On Thu, 24 May 2007, Derek M Jones wrote: > > Saturated arithmetic kills off so many optimizations because reordering > an expression might produce different results. Yes. However, the FP people do have solutions to that, for all the same reasons. (ie, you can add rules like: - parentheses have meaning outside of just precedence, and disable associativity-based ordering optimizations. - the normal C side effect boundaries also act as ordering boundaries for arithmetic. > This is the point of the discussion that has got me confused. > What compiler misfeature? Perhaps I am using the 'wrong' version > of gcc (version 4.0.2), but I get the expected wrapping behavior (ie, > the compiler tries to behave at translate time the same way as st > runtime). The problem that Al was pointing to that sometimes gcc will do *constant folding* at parse time, and treat "1+n-n" as a compile-time constant in situations where that simply isn't valid C. Try this: [torvalds@woody ~]$ cat t.c extern int n; int a[1 + n - n]; [torvalds@woody ~]$ gcc -c -Wall t.c and notice the lack of any error what-so-ever. With sparse, you get [torvalds@woody ~]$ sparse -Wall t.c t.c:2:13: error: bad constant expression now, change the "1+n-n" into "1+n+n-n-n", and notice what gcc says: [torvalds@woody ~]$ gcc -c -Wall t.c t.c:2: error: variably modified ‘a’ at file scope iow, gcc actually thinks that "1+n-n" is somehow different from "1+n+n-n-n". Which was Al's point. Linus
