Hi Thomas, > What is the difference between > > -O0 > > and > > -O1 -fno-defer-pop -fno-merge-constants -fno-thread-jumps -fno-loop-optimize > -fno-if-conversion -fno-if-conversion2 -fno-delayed-branch > -fno-guess-branch-probability -fno-cprop-registers -fno-omit-frame-pointer -O0 Performs no optimizations. It disables (bypasses) all the optimization machinery. Should make compile time faster, and easier to debug (-g), but may hide certain bad (language non-compliant) bugs at runtime (which may give a false impression that the code is correct), and won't generate a few warnings from -Wall -Wextra which depend on optimization analysis. -O1 -fno-yadayada... Enables many optimizations, and the mentioned -fno-yadayada... flags will disable that handful of specific optimizations. (Most of the optimizations have no independently twiddle-able flags.) I like this trick to see which twiddle-able flags were enabled: gcc -O1 -S -fverbose-asm -x c <(echo '') -o O1.s cat O1.s (Your command-line may differ, depending on your platform.) HTH, --Eljay
