Hi:
When compiling following C program:
#include <stdio.h>
int main(int argc, char ** argv)
{
volatile unsigned long long a = 0;
volatile unsigned long long b = a + (1ULL << 63);
if (((long long) a - (long long) b) >= 0)
printf("wrong\n");
else
printf("right\n");
return 0;
}
with mingw gcc compiler:
/home/johannes/.zeranoe/mingw-w64/x86_64/bin/x86_64-w64-mingw32-gcc --version
x86_64-w64-mingw32-gcc (GCC) 12.3.1 20230814
and -O2 the output of the program is:
wrong
When compiling without -O2 or compiling on another gcc (the one that comes
with Linux Ubuntu for example) the result is:
right
Is the above comparision in the C program something that is undefined?
Looking at the assembly code the difference seems to be the
conditional jump expression: it is
js .L4
in the right case and
jl .L4
in the wrong case.
My 8086 assembly is almost about 35-40 years old, however I recall
that jl jumps if operand 2 is less than operand 1 (in System-V
syntax ...).
Is this maybe a mingw gcc bug?
Thanks a lot for looking into this, and best regards,
- Johannes
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