Compiling newlib with fat LTO

[Paulo Matos <pmatos@linki.tools>]

Hi,

I am been attempting to build a riscv toolchain with newlib with
optional LTO. The idea is to use -ffat-lto-objects to include IR
information along with binary in the library so that later I can use lto
optionally when linking to newlib.

The way I am compiling newlib is pretty normal:
$ ../../newlib/configure CFLAGS_FOR_TARGET='-flto -ffat-lto-objects'
LDFLAGS_FOR_TARGET='-flto -ffat-lto-objects'
AR_FOR_TARGET=riscv64-unknown-elf-gcc-ar
RANLIB_FOR_TARGET=riscv64-unknown-elf-gcc-ranlib
NM_FOR_TARGET=riscv64-unknown-elf-gcc-nm
--prefix=/home/pmatos/Projects/RISCV-cross/install
--disable-newlib-fvwrite-in-streamio --disable-newlib-fseek-optimization
--enable-newlib-nano-malloc --disable-newlib-unbuf-stream-opt
--enable-target-optspace --enable-newlib-reent-small
--disable-newlib-wide-orient --disable-newlib-io-float
--enable-newlib-nano-formatted-io --target=riscv64-unknown-elf
--with-sysroot=/home/pmatos/Projects/RISCV-cross/install/riscv64-unknown-elf


The full command lines for a whole toolchain can be found here:
https://gitlab.com/snippets/1833421

All compiles fine but when I try to compile one of the tests from the
gcc testsuite, it fails with undefined '_exit'.
This only happens when I compile newlib with CFLAGS_FOR_TARGET='-flto
-ffat-lto-objects' LDFLAGS_FOR_TARGET='-flto -ffat-lto-objects'

Example:
$ riscv64-unknown-elf-gcc
/home/pmatos/Projects/RISCV-cross/gcc/gcc/testsuite/gcc.c-torture/execute/20000112-1.c
   -fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers
-fdiagnostics-color=never -O1  -w      -lm  -o ./20000112-1.exe
/home/pmatos/Projects/RISCV-cross/install/lib/gcc/riscv64-unknown-elf/9.0.1/../../../../riscv64-unknown-elf/bin/ld:
20000112-1.exe.ltrans0.ltrans.o: in function `.L21':
<artificial>:(.text+0x194): undefined reference to `_exit'
/home/pmatos/Projects/RISCV-cross/install/lib/gcc/riscv64-unknown-elf/9.0.1/../../../../riscv64-unknown-elf/bin/ld:
20000112-1.exe.ltrans0.ltrans.o: in function `.L45':
<artificial>:(.text+0x2f0): undefined reference to `_exit'
/home/pmatos/Projects/RISCV-cross/install/lib/gcc/riscv64-unknown-elf/9.0.1/../../../../riscv64-unknown-elf/bin/ld:
error: could not unlink output file
collect2: error: ld returned 1 exit status

If I dig a bit deeper I notice in the LTO resolution file (use
-save-temps to get it):
$ cat 20000112-1.res | grep '_exit$'
200 bcacc6c809c66af7 UNDEF _exit
199 aa3f5695ee2d0c31 UNDEF _exit

Although, _exit is there in libgloss.a, defined in sys-exit.c.
I am assuming that this is not a bug in newlib or gcc but a mistake on
my side building newlib with LTO.

Has anyone tried this before and can point out anything wrong with the
configure line above?

Regards,

-- 
Paulo Matos