[Bug c++/92717] New: precompiled headers non-deterministic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92717 Bug ID: 92717 Summary: precompiled headers non-deterministic Product: gcc Version: 9.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: gnu.org at mrks dot info Target Milestone: --- I found that introducing precompiled headers to my project causes ccache lookups to fail. I tracked it down to the gcc output not being deterministic: # /usr/bin/c++ -x c++-header -include test.hxx -o test.hxx.gch -c test.hxx.cxx && md5sum test.hxx.gch 3a0efb998351939f4ed8efcfce1c0015 test.hxx.gch # /usr/bin/c++ -x c++-header -include test.hxx -o test.hxx.gch -c test.hxx.cxx && md5sum test.hxx.gch d295cbe3613100e6d989e62e3aafad6c test.hxx.gch Both test.hxx and test.hxx.cxx are empty. The used gcc version is 9.2.1-9ubuntu2. With clang (clang++ -cc1 test.hxx -emit-pch -o test.hxx.pch), the hashes match. When compiling with save-temps, the ii and s files match. As builds of regular C(++) files are deterministic (as expected), I suspect that this might be a bug.
[Bug c++/92717] precompiled headers non-deterministic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92717 --- Comment #2 from Markus Dreseler --- > Try using __DATE__ macro and you will see it is not :). Can't confirm: # /usr/bin/c++ -D__DATE__=0 -D__TIMESTAMP__=0 -D__TIME__=0 -x c++-header -include test.hxx -o test.hxx.gch -c test.hxx.cxx && md5sum test.hxx.gch produces some new warnings, but again different file hashes. Here are some other options that I have tested but that have no effect: * SOURCE_DATE_EPOCH=0 * -fno-pie * -nodefaultlibs -nostartfiles -nostdlib
[Bug c++/92717] precompiled headers non-deterministic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92717 --- Comment #4 from Markus Dreseler --- > By any chance, is your cc1plus built as PIE? PCH doesn't work in that case. I don't think so: # file `find /usr -name cc1plus` /usr/lib/gcc/x86_64-linux-gnu/9/cc1plus: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=ab2940b3dc6bbacbb2b3f15e7cdcdd416e74b3b8, for GNU/Linux 3.2.0, stripped This would say "LSB pie executable" otherwise, correct? gcc 9.2.1 comes from the Ubuntu 19.10 repos.
[Bug c++/92717] precompiled headers non-deterministic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92717 --- Comment #5 from Markus Dreseler --- I took Andrew's __DATE__ suggestion as a reason to look at how much the files actually differ. `cmp -l v1 v2 | wc -l` gives me 692634 differing bytes. This sounds like the difference is bigger than just some macro values. P.S.: Same issue on Mac with g++-8 (Homebrew GCC 8.2.0).
[Bug c++/92717] precompiled headers non-deterministic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92717 --- Comment #8 from Markus Dreseler --- Interesting. Is this implementation documented somewhere? I can confirm that disabling ASLR results in reproducible gchs: # setarch $(uname -m) -R /usr/bin/c++ -x c++-header -include test.hxx -o test.hxx.gch -c test.hxx.cxx && md5sum test.hxx.gch 54486c6ffc08db87d4b58792dcb46091 test.hxx.gch # setarch $(uname -m) -R /usr/bin/c++ -x c++-header -include test.hxx -o test.hxx.gch -c test.hxx.cxx && md5sum test.hxx.gch 54486c6ffc08db87d4b58792dcb46091 test.hxx.gch For CI builds of trusted files, this might be an option, but generally setting the ADDR_NO_RANDOMIZE personality for gcc when gchs are build does not sound like a good approach.
