[Tarantool-patches] [PATCH 1/1] build: turn off LTO for exports.c

[Vladislav Shpilevoy]

There were lots of errors of kind:

/builds/M4RrgQZ3/0/tarantool/tarantool/src/exports.h:395:1: error: variable ‘uuid_nil’ redeclared as function
  EXPORT(uuid_nil)
  ^
 /builds/M4RrgQZ3/0/tarantool/tarantool/src/lib/uuid/tt_uuid.c:39:22: note: previously declared here
  const struct tt_uuid uuid_nil;

when LTO was enabled. That happened because exports.c file, to
take symbol addresses, declared lots of functions and variables
from all the code base as

    extern void <symbol>(void);

This is crazy, but it worked for normal builds. Because symbol is
symbol. The compilers couldn't find conflicting declarations,
because they never met in one compilation unit.

However the lie was revealed by linker with LTO enabled. It could
see, that actual symbol definitions didn't match their exports in
exports.c. It could live with mismatching function-function or
variable-variable cases, but couldn't withstand function-variable
mismatches. When a symbol was declared as a variable in one place
and as a function in another.

This was the case for variables:
- uuid_nil
- tarantool_lua_ibuf
- log_pid
- log_format
- crc32_calc
- _say
- log_level

The errors were false positive, because the symbols were never
used for anything except taking their addresses. To calm the
linker down exports.c now does not participate in LTO.

Closes #5001
---
Branch: http://github.com/tarantool/tarantool/tree/gerold103/gh-5001-lto-exports-fnolto
Issue: https://github.com/tarantool/tarantool/issues/5001

 src/CMakeLists.txt | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 7a718fde9..c2fb7841c 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -246,6 +246,19 @@ if(BUILD_STATIC)
     endif()
 endif()
 
+if (ENABLE_LTO)
+    # Exports compilation unit is entirely a hack. It references
+    # symbols among static libraries and object files declaring
+    # them all as functions. To avoid header dependencies. This is
+    # not detected by the compilers, since they never see
+    # conflicting definitions in one compilation unit. But this
+    # makes LTO mad, because the linker sees all the definitions,
+    # and is especially angry when a variable is declared as a
+    # function. To get rid of these false positive errors the
+    # exports file is not link-time optimized.
+    set_source_files_properties(exports.c PROPERTIES COMPILE_FLAGS -fno-lto)
+endif()
+
 add_executable(
     tarantool main.cc exports.c
     ${LIBUTIL_FREEBSD_SRC}/flopen.c
-- 
2.21.1 (Apple Git-122.3)