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 +1,7 @@
+Add a new ``--enable-prebuilt-jit-stencils`` configure flag that forces the
+build to use the existing provided JIT stencil headers even when the digest
+at the beginning of the file does not match expectations. The JIT shim
+object and unwind info are still rebuilt normally. That allows redistributors
+who prebuilt the JIT stencil headers on a system with a different autoconf
+version to still use them even when ``pyconfig.h`` is slightly different.
+It also allow them to build the JIT shim with different LLVM toolchain.

Tools/jit/_targets.py

@@ -341,6 +341,7 @@ def build(
         *,
         comment: str = "",
         force: bool = False,
+        prebuilt: bool = False,
         jit_stencils: pathlib.Path,
         jit_shim_object: pathlib.Path,
         jit_unwind_info: pathlib.Path,
@@ -355,12 +356,15 @@ def build(
             outline = "=" * len(warning)
             print("\n".join(["", outline, warning, request, outline, ""]))
         digest = f"// {self._compute_digest()}\n"
+        stencils_current = (
+            not force
+            and jit_stencils.exists()
+            and (prebuilt or jit_stencils.read_text().startswith(digest))
+        )
         # The generated headers include the input digest as their first line.
         # If every generated artifact is current, skip the expensive rebuild.
         if (
-            not force
-            and jit_stencils.exists()
-            and jit_stencils.read_text().startswith(digest)
+            stencils_current
             and jit_shim_object.exists()
             and jit_unwind_info.exists()
             and jit_unwind_info.read_text().startswith(digest)
@@ -377,13 +381,14 @@ def build(
             lines=_writer.dump_unwind_info(unwind_info),
         )
         # Build the uop stencils after the shim metadata has been emitted.
-        stencil_groups = ASYNCIO_RUNNER.run(self._build_stencils())
-        self._write_generated_header(
-            jit_stencils,
-            digest=digest,
-            comment=comment,
-            lines=_writer.dump(stencil_groups, self.known_symbols),
-        )
+        if not stencils_current:
+            stencil_groups = ASYNCIO_RUNNER.run(self._build_stencils())
+            self._write_generated_header(
+                jit_stencils,
+                digest=digest,
+                comment=comment,
+                lines=_writer.dump(stencil_groups, self.known_symbols),
+            )
 
 
 class _COFF(

Tools/jit/build.py

@@ -69,6 +69,11 @@ def _write_target_dispatcher(
     parser.add_argument(
         "--llvm-tools-install-dir", help="Installation location of LLVM tools"
     )
+    parser.add_argument(
+        "--prebuilt",
+        action="store_true",
+        help="accept prebuilt stencil headers even if the digest does not match",
+    )
     args = parser.parse_args()
     for target in args.target:
         target.debug = args.debug
@@ -85,6 +90,7 @@ def _write_target_dispatcher(
         target.build(
             comment=comment,
             force=args.force,
+            prebuilt=args.prebuilt,
             jit_stencils=args.output_dir / f"jit_stencils-{target.triple}.h",
             jit_shim_object=args.output_dir / f"jit_shim-{target.triple}.o",
             jit_unwind_info=args.output_dir / f"jit_unwind_info-{target.triple}.h",

configure.ac

@@ -2902,6 +2902,19 @@ 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 stencil headers 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):