http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61123
Bug ID: 61123
Summary: With LTO, -fno-short-enums is ignored, resulting in
ABI mis-matching in linking.
Product: gcc
Version: lto
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Hale.Wang at arm dot com
I have two trivial source files
obj1.cc:
int x(int y)
{
return y - 10;
}
obj2.cc:
int foo(int bar)
{
return bar*10;
}
If I link them without -flto, link time optimization is not invoked, and I get
an object file marked as having int-sized enums:
$ arm-none-eabi-g++ -fno-short-enums -mcpu=arm946e-s obj1.cc obj2.cc
-Wl,-Ur -o partial_link_result.o -nostdlib -Os
$ arm-none-eabi-readelf -a partial_link_result.o | grep enum
Tag_ABI_enum_size: int
But if I simply add -flto to the linker invocation, the output claims that it
has small enums:
$ arm-none-eabi-g++ -fno-short-enums -mcpu=arm946e-s obj1.cc obj2.cc
-Wl,-Ur -o partial_link_result.o -nostdlib -Os -flto
$ arm-none-eabi-readelf -a partial_link_result.o | grep enum
Tag_ABI_enum_size: small
It sure looks to me like something removed `-fno-short-enums` from
`COLLECT_GCC_OPTIONS` during the LTO step. I have reproduced the same behavior
on several different gcc 4.8 distributions (Mentor's Sourcery 4.8.1, Ubuntu's
4.8.2, and now the gcc-arm-embedded 4.8.3).
(The commands above are from Bobby Moretti)
The reason is that: if we add -flto option, link time optimization is invoked.
And the lto need to call the function run_gcc(unsigned argc, char *argv[]) in
lto-wrapper.c. The options in *argv[] are filtered by the following commands:
if(!(cl_options[option->opt_index].flags & (CL_COMMON | CL_TARGET |
CL_DRIVER | CL_LTO)))
continue;
So all the options which do not belong to the
CL_COMMON|CL_TARGET|CL_DRIVER|CL_LTO groups will be ignored by LTO. And
-fno-short-enums is one of these options.
I think it is a bug of LTO. Because the option flag_short_enums can change the
enum size in the generated object file. We should allow the users to disable
this optimization as they like.
I suggest we can add the option flag_short_enums to the CL_COMMON group(or
CL_LTO group, I am not sure which group is more reasonable). And I have
confirmed that this change works.
There are several other options that are similar with flag_short_enums which
may change the ABI interface. And these options may cause the similar failure
in LTO. So how should we deal with these options? Should we develop a new
option group which could not be ignored by LTO?
