From 2c0f20cf6ccefa69000bafa73446c86274bd0fe3 Mon Sep 17 00:00:00 2001
From: Nico Weber <thakis@chromium.org>
Date: Fri, 14 Feb 2020 21:55:33 -0500
Subject: [PATCH] [gn build] Make build locally deterministic

This follows http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
to make the GN build locally deterministic.

With this, I've built lld at two different build paths on my Windows box and got
identical binaries. (I'd expect the same to happen on Linux, and with other
binaries.)

This doesn't have the bits to get universal determinism yet.

Differential Revision: https://reviews.llvm.org/D74519
---
 utils/gn/build/BUILD.gn           | 39 +++++++++++++++++++++++++++++++
 utils/gn/build/toolchain/BUILD.gn |  2 +-
 utils/gn/docs/deterministic.md    | 18 ++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 utils/gn/docs/deterministic.md

diff --git a/utils/gn/build/BUILD.gn b/utils/gn/build/BUILD.gn
index b74268996f1..488e248da3a 100644
--- a/utils/gn/build/BUILD.gn
+++ b/utils/gn/build/BUILD.gn
@@ -11,6 +11,14 @@ declare_args() {
   #        out/gn/bin/llvm-undname ...`
   # to generate a HTML report for the binaries passed in the last line.
   llvm_build_instrumented_coverage = false
+
+  # If set, puts relative paths in debug info.
+  # Makes the build output independent of the build directory, but makes
+  # most debuggers harder to use. See "Getting to local determinism" and
+  # "Getting debuggers to work well with locally deterministic builds" in
+  # http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
+  # for more information.
+  use_relative_paths_in_debug_info = false
 }
 
 assert(!llvm_build_instrumented_coverage || is_clang,
@@ -168,6 +176,37 @@ config("compiler_defaults") {
     ]
     ldflags += [ "-fprofile-instr-generate" ]
   }
+
+  # Deterministic build setup, see
+  # http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
+  if (current_os == "win") {
+    ldflags += [ "/pdbaltpath:%_PDB%" ]
+  }
+  if (is_clang) {
+    cflags += [
+      "-no-canonical-prefixes",
+      "-Werror=date-time",
+    ]
+    if (current_os == "win") {
+      cflags += [
+        "-fmsc-version=1916",
+        "/X",
+      ]
+    }
+    if (current_os == "win" && use_lld) {
+      cflags += [ "/Brepro" ]
+      ldflags += [
+        "/Brepro",
+        "/lldignoreenv",
+      ]
+    }
+    if (use_relative_paths_in_debug_info) {
+      cflags += [
+        "-fdebug-compilation-dir",
+        ".",
+      ]
+    }
+  }
 }
 
 config("no_exceptions") {
diff --git a/utils/gn/build/toolchain/BUILD.gn b/utils/gn/build/toolchain/BUILD.gn
index d435d9c9726..453aab57252 100644
--- a/utils/gn/build/toolchain/BUILD.gn
+++ b/utils/gn/build/toolchain/BUILD.gn
@@ -39,7 +39,7 @@ template("unix_toolchain") {
 
     tool("alink") {
       if (current_os == "mac") {
-        command = "libtool -static -no_warning_for_no_symbols {{arflags}} -o {{output}} {{inputs}}"
+        command = "libtool -D -static -no_warning_for_no_symbols {{arflags}} -o {{output}} {{inputs}}"
       } else {
         # Remove the output file first so that ar doesn't try to modify the
         # existing file.
diff --git a/utils/gn/docs/deterministic.md b/utils/gn/docs/deterministic.md
new file mode 100644
index 00000000000..36d518576ac
--- /dev/null
+++ b/utils/gn/docs/deterministic.md
@@ -0,0 +1,18 @@
+Deterministic builds with LLVM's GN build
+=========================================
+
+Summary: Use the following args.gn.
+
+    use_relative_paths_in_debug_info = true
+
+It is possible to produce [locally deterministic][1] builds of LLVM
+with the GN build. It requires some configuration though.
+
+1. Make debug info use relative paths by setting
+   `use_relative_paths_in_debug_info = true` in your `args.gn` file. With this
+   set, current debuggers need minor configuration to keep working.  See
+   "Getting to local determinism" and "Getting debuggers to work well with
+   locally deterministic builds" in the [deterministic builds][1] documentation
+   for details.
+
+1: http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html