On 29 Apr 2021, at 23:11, Hans Petter Selasky <h...@selasky.org> wrote: > > On 4/29/21 11:06 PM, Hans Petter Selasky wrote: >> Hi, >> Can someone please explain what C-compiler flag I'm missing, to make this >> simple C-program terminate? >> I have a function _abs() which at some point is equal to one, but the >> compiler thinks it is not required to emit the test for the while() at all, >> looking at the assembly code! A printf() clearly verifies that the _abs() >> function has returned one. > >> clang11 -O2 test.c && ./a.out > > Looks like -fwrapv fixes the issue.
Indeed, in this case UBSan can help you diagnose the issue. E.g.: % clang -fsanitize=undefined test.c -o test % ./test Y:0x00000001 ABS:0x00000011 Y:0x00000007 ABS:0x00000041 Y:0x00000019 ABS:0x00000281 Y:0x00000027 ABS:0x00000601 Y:0x000000d9 ABS:0x0000b801 Y:0x00000327 ABS:0x0009f001 Y:0x000004d9 ABS:0x00178001 Y:0x00003b27 ABS:0x0dab0001 Y:0x000044d9 ABS:0x12840001 test.c:20:26: runtime error: signed integer overflow: 46341 * 46341 cannot be represented in type 'int' SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.c:20:26 in Y:0x0001bb27 ABS:0xff200001 Y:0x000e44d9 ABS:0x9a400001 Y:0x0011bb27 ABS:0x64000001 Y:0x01ee44d9 ABS:0xc8000001 ^C -Dimitry
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