hays@isc.intel.com (Kirk Hays) (07/28/90)
In article <9007250601.AA07615@csvax.cs.caltech.edu> daveg@CSVAX.CS.CALTECH.EDU (David Gillespie) writes: >I tried the following program on gcc 1.37.1 under HP-UX 7.0. I was >checking that IsUnsigned would be folded to a constant at compile-time. >It was, but in two cases the result was wrong! Sorry, but the macro is K&R correct(sometimes), ANSI wrong, as the value conversion rules were open in K&R, but defined in ANSI. What you are seeing is ANSI compiler behavior. > > #define IsUnsigned(x) (((x)*0-2)/2+1) > > int a; > unsigned int b; > short c; > unsigned short d; > char e; > unsigned char f; > > main() { > printf("%d, %d, %d, %d, %d, %d\n", > IsUnsigned(a), IsUnsigned(b), > IsUnsigned(c), IsUnsigned(d), > IsUnsigned(e), IsUnsigned(f)); > } > >The output using cc is > > 0, -2147483648, 0, -2147483648, 0, -2147483648 One of the correct non-ANSI answers. There are several. > >but the output using gcc is > > 0, -2147483648, 0, 0, 0, 0 The only correct ANSI answer (on 32 bit machines). See below for explanation. > >(This IsUnsigned macro is due to Karl Heuer on comp.lang.c.) > > -- Dave What you're seeing here is the difference between ANSI and pre-ANSI compilers. In ANSI, before the arithmetic operators are applied to the values, they are promoted to *int*, unless their possible values prevent them from being represented as *int*, in which case, they are converted to *unsigned int* (for all types less than or equal to in size to an *int*). In your program, all of the signed and unsigned types, except _unsigned int_, have values representable as *int*, so they are converted to *int* prior to the operations being performed, and the constants follow suit. That is why ONLY the *unsigned int* case works. Longs and unsigned longs are subject to a different (but equivalent) conversion rule. See section 3.2 of the ANSI standard (ANS 3.159-1989) for more detail. Here is an extension of the above program which demonstrates my assertions. The only correct answers with an ANSI compiler on a 32 bit machine are: 0, -2147483648, 0, 0, 0, 0, 0, -2147483648 0, -2147483648, 0, 0, 0, 0, 0, -2147483648 For any compiler on any architecture, if the two lines are different from each other, COMPLAIN LOUDLY to your vendor until they fix it! ---------------cut here------------------ #define IsUnsigned(x) (((x)*0-2)/2+1) signed int a; unsigned int b; signed short c; unsigned short d; signed char e; unsigned char f; long g; unsigned long h; main() { printf("%d, %d, %d, %d, %d, %d, %d, %d\n", IsUnsigned(a), IsUnsigned(b), IsUnsigned(c), IsUnsigned(d), IsUnsigned(e), IsUnsigned(f), IsUnsigned(g), IsUnsigned(h)); { int zero= 0; int one = 1; int two = 2; #define F(type) (((type)zero-(type)two)/(type)two+(type)one) printf ("%d, %d, %d, %d, %d, %d, %d, %d\n", F(int), F(unsigned int), F(short), F(unsigned short), F(char), F(unsigned char), F(long), F(unsigned long)); } } ---------------cut here------------------ Frankly, I am surprised that Karl Heuer blew the macro; he is the walking lint, after all! Most of this was hashed about in ~1986, I think, when the debate about "value-preserving" vs. "sign-preserving" raged across the net. -- Kirk Hays - I'm the NRA, NRA-ILA, CCRKBA, SAF, and Neal Knox is my lobbyist.
daveg@near.cs.caltech.edu (Dave Gillespie) (07/29/90)
On 27 Jul 90 20:15:17 GMT, hays@isc.intel.com (Kirk Hays) said: > In article <9007250601.AA07615@csvax.cs.caltech.edu> daveg@CSVAX.CS.CALTECH.EDU (David Gillespie) writes: >>I tried the following program on gcc 1.37.1 under HP-UX 7.0. I was >>checking that IsUnsigned would be folded to a constant at compile-time. >>It was, but in two cases the result was wrong! [it worked for unsigned int, but not for unsigned short or unsigned char] > Sorry, but the macro is K&R correct(sometimes), ANSI wrong, as the value > conversion rules were open in K&R, but defined in ANSI. > What you are seeing is ANSI compiler behavior. [explanation of ANSI promotion rules] > Frankly, I am surprised that Karl Heuer blew the macro; he is the walking > lint, after all! Karl's original message said the macro tested the *promoted* type of the argument. I just forgot the distinction, having grown up with pre-ANSI compilers which used different promotion rules. Sorry for the confusion---Karl's track record is intact! Now, whether I *agree* with ANSI's promotion rules is... well, let's not start that again. Can anybody see a way to define IsUnsigned that does check the true type of the argument? -- Dave -- Dave Gillespie 256-80 Caltech Pasadena CA USA 91125 daveg@csvax.cs.caltech.edu, ...!cit-vax!daveg