RFA: Warn when running LTO compiler without any optimization level set

[Nick Clifton <nickc at redhat dot com>]

Hi Guys,

  Running the LTO compiler without enabling any optimizations is not
  very useful, but it can happen if the user forgets to include a -O<n>
  setting on the linker command line.  (Or as happened to me, a
  testsuite harness has separate settings for the compiler and linker
  command lines and although -flto has been added to both, -03 was only
  added to the compiler command line).

  The attached patch adds a warning message for this particular
  circumstance:
  
    lto1: warning: -flto used without an optimization level set - was this intentional ?

  At the moment this warning can only be disabled by adding -O<n> (where
  <n> is not zero), -Os or -w to the command line.  I could add a
  specific warning enable/disable option to control it, but this seemed
  like overkill.

  Tested without regressions on an i686-pc-linux-gnu toolchain.

  OK to apply ?

Cheers
  Nick

gcc/lto/ChangeLog
2010-04-09  Nick Clifton  <nickc@redhat.com>

	* lto-lang.c (lto_init): Warn if the LTO compiler is run without
	any optimizations enabled.

Index: gcc/lto/lto-lang.c
===================================================================
--- gcc/lto/lto-lang.c	(revision 158149)
+++ gcc/lto/lto-lang.c	(working copy)
@@ -1031,6 +1031,22 @@
 static bool
 lto_init (void)
 {
+  /* It is pointless to invoke LTO without any kind of optimization enabled.
+     So if the optimization level is zero we issue a warning message to the
+     user.
+
+     If the user is only interested in enabling specific optimizations and
+     they are bugged by this warning they can add -w to the command line.
+     FIXME: Do we need a -Wlto option to allow this warning to be enabled/
+     disabled explicitly ?
+
+     Note, we have to perform this test here, rather than in
+     opts.c:decode_options, because there is no way to tell that we are
+     running as the LTO compiler in that function.  */
+  if (optimize == 0)
+    warning_at (UNKNOWN_LOCATION, 0, "-flto used on object files without an "
+		"optimization level set - was this intentional ?");
+  
   /* We need to generate LTO if running in WPA mode.  */
   flag_generate_lto = flag_wpa;