Bug 1641674 - Don't use --gc-sections during profile generation r=fro…

…ydnj

For not-well-understood reasons, ld's `--gc-sections` discards a large number of the PGO bookkeeping structures that enable us to keep track of function counters, and the effect gets worse in object files generated by clang-10.

As much as I'd like to understand this better, the investigations take way too much time. As a path of least resistance, we can disable `--gc-sections` for the instrumentation phase of PGO builds. It won't harm anything since users never see those builds, and it will improve the performance of the optimized phase greatly.

Differential Revision: https://phabricator.services.mozilla.com/D78112

build/autoconf/compiler-opts.m4

@@ -109,7 +109,7 @@ dnl ========================================================
 dnl = Automatically remove dead symbols
 dnl ========================================================
 
-if test "$GNU_CC" -a "$GCC_USE_GNU_LD" -a -z "$DEVELOPER_OPTIONS"; then
+if test "$GNU_CC" -a "$GCC_USE_GNU_LD" -a -z "$DEVELOPER_OPTIONS" -a -z "$MOZ_PROFILE_GENERATE"; then
     if test -n "$MOZ_DEBUG_FLAGS"; then
         dnl See bug 670659
         AC_CACHE_CHECK([whether removing dead symbols breaks debugging],

build/moz.configure/lto-pgo.configure

@@ -25,6 +25,9 @@ set_config('MOZ_PROFILE_GENERATE',
 set_define('MOZ_PROFILE_GENERATE',
            depends_if('--enable-profile-generate')(lambda _: True))
 
+add_old_configure_assignment('MOZ_PROFILE_GENERATE', 1,
+                             when='--enable-profile-generate')
+
 js_option('--enable-profile-use',
           env='MOZ_PROFILE_USE',
           nargs='?',