Making it easy to use -flto for bootstrap

[Alexandre Oliva <aoliva at redhat dot com>]

I've been playing with -flto bootstraps.

The first problem I ran into was that LTO sections may differ depending
on the presence of debug stmts, so compare-debug needed some tweaking.

Furthermore, simply adding -flto to BOOT_CFLAGS didn't quite work,
because Ada doesn't seem to be ready for LTO.  I got a crash during
linking and didn't pursue it any further.  Adding -fno-lto to
BOOT_ADAFLAGS was the work-around I ended up with.

To make my partly-automated testing easier, I added bootstrap-lto as a
BUILD_CONFIG option.

I didn't have much luck with -flto in target libraries (bad relocs
creating shared libs on x86_64-linux-gnu IIRC), so I left it out.  Was
-flto supposed to work for shared libs?

I ran into -fcompare-debug failures with -flto on x86_64-linux-gnu, and
also with bootstrap-O3 on ia64-lixnu-gnu, but I haven't looked into them
yet.

This is the patch I'm going to install if there aren't objections within
24 hours or so.

for  contrib/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* compare-debug: Drop LTO sections.

for  config/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* bootstrap-lto.mk: New.

for  gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* doc/install.texi: Document bootstrap-lto.

Index: contrib/compare-debug
===================================================================
--- contrib/compare-debug.orig	2010-06-16 10:30:16.000000000 -0300
+++ contrib/compare-debug	2010-06-16 10:44:18.000000000 -0300
@@ -100,9 +100,11 @@ else
   done
 
   # If we found .eh_frame in one but not the other, or if we could not
-  # find a command to tell, try to strip off the .eh_frame section
-  # from both.
-  if test "x$cmp1" != "x$cmp2" || test "x$cmd" = "x"; then
+  # find a command to tell, or if there are LTO sections, try to strip
+  # off the .eh_frame and LTO sections from both.
+  if test "x$cmp1" != "x$cmp2" || test "x$cmd" = "x" ||
+     $cmd --section-headers "$1.$suf1" | grep '.gnu.lto_' > /dev/null ||
+     $cmd --section-headers "$2.$suf2" | grep '.gnu.lto_' > /dev/null ; then
     suf3=$suf1.
     while test -f "$1.$suf3"; do
       suf3=$suf3.
@@ -115,21 +117,27 @@ else
 
     trap 'rm -f "$1.$suf1" "$2.$suf2" "$1.$suf3" "$2.$suf4"' 0 1 2 15
 
-    echo stripping off .eh_frame, then retrying >&2
+    echo stripping off .eh_frame and LTO sections, then retrying >&2
+
+    seclist=".eh_frame .rel.eh_frame .rela.eh_frame"
+    if test "x$cmd" != "x"; then
+      seclist="$seclist "`{ $cmd --section-headers "$1.$suf1"; $cmd --section-headers "$2.$suf2"; } | sed -n 's,.* \(\.gnu\.lto_[^ 	]*\).*,\1,p' | sort -u`
+    fi
+    rsopts=`for sec in $seclist; do echo " --remove-section $sec"; done`
 
     if (objcopy -v) 2>&1 | grep ' --remove-section' > /dev/null; then
-      objcopy --remove-section .eh_frame --remove-section .rel.eh_frame --remove-section .rela.eh_frame "$1.$suf1" "$1.$suf3"
+      objcopy $rsopts "$1.$suf1" "$1.$suf3"
       mv "$1.$suf3" "$1.$suf1"
 
-      objcopy --remove-section .eh_frame --remove-section .rel.eh_frame --remove-section .rela.eh_frame "$2.$suf2" "$2.$suf4"
+      objcopy $rsopts "$2.$suf2" "$2.$suf4"
       mv "$2.$suf4" "$2.$suf2"
     elif (strip --help) 2>&1 | grep ' --remove-section' > /dev/null; then
       cp "$1.$suf1" "$1.$suf3"
-      strip --remove-section .eh_frame --remove-section .rel.eh_frame --remove-section .rela.eh_frame "$1.$suf3"
+      strip $rsopts "$1.$suf3"
       mv "$1.$suf3" "$1.$suf1"
 
       cp "$2.$suf2" "$2.$suf4"
-      strip --remove-section .eh_frame --remove-section .rel.eh_frame --remove-section .rela.eh_frame "$2.$suf4"
+      strip $rsopts "$2.$suf4"
       mv "$2.$suf4" "$2.$suf2"
     else
       echo failed to strip off .eh_frame >&2
Index: config/bootstrap-lto.mk
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ config/bootstrap-lto.mk	2010-06-16 16:56:28.000000000 -0300
@@ -0,0 +1,8 @@
+# This option enables LTO for stage2 and stage3.  It requires lto to
+# be enabled for stage1 with --enable-stage1-languages.
+
+STAGE2_CFLAGS += -flto
+STAGE3_CFLAGS += -flto
+
+# Ada fails to build with LTO, turn it off for now.
+BOOT_ADAFLAGS += -fno-lto
Index: gcc/doc/install.texi
===================================================================
--- gcc/doc/install.texi.orig	2010-06-16 16:56:26.000000000 -0300
+++ gcc/doc/install.texi	2010-06-16 16:58:10.000000000 -0300
@@ -2150,6 +2150,11 @@ Removes any @option{-O}-started option f
 @item @samp{bootstrap-O3}
 Analogous to @code{bootstrap-O1}.
 
+@item @samp{bootstrap-lto}
+Enables Link-Time Optimization for host tools during bootstrapping.
+@samp{BUILD_CONFIG=bootstrap-lto} is equivalent to adding
+@option{-flto} to @samp{BOOT_CFLAGS}.
+
 @item @samp{bootstrap-debug}
 Verifies that the compiler generates the same executable code, whether
 or not it is asked to emit debug information.  To this end, this


-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer