On 04/24/2017 05:25 AM, Richard Biener wrote:
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).
Minimizing the differences between the guarantees provided at different optimization levels is a good thing. It will help uncover bugs that would go undetected during development (with -O0) and only manifest when building with optimization (which may be less frequent). I find the -Wstrict-overflow warning with all its levels over- engineered but I'm not sure I'm in favor of completely eliminating it. It has helped illuminate the signed integer overflow problem for many users who were otherwise completely unaware of it. I'd be concerned that by getting rid of it users might be lulled back into assuming that it has the same wrapping semantics as common hardware (or simply doesn't happen). It sounds like you'd like to get rid of it to simplify GCC code. Would it make sense to preserve it for at least the most egregious instances of overflow (like in 'if (i + 1 < i)' and similar)? Martin
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 },