The following makes signed overflow undefined for all (non-)optimization levels. The intent is to remove -fno-strict-overflow signed overflow behavior as that is not a sensible option to the user (it ends up with the worst of both -fwrapv and -fno-wrapv). The implementation details need to be preserved for the forseeable future to not wreck UBSAN with either associating (-fwrapv behavior) or optimizing (-fno-wrapv behavior).
The other choice would be to make -fwrapv the default for -O[01]. A second patch in this series would unify -f[no-]wrapv, -f[no-]trapv and -f[no-]strict-overflow with a -fsigned-integer-overflow={undefined,wrapping,trapping[,sanitized]} option, making conflicts amongst the options explicit (and reduce the number of flag_ variables). 'sanitized' would essentially map to todays flag_strict_overflow = 0. There's another sole user of flag_strict_overflow, POINTER_TYPE_OVERFLOW_UNDEFINED - not sure what to do about that, apart from exposing it as different flag alltogether. Further patches in the series would remove -Wstrict-overflow (and cleanup VRP for example). Anyway, most controversical part(?) below. Any comments on this particular patch (and the overall proposal)? Cleaning up the options is probably a no-brainer anyways. Thanks, Richard. 2017-04-24 Richard Biener <rguent...@suse.de> * common.opt (fstrict-overflow): Enable by default. * opts.c (default_options_table): Remove OPT_fstrict_overflow entry. Index: gcc/common.opt =================================================================== --- gcc/common.opt (revision 247091) +++ gcc/common.opt (working copy) @@ -2342,7 +2342,7 @@ Common Report Var(flag_strict_aliasing) Assume strict aliasing rules apply. fstrict-overflow -Common Report Var(flag_strict_overflow) Optimization +Common Report Var(flag_strict_overflow) Init(1) Optimization Treat signed overflow as undefined. fsync-libcalls Index: gcc/opts.c =================================================================== --- gcc/opts.c (revision 247091) +++ gcc/opts.c (working copy) @@ -496,7 +496,6 @@ static const struct default_options defa { OPT_LEVELS_2_PLUS, OPT_fschedule_insns2, NULL, 1 }, #endif { OPT_LEVELS_2_PLUS, OPT_fstrict_aliasing, NULL, 1 }, - { OPT_LEVELS_2_PLUS, OPT_fstrict_overflow, NULL, 1 }, { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_freorder_blocks_algorithm_, NULL, REORDER_BLOCKS_ALGORITHM_STC }, { OPT_LEVELS_2_PLUS, OPT_freorder_functions, NULL, 1 },