[Bug inline-asm/84861] New: -flto with asm() optimizes too much

[dilyan.palauzov at aegee dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84861

            Bug ID: 84861
           Summary: -flto with asm() optimizes too much
           Product: gcc
           Version: lto
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dilyan.palauzov at aegee dot org
  Target Milestone: ---

When compiling valgrind's coregrind/m_libcsetjmp.c
(https://sourceware.org/git/?p=valgrind.git;a=blob;f=coregrind/m_libcsetjmp.c)
with -lto the functions VG_MINIMAL_LONGJMP and VG_MINIMAL_SETJMP disappear on
amd64 from the coregrind/libcoregrind_amd64_linux_a-m_libcsetjmp.o file:

cd coregrind

WITH LTO:
$ make V=1 libcoregrind_amd64_linux_a-m_libcsetjmp.o
gcc -DHAVE_CONFIG_H -I. -I..  -I.. -I../include -I../include -I../VEX/pub
-I../VEX/pub -DVGA_amd64=1 -DVGO_linux=1 -DVGP_amd64_linux=1
-DVGPV_amd64_linux_vanilla=1 -I../coregrind
-DVG_LIBDIR="\"/usr/local/lib/valgrind"\" -DVG_PLATFORM="\"amd64-linux\""  -m64
-O2 -finline-functions -g -Wall -Wmissing-prototypes -Wshadow -Wpointer-arith
-Wstrict-prototypes -Wmissing-declarations -Wcast-align -Wcast-qual
-Wwrite-strings -Wempty-body -Wformat -Wformat-security -Wignored-qualifiers
-Wmissing-parameter-type -Wlogical-op -Wold-style-declaration
-fno-stack-protector -fno-strict-aliasing -fno-builtin  -fomit-frame-pointer
-DENABLE_LINUX_TICKET_LOCK -flto -MT libcoregrind_amd64_linux_a-m_libcsetjmp.o
-MD -MP -MF .deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Tpo -c -o
libcoregrind_amd64_linux_a-m_libcsetjmp.o `test -f 'm_libcsetjmp.c' || echo
'./'`m_libcsetjmp.c
mv -f .deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Tpo
.deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Po


$ nm libcoregrind_amd64_linux_a-m_libcsetjmp.o
nm: libcoregrind_amd64_linux_a-m_libcsetjmp.o: no symbols
(for the other coregrind/libcoregrind_amd64_linux_a-*.o  files `nm` shows
exported symbols).


WITHOUT LTO:
$ make V=1 libcoregrind_amd64_linux_a-m_libcsetjmp.o
gcc -DHAVE_CONFIG_H -I. -I..  -I.. -I../include -I../include -I../VEX/pub
-I../VEX/pub -DVGA_amd64=1 -DVGO_linux=1 -DVGP_amd64_linux=1
-DVGPV_amd64_linux_vanilla=1 -I../coregrind
-DVG_LIBDIR="\"/usr/local/lib/valgrind"\" -DVG_PLATFORM="\"amd64-linux\""  -m64
-O2 -finline-functions -g -Wall -Wmissing-prototypes -Wshadow -Wpointer-arith
-Wstrict-prototypes -Wmissing-declarations -Wcast-align -Wcast-qual
-Wwrite-strings -Wempty-body -Wformat -Wformat-security -Wignored-qualifiers
-Wmissing-parameter-type -Wlogical-op -Wold-style-declaration
-fno-stack-protector -fno-strict-aliasing -fno-builtin  -fomit-frame-pointer
-DENABLE_LINUX_TICKET_LOCK       -MT libcoregrind_amd64_linux_a-m_libcsetjmp.o
-MD -MP -MF .deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Tpo -c -o
libcoregrind_amd64_linux_a-m_libcsetjmp.o `test -f 'm_libcsetjmp.c' || echo
'./'`m_libcsetjmp.c
mv -f .deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Tpo
.deps/libcoregrind_amd64_linux_a-m_libcsetjmp.Po


$ nm libcoregrind_amd64_linux_a-m_libcsetjmp.o
0000000000000052 T VG_MINIMAL_LONGJMP
0000000000000000 T VG_MINIMAL_SETJMP

The only difference is "-flto".

It this a problem of LTO, or is the assembler supposed to be tweaked (how?) so
that the symbols get exported?

Having a file containing
  asm ("");
does compile, but a file containing only:
  asm volatile ("");
fails with "error: expected ‘(’ before ‘volatile’".

Happens with gcc 8.0.1 20180306 (experimental).