Document PyUnstable_WritePerfMapEntry signature change in 3.15

The code_size widening from unsigned int to size_t is an LP64 ABI
break; add a whatsnew Porting note, a versionchanged directive, and
mention it in the NEWS entry so extension authors know to recompile.

Author: pablogsal

Doc/c-api/perfmaps.rst

@@ -43,6 +43,12 @@ Note that holding an :term:`attached thread state` is not required for these API
    the perf map file is not already opened. Returns ``0`` on success, or the
    same error codes as :c:func:`PyUnstable_PerfMapState_Init` on failure.
 
+   .. versionchanged:: 3.15
+
+      The *code_size* parameter was widened from ``unsigned int`` to
+      :c:type:`size_t`. On LP64 platforms this is an ABI-incompatible
+      change; see the :ref:`porting notes <whatsnew315-c-api-porting>`.
+
 .. c:function:: void PyUnstable_PerfMapState_Fini(void)
 
    Close the perf map file opened by :c:func:`PyUnstable_PerfMapState_Init`.

Doc/whatsnew/3.15.rst

@@ -1767,6 +1767,14 @@ Changed C APIs
   for ``NULL`` should be updated to call :c:macro:`PyDateTime_IMPORT` instead.
   (Contributed by Kumar Aditya in :gh:`141563`.)
 
+* The *code_size* parameter of :c:func:`PyUnstable_WritePerfMapEntry` was
+  widened from ``unsigned int`` to :c:type:`size_t` to support JIT code
+  regions larger than 4 GiB. On LP64 platforms this widens the
+  argument slot from 32 to 64 bits; see the Porting section below.
+  (Contributed by Diego Russo in :gh:`126910`.)
+
+.. _whatsnew315-c-api-porting:
+
 Porting to Python 3.15
 ----------------------
 
@@ -1775,6 +1783,16 @@ Porting to Python 3.15
   The |pythoncapi_compat_project| can be used to get most of these new
   functions on Python 3.14 and older.
 
+* The signature of :c:func:`PyUnstable_WritePerfMapEntry` changed in an
+  ABI-incompatible way: the *code_size* parameter is now :c:type:`size_t`
+  instead of ``unsigned int``. On LP64 platforms (Linux x86_64, AArch64,
+  etc.) this widens the argument from 32 to 64 bits. Extensions that call
+  this function must be recompiled against CPython 3.15 headers; mixing a
+  3.14-compiled caller with the 3.15 runtime will pass garbage in the
+  upper 32 bits of *code_size*. As the ``PyUnstable_`` prefix indicates,
+  this API has no backward-compatibility guarantee across feature
+  releases. (Contributed by Diego Russo in :gh:`126910`.)
+
 
 Removed C APIs
 --------------

Misc/NEWS.d/next/Core_and_Builtins/2026-03-17-20-30-17.gh-issue-126910.NaUwmD.rst

@@ -1 +1,5 @@
-Add support for unwinding JIT frames using GDB. Patch by Diego Russo
+Add support for unwinding JIT frames using GDB. As part of this change, the
+*code_size* parameter of :c:func:`PyUnstable_WritePerfMapEntry` was widened
+from ``unsigned int`` to :c:type:`size_t`; this is an ABI-incompatible
+change on LP64 platforms and requires extensions that use that function to
+be recompiled against CPython 3.15 headers. Patch by Diego Russo.