Hi Kewen,
The ticket you linked does sound similar. And thanks for your suggestions.
I tried them out and summarized the results below:
1) Don't use linker-plugin, use -fno-use-linker-plugin explicitly.
The build finishes and outputs the result I expect. My custom
implementation of _write is pulled into the final binary. But wouldn't
-fno-use-linker-plugin effectively disable LTO in the sense that my project
won't be able to take advantage of more aggressive
interprocedural optimizations?
2) Use shared library instead of static library for _write library.
I can't use a shared library in my case since I'm compiling for an embedded
target without any sort of dynamic linker.
3) -Wl,--whole-archive -l<your_static_write_library> -Wl,--no-whole-archive
I changed the build command to the following:
@$(CC) $(FLAGS) -fuse-linker-plugin -fno-common -Wl,-Map=output.map
-Wl,--gc-sections -o $@ -Wl,--start-group main.a -lc -Wl,--end-group
-Wl,--whole-archive _my_write.a -Wl,--no-whole-archive
Result:
*```*
> LTO=1 make
.cmake/install/GNUARM-9.2.1-Darwin/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/bin/ld:
/Users/aw/.cmake/install/GNUARM-9.2.1-Darwin/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib/thumb/v7e-m/nofp/libc_nano.a(lib_a-writer.o):
in function `_write_r':
writer.c:(.text._write_r+0x10): undefined reference to `_write'
collect2: error: ld returned 1 exit status
make: *** [main.elf] Error 1
```
Can't seem to link when using whole archives.
If you have any more suggestions, it would be much appreciated!
From,
Aw
On Sun, Jun 28, 2020 at 4:54 AM Kewen.Lin <linkw@xxxxxxxxxxxxx> wrote:
> on 2020/6/16 上午10:56, Alice Wang via Gcc-help wrote:
> > Hi,
> >
> > I've come across some unexpected behavior and I'd appreciate your input
> on
> > the issue. I have an example project below that reproduces the issue. In
> > short, when building the project without LTO, a function will resolve to
> my
> > expected version, but when building with LTO, the symbol is resolved to a
> > different definition that I do not want.
> >
> > Specifically, when building without LTO (which results in the behavior I
> > want), the final binary will pull in the definition of `_write` from my
> > custom static library that is listed first in the library list. With LTO
> > enabled (the bad case), the final binary will pull in the definition of
> > `_write` from libnosys, which comes later in the list of static
> libraries.
> >
> > The output of my test project is pasted below. `make` builds without LTO
> > and dumping the resulting binary shows that `_write` is my stub
> > implementation. `LTO=1 make` builds with LTO enabled, and dumping the
> > resulting binary shows the implementation comes from libnosys.
> >
> > How can I enable LTO in my build while providing custom definitions for
> > library calls (like _write)?
> >
> > ~/D/lto_test ❯❯❯ make
> >
> > Final binary:
> > 00008ae0 <_write>:
> > 8ae0: 4610 mov r0, r2
> > 8ae2: 4770 bx lr
> >
> > ~/D/lto_test ❯❯❯ make clean
> > ~/D/lto_test ❯❯❯ LTO=1 make
> >
> > Final binary:
> > 00008b50 <_write>:
> > 8b50: 4b02 ldr r3, [pc, #8] ; (8b5c <_write+0xc>)
> > 8b52: 2258 movs r2, #88 ; 0x58
> > 8b54: 601a str r2, [r3, #0]
> > 8b56: f04f 30ff mov.w r0, #4294967295 ; 0xffffffff
> > 8b5a: 4770 bx lr
> > 8b5c: 00018c84 .word 0x00018c84
> >
> > ------------------------------------------------
> >
> > Toolchain version:
> > ARM 9-2020-q2-update (based on the 9 series)
> >
> > Repro steps:
> > make
> > make clean
> > LTO=1 make
> >
> > Repro project:
> >
> > main.c
> > ```
> > #include <stdint.h>
> > #include <stdlib.h>
> > #include <stdio.h>
> > #include <string.h>
> >
> > int main(int argc, char *argv[]) {
> > printf("Hello.\n");
> > return 0;
> > }
> > ```
> >
> > write.c
> > ```
> > #include <unistd.h>
> >
> > ssize_t _write (int fd,
> > void *p_buf,
> > size_t cnt)
> > {
> > return cnt;
> > }
> > ```
> >
> > Makefile
> > Note: You'll have to change the path to point to your local installation
> of
> > the ARM 2020 Q2 toolchain.
> > ```
> > TOOLCHAIN=<YOUR_PATH_TO_TOOLCHAIN>
> > LTO_PLUGIN=$(TOOLCHAIN)/lib/gcc/arm-none-eabi/9.3.1/liblto_plugin.so
> >
> > CC=$(TOOLCHAIN)/bin/arm-none-eabi-gcc
> > AR=$(TOOLCHAIN)/bin/arm-none-eabi-ar
> > DUMP=$(TOOLCHAIN)/bin/arm-none-eabi-objdump
> > NM=$(TOOLCHAIN)/bin/arm-none-eabi-gcc-nm
> >
> > FLAGS=-g -Os -mthumb -mcpu=cortex-m4 -ffunction-sections -fdata-sections
> > --specs=nano.specs --specs=nosys.specs
> > ifeq ($(LTO),1)
> > FLAGS+=-flto -ffat-lto-objects
> > endif
> >
> >
> > .PHONY: dump
> > dump: main.elf _my_write.o
> > @echo "\n======================== Dump ========================"
> > @$(NM) -nS _my_write.o
> > @echo "\n==================== Final binary ===================="
> > @$(DUMP) -d $< | grep "<_write>:" -A 6
> > @echo "\n====================== _my_write.o ======================"
> > @$(DUMP) -d $(word 2,$^) | grep "<_write>:" -A 6
> > @echo "\n======================= libnosys ========================"
> > @$(DUMP) -d $(TOOLCHAIN)/arm-none-eabi/lib/thumb/v7e-m/nofp/libnosys.a |
> > grep "<_write>:" -A 6
> >
> > main.elf: main.a _my_write.a
> > @$(CC) $(FLAGS) -fuse-linker-plugin -fno-common -Wl,-Map=output.map
> > -Wl,--gc-sections -o $@ -Wl,--start-group $^ -lc -Wl,--end-group
> >
> > %.a: %.o
> > @$(AR) --plugin $(LTO_PLUGIN) -rcs $@ $<
> >
> > _my_write.o: ./write.c Makefile
> > @$(CC) $(FLAGS) -c $< -o $@
> >
> > main.o: main.c Makefile
> > @$(CC) $(FLAGS) -c $< -o $@
> >
> > .PHONY: clean
> > clean:
> > @rm -rf _write.a main.a main.elf *.res *.out *.o *.s *.map *.dump *.a
> > ```
> >
> > Once again, any insight would be greatly appreciated. My goal is to
> enable
> > LTO in my project while being able to provide custom definitions of
> > specific standard library calls.
> >
> > Thanks in advance.
> >
> > From,
> > AW
> >
>
> Hi Alice,
>
> This symptom made me recall one PR
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91287
>
> Sorry that I don't have the environment for reproduction locally.
>
> Do you mind to try each of below:
> 1) Don't use linker-plugin, use -fno-use-linker-plugin explicitly.
> 2) Use shared library instead of static library for _write library.
> 3) -Wl,--whole-archive -l<your_static_write_library>
> -Wl,--no-whole-archive
>
> HTH.
>
> BR,
> Kewen
>
