https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84847
--- Comment #1 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 13 Mar 2018, dmalcolm at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84847 > > Bug ID: 84847 > Summary: [8 Regression] Incompatibility between early LTO debug > and "-Wl,--gc-sections" leads to corrupt DWARF > debuginfo > Product: gcc > Version: 8.0 > Status: UNCONFIRMED > Severity: normal > Priority: P3 > Component: lto > Assignee: unassigned at gcc dot gnu.org > Reporter: dmalcolm at gcc dot gnu.org > CC: marxin at gcc dot gnu.org, nickc at gcc dot gnu.org, > rguenth at gcc dot gnu.org > Target Milestone: --- > > The downstream bug report: > https://bugzilla.redhat.com/show_bug.cgi?id=1543912 > describes a problem building systemd and systemd-bootchart with gcc 8, which > turned out to be due to those packages' configure scripts injecting both > -flto > and > -Wl,--gc-sections > > They build fine with just one of them, but with both, the binaries have > corrupt > DWARF debuginfo. > > Adding "-Wl,--print-gc-sections" shows the issue more clearly. > > I created a minimal reproducer here: > https://github.com/davidmalcolm/rhbz-1543912 > with which I can reproduce the issue, using: > gcc-8.0.1-0.16.fc29.x86_64 > binutils-2.30-6.fc29.x86_64 > (this is on Fedora in a chroot via "mock -r fedora-rawhide-x86_64 shell"; I'm > still working on reproducing this outside a chroot) > > $ make > gcc -I ./src -flto -O2 -g -c src/bootchart.c -o build/bootchart.o > gcc -I ./src -flto -O2 -g -c src/log.c -o build/log.o > gcc -flto -g -Wl,--gc-sections -Wl,--print-gc-sections \ > build/bootchart.o build/log.o \ > -o build/systemd-bootchart > /usr/bin/ld: Removing unused section '.rodata.cst4' in file > '/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o' > /usr/bin/ld: Removing unused section '.data' in file > '/usr/lib/gcc/x86_64-redhat-linux/8/../../../../lib64/crt1.o' > /usr/bin/ld: Removing unused section '.rodata' in file > '/usr/lib/gcc/x86_64-redhat-linux/8/crtbegin.o' > /usr/bin/ld: Removing unused section '.debug_abbrev' in file > '/tmp/cc1vnNPxdebugobj' > /usr/bin/ld: Removing unused section '.debug_str' in file > '/tmp/cc1vnNPxdebugobj' > /usr/lib/rpm/debugedit build/systemd-bootchart > /usr/lib/rpm/debugedit: build/systemd-bootchart: Invalid .line_table offset > 0x2b0803 > /usr/lib/rpm/debugedit: build/systemd-bootchart: Could not find DWARF > abbreviation 7 > > In the above, note how ld has garbage-collected the '.debug_abbrev' and > '.debug_str' sections from the /tmp/cc*debugobj file. > > Examining the binary via > eu-readelf -w build/systemd-bootchart > shows bogus-looking compile_unit DIEs in the binary: > > DWARF section [23] '.debug_info' at offset 0x10a6: > [Offset] > Compilation unit at offset 0: > Version: 4, Abbreviation section offset: 0, Address size: 8, Offset size: 4 > [ b] compile_unit > producer (strp) "/builddir/build/BUILD/minimizing-2" > language (data1) C99 (12) > name (strp) "/builddir/build/BUILD/minimizing-2" > comp_dir (strp) "/builddir/build/BUILD/minimizing-2" > ranges (sec_offset) range list [ 2] > low_pc (addr) 0x000000250e030100 > Compilation unit at offset 105: > Version: 4, Abbreviation section offset: 0, Address size: 8, Offset size: 4 > [ 74] compile_unit > producer (strp) "/builddir/build/BUILD/minimizing-2" > language (data1) C99 (12) > name (strp) "/builddir/build/BUILD/minimizing-2" > comp_dir (strp) "/builddir/build/BUILD/minimizing-2" > ranges (sec_offset) range list [ 2] > low_pc (addr) 0x0000870006030100 > > Note how every string is using string 0. > > Removing the -Wl,--gc-sections leads to sane-looking DIEs. > > The issue seems to be that lto-wrapper generates /tmp/cc*debugobj files > containing "early LTO debug info", but, quoting Nick in: > https://bugzilla.redhat.com/show_bug.cgi?id=1543912#c22 > > I have now found out what it happening, although I am still a little bit > > confused as to why it should be occurring. The LTO pass is producing an > > object file that only contains debug information: > > > > /dev/shm/ccqhNEybdebugobj > > > > (There was a clue in the filename, but I did not pay attention to this). > > > > The linker's garbage collection code reasons that any file that does not > > have *any* allocatable sections[1] can automatically be discarded, since > > it > > cannot have any effect on the execution of the resulting binary, right ? > > ... > > > > So this is why the .debug_abbrev and .debug_str sections are being > > discarded. > > I do not know why the LTO pass is producing this file. Perhaps someone > > else > > more familiar with LTO can explain. > > > > A simple fix would be remove the code from the linker's garbage collector > > that discards non-allocatable input files, but this seems rather heavy > > handed. I would rather know if there is some way to deduce that a given > > file really is needed, despite it only containing debug information, and > > then choose to keep it. I guess it all comes down to why the LTO pass is > > generating this file, and what it contains. > > > > Cheers > > Nick > > > > [1]. Actually the linker first checks to see if any allocatable sections > > can be discarded. Then if the file did not contain any allocatable > > sections, or all of the allocatable sections can be discarded, then the > > whole file is discarded. > > Nick has posted a patch for lto-wrapper at: > https://bugzilla.redhat.com/show_bug.cgi?id=1543912#c28 > as a workaround for this (actually, a 2nd iteration, based on some nit-picking > by me - but Richard is the expert at this). Isn't this the same as https://sourceware.org/bugzilla/show_bug.cgi?id=20882 which was appearantly set to fixed? See libstdc++-v3/testsuite/libstdc++-prettyprinters/prettyprinters.exp which has if { [check_effective_target_lto] } { append cxxflags " -flto" # work around sourceware.org 20882 regsub {^(.*)-Wl,--gc-sections(.*)$} $cxxldflags {\1\2} cxxldflags gdb-dg-runtest [lsort [glob $srcdir/$subdir/*.cc]] \ "" "$DEFAULT_CXXFLAGS -flto $PCH_CXXFLAGS" } where I didn't yet verify if the "fixed" binutils works without pruning --gc-sections.
