On Thu, Nov 24, 2022 at 05:53:53PM +0100, Richard Biener wrote: > > > > Am 24.11.2022 um 17:28 schrieb Stefan Schulze Frielinghaus via Gcc > > <gcc@gcc.gnu.org>: > > > > Hi everyone, > > > > Currently I'm looking into a wrong-code bug and would like to understand > > a certain optimization done by combine during local transformation. > > Without LTO I would simply debug cc1 and step through combine. However, > > with LTO enabled AFAIK I have to debug lto1 instead. In order to get > > the lto1 command line of interest according to > > https://gcc.gnu.org/legacy-ml/gcc/2009-11/msg00047.html > > I have to pass -Wl,-debug to gcc in order to get the command for > > collect2 to which itself I have to pass -plugin-opt=-debug in order to > > get the command for lto-wrapper. According to the aforementioned mail I > > should add option -debug to lto-wrapper, however, it appears to me that > > option -debug was removed. I gave options -v and -### a chance without > > luck, i.e., those only print the usual environment variables and > > afterwards a list of object files like > > > > /tmp/ccPEIV35.ltrans0.ltrans.o > > /tmp/ccNmpKfS.debug.temp.o > > /tmp/cceiCIFg.debug.temp.o > > /tmp/ccZ4Qc7E.debug.temp.o > > ... > > > > but no lto1 command. Thus, how do you retrieve the lto1 command? > > > > While desperate I retrieved it manually via strace. However, the lto1 > > command refers to temporary files which have been erased meanwhile. I > > actually didn't expect that because I added -save-temps to all the > > intermediate commands which is also reflected in the environment > > variable COLLECT_GCC_OPTIONS. Thus, how do you keep temporary files? > > Adding -v -save-temps and then running gdb on the lto1 command works and is > what I usually do.
Strangely this is not the case for the current project I'm looking at. The link step is done via /usr/local/bin/s390x-linux-gnu-gcc -save-temps \ -O3 -DNDEBUG -flto=auto -fno-fat-lto-objects \ -march=z15 -static -nostdlib \ -Wl,--fatal-warnings -Wl,--no-warn-rwx-segments \ -Werror -Wall -Wundef -T/devel/foo.ld \ obj1.o ... objN.o where -save-temps is given and was also given during compile time of each object obj1.o ... objN.o file. Anyhow, adding -Wl,-debug to the link command above I see that -save-temps is also included in the environment variable COLLECT_GCC_OPTIONS="... -save-temps ..." and I get the linker command /usr/local/lib/gcc/s390x-linux-gnu/13.0.0/../../../../s390x-linux-gnu/bin/ld \ -plugin /usr/local/libexec/gcc/s390x-linux-gnu/13.0.0/liblto_plugin.so \ -plugin-opt=/usr/local/libexec/gcc/s390x-linux-gnu/13.0.0/lto-wrapper \ -plugin-opt=-fresolution=foo.res \ -m elf64_s390 -static -o foo \ -L/usr/local/lib/gcc/s390x-linux-gnu/13.0.0 \ -L/usr/local/lib/gcc/s390x-linux-gnu/13.0.0/../../../../s390x-linux-gnu/lib \ --fatal-warnings --no-warn-rwx-segments \ obj1.o ... objN.o -lgcc -T /devel/foo.ld to which I add -plugin-opt=-debug in order to get the lto-wrapper command /usr/local/libexec/gcc/s390x-linux-gnu/13.0.0/lto-wrapper \ -fresolution=foo.res -flinker-output=exec \ obj1.o ... objN.o to which I add -v -save-temps in order to actually get the lto1 command. From the output printed to stderr I see one lto1 command: /usr/local/libexec/gcc/s390x-linux-gnu/13.0.0/lto1 -quiet -dumpbase ./a.wpa \ -march=z15 -m64 -mzarch -g -O3 -version -fno-openmp -fno-openacc -fno-pie \ -fcf-protection=none -fltrans-output-list=/tmp/ccsuw4Nj.ltrans.out \ -fwpa=32 -fresolution=foo.res -flinker-output=exec @./a.wpa.args.0 However, this lto1 invocation does no local transformation. Having a look at the strace output I see a second lto1 invocation: /usr/local/libexec/gcc/s390x-linux-gnu/13.0.0/lto1 -quiet \ -dumpbase ./a.ltrans0.ltrans -march=z15 -m64 -mzarch -g -O3 \ -fno-openmp -fno-openacc -fno-pie -fcf-protection=none \ -fltrans /tmp/ccsuw4Nj.ltrans0.o -o /tmp/cc69wMp3.s This lto1 invocation seems to originate from Makefile /tmp/cc5piuXT.mk generated by lto-wrapper. To my surprise the Makefile as well as the temporary file /tmp/ccsuw4Nj.ltrans0.o are removed although -save-temps was given all the time. Stepping through lto-wrapper with gdb I see that save_temps = 1; is never executed while walking over decoded_options. For the time being I hard coded save_temps=1 into lto-wrapper.cc and I am now able to debug the lto1 invocation of interest. Not sure whether there is a more elegant way? All this is based upon a rather huge CMake based project and so far I didn't find the time to come up with a small reproducer. Still hoping this might be useful for someone else. Cheers, Stefan
