From b10785c1be469319a09b10bc69db21159b0599ee Mon Sep 17 00:00:00 2001
From: Sergei Trofimovich <siarheit@google.com>
Date: Fri, 22 Sep 2023 22:41:49 +0100
Subject: [PATCH] gcc/file-prefix-map.cc: always mangle __FILE__ into invalid
 store path

Without the change `__FILE__` used in static inline functions in headers
embed paths to header files into executable images. For local headers
it's not a problem, but for headers in `/nix/store` this causes `-dev`
inputs to be retained in runtime closure.

Typical examples are `nix` -> `nlohmann_json` and `pipewire` ->
`lttng-ust.dev`.

Ideally we would like to use `-fmacro-prefix-map=` feature of `gcc` as:

  -fmacro-prefix-map=/nix/store/$hash1-nlohmann-json-ver=/nix/store/eeee.eee-nlohmann-json-ver
  -fmacro-prefix-map=/nix/...

In practice it quickly exhausts argument lengtth limit due to `gcc`
deficiency: https://gcc.gnu.org/PR111527

Until it;s fixed let's hardcode header mangling if $NIX_STORE variable
is present in the environment.

Tested as:

    $ printf "# 0 \"/nix/store/01234567890123456789012345678901-pppppp-vvvvvvv\" \nconst char * f(void) { return __FILE__; }" | NIX_STORE=/nix/store ./gcc/xgcc -Bgcc -x c - -S -o -
    ...
    .string "/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-pppppp-vvvvvvv"
    ...

Mangled successfully.
--- a/gcc/file-prefix-map.cc
+++ b/gcc/file-prefix-map.cc
@@ -60,6 +60,9 @@ add_prefix_map (file_prefix_map *&maps, const char *arg, const char *opt)
   maps = map;
 }
 
+/* Forward declaration for a $NIX_STORE remap hack below. */
+static file_prefix_map *macro_prefix_maps; /* -fmacro-prefix-map  */
+
 /* Perform user-specified mapping of filename prefixes.  Return the
    GC-allocated new name corresponding to FILENAME or FILENAME if no
    remapping was performed.  */
@@ -76,7 +79,30 @@ remap_filename (file_prefix_map *maps, const char *filename)
     if (filename_ncmp (filename, map->old_prefix, map->old_len) == 0)
       break;
   if (!map)
-    return filename;
+    {
+      if (maps == macro_prefix_maps)
+	{
+	  /* Remap all fo $NIX_STORE/.{32} paths to
+	  * equivalent $NIX_STORE/e{32}.
+	  *
+	  * That way we avoid argument parameters explosion
+	  * and still avoid embedding headers into runtime closure:
+	  *   https://gcc.gnu.org/PR111527
+	  */
+	  char * nix_store = getenv("NIX_STORE");
+	  size_t nix_store_len = nix_store ? strlen(nix_store) : 0;
+	  const char * name = filename;
+	  size_t name_len = strlen(name);
+	  if (nix_store && name_len >= nix_store_len + 1 + 32 && memcmp(name, nix_store, nix_store_len) == 0)
+	    {
+	       s = (char *) ggc_alloc_atomic (name_len + 1);
+	       memcpy(s, name, name_len + 1);
+	       memset(s + nix_store_len + 1, 'e', 32);
+	       return s;
+	    }
+	}
+      return filename;
+    }
   name = filename + map->old_len;
   name_len = strlen (name) + 1;
 
@@ -90,7 +116,6 @@ remap_filename (file_prefix_map *maps, const char *filename)
    ignore it in DW_AT_producer (dwarf2out.cc).  */
 
 /* Linked lists of file_prefix_map structures.  */
-static file_prefix_map *macro_prefix_maps; /* -fmacro-prefix-map  */
 static file_prefix_map *debug_prefix_maps; /* -fdebug-prefix-map  */
 static file_prefix_map *profile_prefix_maps; /* -fprofile-prefix-map  */