[Bazel][GN] Reuse the GN LLVM config file generation code

Currently, the Bazel build uses static, checked in [llvm-]config.h files
in combination with global macro definitions to mimic CMake's generated
headers. This change reuses the write_cmake_config.py script from the GN
build to generate the headers from source in the same way. The purpose
is to ensure that the Bazel build stays up to date with any changes to
the CMake config files. The write_cmake_config.py script has good error
checking to ensure that unneeded, stale variables are not passed, and
that any missing variables are reported as errors.

I tried to closely follow the logic in the GN build here:
  llvm/utils/gn/secondary/llvm/include/Config/BUILD.gn
The duplication between this file and config.bzl is significant, and we
could consider going further, but I'd like to hold off on it for now.

The GN build changes are to move the write_cmake_config.py script up to
//llvm/utils/write_cmake_config.py, and update the paths accordingly.

The next logical change is to generate Clang's config.h header.

Differential Revision: https://reviews.llvm.org/D126581

Author: rnk

llvm/utils/gn/build/write_cmake_config.gni

@@ -28,7 +28,7 @@ template("write_cmake_config") {
   assert(defined(invoker.values), "must set 'values' in $target_name")
 
   action(target_name) {
-    script = "//llvm/utils/gn/build/write_cmake_config.py"
+    script = "//llvm/utils/write_cmake_config.py"
 
     sources = [ invoker.input ]
     outputs = [ invoker.output ]

llvm/utils/gn/build/write_cmake_config.py renamed to llvm/utils/write_cmake_config.py

@@ -2,8 +2,8 @@
 r"""Emulates the bits of CMake's configure_file() function needed in LLVM.
 
 The CMake build uses configure_file() for several things.  This emulates that
-function for the GN build.  In the GN build, this runs at build time instead
-of at generator time.
+function for alternative build systems such as GN and Bazel.  In both GN and
+Bazel, config file generation happens during the build rather than before it.
 
 Takes a list of KEY=VALUE pairs (where VALUE can be empty).
 
@@ -28,8 +28,9 @@
         #define FOO [...]
         /* #undef FOO */
 
-Fails if any of the KEY=VALUE arguments aren't needed for processing the
-input file, or if the input file references keys that weren't passed in.
+To ensure that config file changes do not go unnoticed, this script fails if
+any of the KEY=VALUE arguments aren't needed for processing the input file, or
+if the input file references keys that weren't passed in.
 """
 
 import argparse

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -2,9 +2,9 @@
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-load(":template_rule.bzl", "template_rule")
+load(":write_cmake_config.bzl", "write_cmake_config")
 load(":tblgen.bzl", "gentbl")
-load(":config.bzl", "llvm_config_defines")
+load(":config.bzl", "config_h_values", "llvm_config_h_values", "llvm_global_defines")
 load(":targets.bzl", "llvm_targets")
 load(":enum_targets_gen.bzl", "enum_targets_gen")
 load(":binary_alias.bzl", "binary_alias")
@@ -101,17 +101,30 @@ genrule(
     cmd = "echo '#define LLVM_VERSION_INFO \"git\"' > $@",
 )
 
-template_rule(
+write_cmake_config(
     name = "abi_breaking_h_gen",
     src = "include/llvm/Config/abi-breaking.h.cmake",
     out = "include/llvm/Config/abi-breaking.h",
-    substitutions = {
-        # Define to enable checks that alter the LLVM C++ ABI
-        "#cmakedefine01 LLVM_ENABLE_ABI_BREAKING_CHECKS": "#define LLVM_ENABLE_ABI_BREAKING_CHECKS 0",
+    # Currently, in the Bazel build, we opt out of ABI breaking checks and
+    # reverse iteration.
+    values = [
+        "LLVM_ENABLE_ABI_BREAKING_CHECKS=",
+        "LLVM_ENABLE_REVERSE_ITERATION=",
+    ],
+)
 
-        # Define to enable reverse iteration of unordered llvm containers
-        "#cmakedefine01 LLVM_ENABLE_REVERSE_ITERATION": "#define LLVM_ENABLE_REVERSE_ITERATION 0",
-    },
+write_cmake_config(
+    name = "config_h_gen",
+    src = "include/llvm/Config/config.h.cmake",
+    out = "include/llvm/Config/config.h",
+    values = config_h_values,
+)
+
+write_cmake_config(
+    name = "llvm_config_h_gen",
+    src = "include/llvm/Config/llvm-config.h.cmake",
+    out = "include/llvm/Config/llvm-config.h",
+    values = llvm_config_h_values,
 )
 
 # To enable diff testing out of tree
@@ -129,7 +142,7 @@ cc_library(
         "include/llvm/Config/llvm-config.h",
     ],
     copts = llvm_copts,
-    defines = llvm_config_defines,
+    defines = llvm_global_defines,
     includes = ["include"],
     textual_hdrs = [
         "include/llvm/Config/AsmParsers.def",
@@ -2907,11 +2920,11 @@ cc_binary(
     copts = llvm_copts,
     stamp = 0,
     deps = [
-        ":Symbolize",
         ":BitReader",
         ":Core",
-        ":Support",
         ":Debuginfod",
+        ":Support",
+        ":Symbolize",
     ],
 )
 
@@ -4429,3 +4442,8 @@ cc_binary(
         ":Support",
     ],
 )
+
+py_binary(
+    name = "write_cmake_config",
+    srcs = ["utils/write_cmake_config.py"],
+)