gh-148646: Add --enable-prebuilt-jit-stencils configure flag

Misc/NEWS.d/next/Build/2026-04-16-14-39-33.gh-issue-148646.iIIgWF.rst

@@ -0,0 +1,6 @@
+Add a new ``--enable-prebuilt-jit-stencils`` configure flag that forces the
+build to use the existing provided JIT stencils even when the digest at the
+beginning of the file does not match expectations. That allows
+redistributors who prebuilt the JIT stencils on a system with a different
+autoconf version to still use them even when ``pyconfig.h`` is slightly
+different.

Tools/jit/_targets.py

@@ -235,6 +235,7 @@ def build(
         *,
         comment: str = "",
         force: bool = False,
+        prebuilt: bool = False,
         jit_stencils: pathlib.Path,
     ) -> None:
         """Build jit_stencils.h in the given directory."""
@@ -250,7 +251,7 @@ def build(
         if (
             not force
             and jit_stencils.exists()
-            and jit_stencils.read_text().startswith(digest)
+            and (prebuilt or jit_stencils.read_text().startswith(digest))
         ):
             return
         stencil_groups = ASYNCIO_RUNNER.run(self._build_stencils())

Tools/jit/build.py

@@ -43,6 +43,11 @@
         "--cflags", help="additional flags to pass to the compiler", default=""
     )
     parser.add_argument("--llvm-version", help="LLVM version to use")
+    parser.add_argument(
+        "--prebuilt",
+        action="store_true",
+        help="accept prebuilt stencils even if the digest does not match",
+    )
     args = parser.parse_args()
     for target in args.target:
         target.debug = args.debug
@@ -55,6 +60,7 @@
         target.build(
             comment=comment,
             force=args.force,
+            prebuilt=args.prebuilt,
             jit_stencils=args.output_dir / f"jit_stencils-{target.triple}.h",
         )
     jit_stencils_h = args.output_dir / "jit_stencils.h"

configure.ac

@@ -2836,6 +2836,19 @@ AS_VAR_IF([jit_flags],
 AC_SUBST([REGEN_JIT_COMMAND])
 AC_MSG_RESULT([$tier2_flags $jit_flags])
 
+# Check for --enable-prebuilt-jit-stencils:
+AC_MSG_CHECKING([for --enable-prebuilt-jit-stencils])
+AC_ARG_ENABLE([prebuilt-jit-stencils],
+              [AS_HELP_STRING([--enable-prebuilt-jit-stencils],
+                              [accept prebuilt JIT stencils even if the digest does not match (default is no)])],
+              [],
+              [enable_prebuilt_jit_stencils=no])
+AS_VAR_IF([enable_prebuilt_jit_stencils],
+          [no],
+          [],
+          [AS_VAR_APPEND([REGEN_JIT_COMMAND], [" --prebuilt"])])
+AC_MSG_RESULT([$enable_prebuilt_jit_stencils])
+
 if test "$disable_gil" = "yes" -a "$enable_experimental_jit" != "no"; then
   # GH-133171: This configuration builds the JIT but never actually uses it,
   # which is surprising (and strictly worse than not building it at all):