https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104237
--- Comment #13 from Chengnian Sun <cnsun at uwaterloo dot ca> --- (In reply to Jakub Jelinek from comment #10) > Because -fcompare-debug tells the driver to compile the TU twice, once > without and once with -gtoggle, and compare the result. > So, if there is a difference in the generated IL e.g. for -flto > -ffat-lto-objects, it will detect it. > Just watch > gcc -flto -O1 pr104237.c -o pr104237 -g -fcompare-debug -v 2>&1 | grep > 'cc1\|lto1' > to see that, cc1 is invoked twice, once without -gtoggle, once with it, but > lto1 is invoked just twice, once for wpa and once for ltrans. > Even if you convince the linker plugin to pass -fcompare-debug even to the > driver that invokes lto1, that would again compare lto1 with and without -g > but only > on a single input (depending on whether the original command line has -g > with or without debug stmts in it). > To reproduce this bug, one needs to do a "manual -fcompare-debug", which is > gcc -flto -O1 pr104237.c -o pr104237 -g // + force lto1 to pass > -fdump-final-insns=1 > gcc -flto -O1 pr104237.c -o pr104237 // + force lto1 to pass > -fdump-final-insns=2 > diff -up 1 2 > Now, in theory the driver could arrange that, but it would only handle the > trivial case when the compilation and linking are from the same command. > Typically with LTO, compilation is done separately and linking is done > separately, and I don't see how the driver could arrange for that to work, > we need in that case two sets of *.o files with the IL, one with -g0 and one > with -g, link twice and compare the result at the end. Thank you so much for the detailed explanation.
