Re: [PATCH roland/arm-mcount] ARM: Disable compat mcount code when unneeded.

[Roland McGrath <roland at hack dot frob dot com>]

[This new patch is on the replaced branch roland/arm-mcount.]

> It does occur to me that for the hard-float ABI (__ARM_PCS_VFP) it would 
> be OK in principle to make the old entry point into a compat symbol in the 
> shared library and remove it from the static library, so not available for 
> new links, given that the hard-float ABI wasn't supported before GCC 4.5 
> (and GCC has never supported marking objects that don't use floating point 
> as "don't care" for the ABI, so soft-float objects built with older GCC 
> couldn't be used with a hard-float libc).  But that's probably not 
> worthwhile (and conditioning something that's nothing to do with floating 
> point on the floating-point ABI would at least be rather confusing).

I've finally gotten back to this.  I made it a bit cleaner than the minimal
requirement, by introducing gcc-compat.h and GCC_COMPAT as a rough analogue
to shlib-compat.h and SHLIB_COMPAT.

I've done an arm-linux-gnueabi build and verified that arm-mcount.o{,s}
have no differences except for the addition of the __mcount_arm_compat
symbol, and that 'make check-abi' still passes.

I've done an arm-linux-gnueabihf build and verified that arm-mcount.o has
no differences except the removal of the _mcount code and the mcount alias,
and that arm-mcount.os has no differences except in its symbol table, and
that 'make check-abi' still passes.  That is, the one user-visible change
here is that the symbols _mcount and mcount are no longer available at link
time (either static or dynamic) for arm-linux-gnueabihf.  Does that merit a
NEWS mention?

I've also done a build of my unfinished arm-nacl port (which has earliest
symbol set GLIBC_2.19), and verified that the _mcount code and the symbols
_mcount and mcount are entirely gone from everywhere.

OK?

I have a mild preference for putting this in before the freeze, just to
avoid having to push the SHLIB_COMPAT tests to GLIBC_2.20.


Thanks,
Roland


2014-01-10  Roland McGrath  <roland@hack.frob.com>

	* sysdeps/generic/gcc-compat.h: New file.

ports/ChangeLog.arm
2014-01-10  Roland McGrath  <roland@hack.frob.com>

	* sysdeps/arm/arm-mcount.S:
	#include <shlib-compat.h> and <gcc-compat.h>.
	(_mcount): Renamed to __mcount_arm_compat.  Made conditional on
	[GCC_COMPAT (4, 3) || SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_19)].
	(_mcount, mcount): Define (as aliases) only under [GCC_COMPAT (4, 3)],
	with compat_symbol under [SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_19)].

	* sysdeps/arm/gcc-compat.h: New file.

--- a/ports/sysdeps/arm/arm-mcount.S
+++ b/ports/sysdeps/arm/arm-mcount.S
@@ -21,6 +21,7 @@
 
 #include <sysdep.h>
 
+#undef mcount
 
 #ifdef __thumb2__
 	.thumb
@@ -65,10 +66,20 @@ ENTRY(__gnu_mcount_nc)
 END(__gnu_mcount_nc)
 
 
+#include <gcc-compat.h>
+#include <shlib-compat.h>
+
+/* The new __gnu_mcount_nc entry point was introduced in 4.4, so the
+   static library needs the old one only to support older compilers.
+   Even in a configuration that only cares about newer compilers, the
+   shared library might need it only for strict ABI compatibility.  */
+
+#if GCC_COMPAT (4, 3) || SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_19)
+
 /* Provide old mcount for backwards compatibility.  This requires
    code be compiled with APCS frame pointers.  */
 
-ENTRY(_mcount)
+ENTRY(__mcount_arm_compat)
 	push	{r0, r1, r2, r3, fp, lr}
 	cfi_adjust_cfa_offset (24)
 	cfi_rel_offset (r0, 0)
@@ -83,7 +94,7 @@ ENTRY(_mcount)
 	ldrne r0, [\B, #-4]
 	movsne r1, lr
 	blne __mcount_internal
-#if defined (__ARM_ARCH_4T__) && defined (__THUMB_INTERWORK__)
+# if defined (__ARM_ARCH_4T__) && defined (__THUMB_INTERWORK__)
 	pop	{r0, r1, r2, r3, fp, lr}
 	cfi_adjust_cfa_offset (-24)
 	cfi_restore (r0)
@@ -93,12 +104,26 @@ ENTRY(_mcount)
 	cfi_restore (fp)
 	cfi_restore (lr)
 	bx lr
-#else
+# else
 	pop	{r0, r1, r2, r3, fp, pc}
+# endif
+END(__mcount_arm_compat)
+
 #endif
-END(_mcount)
+
+#if GCC_COMPAT (4, 3)
+
+strong_alias (__mcount_arm_compat, _mcount)
 
 /* The canonical name for the function is `_mcount' in both C and asm,
    but some old asm code might assume it's `mcount'.  */
-#undef mcount
 weak_alias (_mcount, mcount)
+
+#elif SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_19)
+
+compat_symbol (libc, __mcount_arm_compat, _mcount, GLIBC_2_0)
+
+strong_alias (__mcount_arm_compat, __mcount_arm_compat_1)
+compat_symbol (libc, __mcount_arm_compat_1, mcount, GLIBC_2_0)
+
+#endif
--- /dev/null
+++ b/ports/sysdeps/arm/gcc-compat.h
@@ -0,0 +1,35 @@
+/* Macros for checking required GCC compatibility.  ARM version.
+   Copyright (C) 2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef _ARM_GCC_COMPAT_H
+#define _ARM_GCC_COMPAT_H 1
+
+#ifndef GCC_COMPAT_VERSION
+# ifdef __ARM_PCS_VFP
+/* The hard-float ABI was first supported in 4.5.  */
+#  define GCC_COMPAT_VERSION    GCC_VERSION (4, 5)
+# else
+/* The EABI configurations (the only ones we handle) were first supported
+   in 4.1.  */
+#  define GCC_COMPAT_VERSION    GCC_VERSION (4, 1)
+# endif
+#endif
+
+#include_next <gcc-compat.h>
+
+#endif
--- /dev/null
+++ b/sysdeps/generic/gcc-compat.h
@@ -0,0 +1,42 @@
+/* Macros for checking required GCC compatibility.  Generic version.
+   Copyright (C) 2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* This is the base file.  More-specific sysdeps/.../gcc-compat.h files
+   can define GCC_COMPAT_VERSION and then #include_next this file.  */
+
+#ifndef _GENERIC_GCC_COMPAT_H
+#define _GENERIC_GCC_COMPAT_H 1
+
+/* This is the macro that gets used in #if tests in code: true iff
+   the library we build must be compatible with user code built by
+   GCC version MAJOR.MINOR.  */
+#define GCC_COMPAT(major, minor)        \
+  (GCC_COMPAT_VERSION <= GCC_VERSION (major, minor))
+
+/* This is how we compose an integer from major and minor version
+   numbers, for comparison.  */
+#define GCC_VERSION(major, minor)       \
+  (((major) << 16) + (minor))
+
+#ifndef GCC_COMPAT_VERSION
+/* GCC 2.7.2 was current at the time of the glibc-2.0 release.
+   We assume nothing before that ever mattered.  */
+# define GCC_COMPAT_VERSION     GCC_VERSION (2, 7)
+#endif
+
+#endif