions tested on: Python 3.12.0b3
• Operating system and architecture: Linux Fedora 38, GCC 13.1.1

Linked PRs

• gh-106550: Fix sign conversion in pycore_code.h #112613
• [3.12] gh-106550: Fix sign conversion in pycore_code.h (GH-112613) #112696

[vstinner]

Thanks for the bug report, it should not be fixed. Until a Python version including the change is released, you should avoid the -Wconversion compiler flag (or find a way to ignore warnings from Python header files).