On 16/01/2021 20:48, Brent Roman wrote: > My very old, highly modified Matz Ruby 1.87 interpreter stopped working > when Debian switched from GCC-9 to GCC-10 > > The Ruby interpreter binary output from GCC-10 segfaults. > > However, if I reduce optimization from -O2 to -O1, the resulting > binaries work fine. > > I'd like to find which specific -O2 optimization is causing the failure > when run through gcc-10. > > But, when I specified -O1 followed by options explicitly enabling all > the specific optimizations that are supposed to be enabled by -O2, > the resulting binary works fine. Conversely, when I specify -O2, > followed by explicit options to *disable* all those options, the > resulting binary fails. > > Does anyone know what -O2 enables aside from the options documented on: > > https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Optimize-Options.html#Optimize-Options > > > Here's an example of a gcc invocation with -O2 followed by disabling all > the -O2 specific optimizations: > > gcc -O2 -g -Wclobbered -fno-stack-protector -fno-align-functions > -fno-align-jumps -fno-align-labels -fno-align-loops -fno-caller-saves > -fno-code-hoisting -fno-crossjumping -fno-cse-follow-jumps > -fno-cse-skip-blocks -fno-delete-null-pointer-checks -fno-devirtualize > -fno-devirtualize-speculatively -fno-expensive-optimizations > -fno-finite-loops -fno-gcse -fno-gcse-lm -fno-hoist-adjacent-loads > -fno-inline-functions -fno-inline-small-functions > -fno-indirect-inlining -fno-ipa-bit-cp -fno-ipa-cp -fno-ipa-icf > -fno-ipa-ra -fno-ipa-sra -fno-ipa-vrp > -fno-isolate-erroneous-paths-dereference -fno-lra-remat > -fno-optimize-sibling-calls -fno-optimize-strlen -fno-partial-inlining > -fno-peephole2 -fno-reorder-blocks-and-partition -fno-reorder-functions > -fno-rerun-cse-after-loop -fno-schedule-insns -fno-schedule-insns2 > -fno-sched-interblock -fno-sched-spec -fno-store-merging > -fno-strict-aliasing -fno-thread-jumps -fno-tree-builtin-call-dce > -fno-tree-pre -fno-tree-switch-conversion -fno-tree-tail-merge > -fno-tree-vrp -DRUBY_EXPORT -D_GNU_SOURCE=1 -I. -I. -c main.c > > > Thanks! > > Sorry, it's not as simple as that. There are places in the compiler where the optimization level (O1, O2, O3) is just tested with something like if (optimize >= level) for some level. R.
