Improve building with LTO

Use symver attribute for symbol versioning instead of .symver
assembler directive when available. Convert to use double @ syntax
for default version in all cases (required when using the attribute).

Add the attributes externally_visible, no_reorder if available when
using assembler directives to improve the situation for < gcc-10.
This is not 100% reliable, though; -flto-partition=none may still be
needed in some cases.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24498
Signed-off-by: Alexander Miller <[email protected]>

2 files changed, 66 insertions, 12 deletions

diff --git a/lib/ChangeLog b/lib/ChangeLog
index c72452b1..8f4d4d9f 100644
--- a/lib/ChangeLog
+++ b/lib/ChangeLog
@@ -1,3 +1,16 @@
+2021-02-14  Alexander Miller  <[email protected]>
+
+	* eu-config.h (used_in_asm): New macro.
+	(NEW_INTDEF): New macro.
+	(NEW_VERSION): Mark symbol as used_in_asm.  Use @@ symver and change
+	asm name instead.  New variant using symver attribute if available.
+	(OLD_VERSION): Update new symbol name. Indent asm directives.  New
+	variant using symver attribute.
+	(COMPAT_VERSION_NEWPROTO): Mark symbol as used_in_asm.  Reorder
+	lines.  Replace asm with __asm__ in declaration.  New variant using
+	symver attribute.
+	(COMPAT_VERSION): Likewise.
+
 2021-09-10  Colin Cross <[email protected]>
 
 	* error.c (error): Call fflush on stdout and stderr. Setup errno and
diff --git a/lib/eu-config.h b/lib/eu-config.h
index f0e3d07a..c7d7cbb2 100644
--- a/lib/eu-config.h
+++ b/lib/eu-config.h
@@ -176,27 +176,68 @@ asm (".section predict_data, \"aw\"; .previous\n"
 /* This macro is used by the tests conditionalize for standalone building.  */
 #define ELFUTILS_HEADER(name) <lib##name.h>
 
+/* Don't reorder with global asm blocks or optimize away. (Doesn't reliably
+   keep it in the same LTO partition, though; -flto-partition=none may be
+   still needed for some gcc versions < 10.) */
+#ifdef __has_attribute
+# if __has_attribute(no_reorder)
+#  define used_in_asm __attribute__ ((externally_visible, no_reorder))
+# endif
+#endif
+#ifndef used_in_asm
+# define used_in_asm /* empty */
+#endif
 
 #ifdef SYMBOL_VERSIONING
-# define OLD_VERSION(name, version) \
-  asm (".globl _compat." #version "." #name "\n" \
-       "_compat." #version "." #name " = " #name "\n" \
-       ".symver _compat." #version "." #name "," #name "@" #version);
-# define NEW_VERSION(name, version) \
-  asm (".symver " #name "," #name "@@@" #version);
-# define COMPAT_VERSION_NEWPROTO(name, version, prefix) \
-  asm (".symver _compat." #version "." #name "," #name "@" #version); \
+# define NEW_INTDEF(name) __typeof (name) INTUSE(name) \
+  __attribute__ ((alias ("_new." #name))) attribute_hidden;
+# ifdef __has_attribute
+#  if __has_attribute(symver)
+#   define NEW_VERSION(name, version) \
+  __typeof (name) name __asm__ ("_new." #name) \
+    __attribute__ ((symver (#name "@@" #version)));
+#   define OLD_VERSION(name, version) _OLD_VERSION1(name, __COUNTER__, version)
+#   define _OLD_VERSION1(name, num, version) _OLD_VERSION2(name, num, version)
+#   define _OLD_VERSION2(name, num, version) \
+  __typeof (name) _compat_old##num##_##name \
+    __asm__ ("_compat." #version "." #name) \
+    __attribute__ ((alias ("_new." #name), symver (#name "@" #version)));
+#   define COMPAT_VERSION_NEWPROTO(name, version, prefix) \
   __typeof (_compat_##prefix##_##name) _compat_##prefix##_##name \
-    asm ("_compat." #version "." #name);
-# define COMPAT_VERSION(name, version, prefix) \
+    __asm__ ("_compat." #version "." #name) \
+    __attribute__ ((symver (#name "@" #version)));
+#   define COMPAT_VERSION(name, version, prefix) \
   asm (".symver _compat." #version "." #name "," #name "@" #version); \
-  __typeof (name) _compat_##prefix##_##name asm ("_compat." #version "." #name);
+  __typeof (name) _compat_##prefix##_##name \
+    __asm__ ("_compat." #version "." #name) \
+    __attribute__ ((symver (#name "@" #version)));
+#  endif
+# endif
+# ifndef NEW_VERSION
+#  define OLD_VERSION(name, version) \
+  asm (".globl _compat." #version "." #name "\n\t" \
+       "_compat." #version "." #name " = _new." #name "\n\t" \
+       ".symver _compat." #version "." #name "," #name "@" #version);
+#  define NEW_VERSION(name, version) \
+  __typeof (name) name __asm__ ("_new." #name) used_in_asm; \
+  asm (".symver _new." #name ", " #name "@@" #version);
+#  define COMPAT_VERSION_NEWPROTO(name, version, prefix) \
+  __typeof (_compat_##prefix##_##name) _compat_##prefix##_##name \
+    __asm__ ("_compat." #version "." #name) used_in_asm; \
+  asm (".symver _compat." #version "." #name ", " #name "@" #version);
+#  define COMPAT_VERSION(name, version, prefix) \
+  __typeof (name) _compat_##prefix##_##name \
+    __asm__ ("_compat." #version "." #name) used_in_asm; \
+  asm (".symver _compat." #version "." #name ", " #name "@" #version);
+# endif
 #else
+# define NEW_INTDEF(name) INTDEF(name)
 # define OLD_VERSION(name, version) /* Nothing for static linking.  */
 # define NEW_VERSION(name, version) /* Nothing for static linking.  */
 # define COMPAT_VERSION_NEWPROTO(name, version, prefix) \
   error "should use #ifdef SYMBOL_VERSIONING"
-# define COMPAT_VERSION(name, version, prefix) error "should use #ifdef SYMBOL_VERSIONING"
+# define COMPAT_VERSION(name, version, prefix) \
+  error "should use #ifdef SYMBOL_VERSIONING"
 #endif
 
 #ifndef FALLTHROUGH