https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83133
--- Comment #10 from Maxim Egorushkin <maxim.yegorushkin at gmail dot com> --- (In reply to Uroš Bizjak from comment #8) > (In reply to jos...@codesourcery.com from comment #5) > > Both 32-bit and 64-bit ABIs make the values of flags in EFLAGS (other than > > DF) undefined on function entry and return. Thus, a function can never > > assume anything about the value of OF unless it has executed an > > instruction known to set/clear it and has not called any other functions > > since then. > > In the testcase, there is nothing that violates ABI. Not sure why ABI got involved here. > It all happens in "g" > that passes calculated result to a function. Selected function depends on > flags, calculated from the subtraction, and wrong function gets selected > when overflow happens. The underflow happens as a result of that subtraction. That underflow leads to UB. In the case of UB, both gcc and clang behaviours are valid. The clang behaviour is one instruction cheaper though.
