Re: link-time optimization

[Ralf Wildenhues]

Hello Simon, Török, Åke,

* Simon Richter wrote on Wed, Mar 24, 2010 at 12:11:13PM CET:
> I'm trying to use link time optimization with gcc 4.5, which somewhat
> works :), but requires the compiler flags to be passed to the linking
> step as well. libtool however removes them from the command line.

Let's address three reports in one go.

I'm still working on a patch to support -flto in libtool, it needs
changes in several places.

> Is there any harm in passing them to the linking step?

Yes.  Let me answer with a patch to add a FAQ and FAQ entry.
OK to commit?

Thanks,
Ralf

    Add FAQ, and FAQ entry for flags stripped at library link time.
    
    * doc/libtool.texi (FAQ, Stripped link flags): New nodes.
    (Wrapper executables): Replace bug address with macro.
    (Compile mode, Link mode): Document `-Wc,' and `-Xcompiler '
    semantics better.
    * libltdl/config/ltmain.m4sh (func_mode_link): Rewrite comment
    for the flags that are passed through.
    Prompted by reports from Simon Richter <address@hidden>,
    Török Edwin <address@hidden> and
    Åke Sandgren <address@hidden>.

diff --git a/doc/libtool.texi b/doc/libtool.texi
index f07eaf2..f0dd24b 100644
--- a/doc/libtool.texi
+++ b/doc/libtool.texi
@@ -95,6 +95,7 @@ version @value{VERSION}.
 * Dlopened modules::            @code{dlopen}ing libtool-created libraries.
 * Using libltdl::               Libtool's portable @code{dlopen} wrapper 
library.
 * Trace interface::             Libtool's trace interface.
+* FAQ::                         Frequently Asked Questions
 * Troubleshooting::             When libtool doesn't work as advertised.
 * Maintaining::                 Information used by the libtool maintainer.
 * GNU Free Documentation License:: License for this manual.
@@ -186,6 +187,10 @@ Using libltdl
 * Module loaders for libltdl::  Creating user defined module loaders.
 * Distributing libltdl::        How to distribute libltdl with your package.
 
+Frequently Asked Questions
+
+* Stripped link flags::         Dropped flags when creating a library
+
 Troubleshooting
 
 * Libtool test suite::          Libtool's self-tests.
@@ -886,7 +891,7 @@ employ command line options that begin with @code{--lt-}. 
(In fact, the
 wrapper executable will detect any command line options that begin with
 @code{--lt-} and abort with an error message if the option is not
 recognized). If this presents a problem, please contact the Libtool
-team at @email{bug-libtool@@gnu.org}.
+team at @value{BUGADDR}.
 
 These command line options include:
 
@@ -1414,7 +1419,9 @@ linking.
 
 @item -Wc,@var{flag}
 @itemx -Xcompiler @var{flag}
-Pass a flag directly to the compiler.
+Pass a flag directly to the compiler.  With @code{-Wc,}, multiple flags
+may be separated by commas, whereas @code{-Xcompiler } passes through
+commas unchanged.
 @end table
 
 @node Link mode
@@ -1602,7 +1609,9 @@ supplies the same interface (@pxref{Linking with dlopened 
modules}).
 
 @item -Wc,@var{flag}
 @itemx -Xcompiler @var{flag}
-Pass a linker-specific flag directly to the compiler.
+Pass a linker-specific flag directly to the compiler.  With @code{-Wc,},
+multiple flags may be separated by commas, whereas @code{-Xcompiler }
+passes through commas unchanged.
 
 @item -Wl,@var{flag}
 @itemx -Xlinker @var{flag}
@@ -4946,6 +4955,46 @@ autoconf --trace 'LT_SUPPORTED_TAG:$1'
 @end defmac
 
 
address@hidden FAQ
address@hidden Frequently Asked Questions about libtool
+
+This chapter covers some questions that often come up on the mailing
+lists.
+
address@hidden
+* Stripped link flags::         Dropped flags when creating a library
address@hidden menu
+
address@hidden Stripped link flags
address@hidden Why does libtool strip link flags when creating a library?
+
+When creating a shared library, but not when compiling or creating
+a program, @command{libtool} drops some flags from the command line
+provided by the user.  This is done because flags unknown to
address@hidden may interfere with library creation or require
+additional support from @command{libtool}, and because omitting
+flags is usually the conservative choice for a successful build.
+
+If you encounter flags that you think are useful to pass, as a
+work-around you can prepend flags with @code{-Wc,} or @code{-Xcompiler }
+to allow them to be passed through to the compiler driver
+(@pxref{Link mode}).  Another possibility is to add flags already
+to the compiler command at @command{configure} run time:
+
address@hidden
+./configure CC='gcc -m64'
address@hidden example
+
+If you think @command{libtool} should let some flag through by default,
+here's how you can test such an inclusion: grab the Libtool development
+tree, edit the @file{ltmain.m4sh} file in the @file{libltdl/config}
+subdirectory to pass through the flag (search for @samp{Flags to be
+passed through}), re-bootstrap and build with the flags in question
+added to @code{LDFLAGS}, @code{CFLAGS}, @code{CXXFLAGS}, etc. on the
address@hidden command line as appropriate.  Run the testsuite
+as described in the @file{README} file and report results to
address@hidden
+
 @node Troubleshooting
 @chapter Troubleshooting
 @cindex troubleshooting
diff --git a/libltdl/config/ltmain.m4sh b/libltdl/config/ltmain.m4sh
index 9790579..b915203 100644
--- a/libltdl/config/ltmain.m4sh
+++ b/libltdl/config/ltmain.m4sh
@@ -4489,17 +4489,17 @@ func_mode_link ()
        arg="$func_quote_for_eval_result"
        ;;
 
-      # -64, -mips[0-9] enable 64-bit mode on the SGI compiler
-      # -r[0-9][0-9]* specifies the processor on the SGI compiler
-      # -xarch=*, -xtarget=* enable 64-bit mode on the Sun compiler
-      # +DA*, +DD* enable 64-bit mode on the HP compiler
-      # -q* pass through compiler args for the IBM compiler
-      # -m*, -t[45]*, -txscale* pass through architecture-specific
-      # compiler args for GCC
-      # -F/path gives path to uninstalled frameworks, gcc on darwin
-      # -p, -pg, --coverage, -fprofile-* pass through profiling flag for GCC
-      # @file GCC response files
-      # -tp=* Portland pgcc target processor selection
+      # Flags to be passed through unchanged, with rationale:
+      # -64, -mips[0-9]      enable 64-bit mode for the SGI compiler
+      # -r[0-9][0-9]*        specify processor for the SGI compiler
+      # -xarch=*, -xtarget=* enable 64-bit mode for the Sun compiler
+      # +DA*, +DD*           enable 64-bit mode for the HP compiler
+      # -q*                  compiler args for the IBM compiler
+      # -m*, -t[45]*, -txscale* architecture-specific flags for GCC
+      # -F/path              path to uninstalled frameworks, gcc on darwin
+      # -p, -pg, --coverage, -fprofile-*  profiling flag for GCC
+      # @file                GCC response files
+      # -tp=*                Portland pgcc target processor selection
       -64|-mips[0-9]|-r[0-9][0-9]*|-xarch=*|-xtarget=*|+DA*|+DD*|-q*|-m*| \
       -t[45]*|-txscale*|-p|-pg|--coverage|-fprofile-*|-F*|@*|-tp=*)
         func_quote_for_eval "$arg"