On Dienstag, 7. Januar 2025 14:54:54 MEZ Kjetil Oftedal wrote: > On Tue, 7 Jan 2025 at 14:39, Frank Mehnert > <frank.mehn...@kernkonzept.com> wrote: > > > > Hi Kjetil, > > > > On Dienstag, 7. Januar 2025 14:00:16 MEZ Kjetil Oftedal wrote: > > > How can the compiler in this case assume that endptr and str do not > > > point to the same array object? > > > > > > ISO C99 6.5,8 Relational operators > > > 5) > > > ... > > > "If the expression P points to an element of an array object and the > > > expression Q points to the last element of the same array object, > > > the pointer expression Q+1 compares greater than P. In all other > > > cases, the behavior is undefined" > > > > > > How does the compiler at compile time know that endptr which is > > > equivalent is to str[maxlen] is not within the > > > range of string declared by str? > > > Or in the terms of the above standard, how does it know that endptr is > > > not Q+1? > > > > > > [...] > > > > The compiler does not need to know! > > > > Let's consider the pointer 'x', the offset 'offs' and > > > > y = x + offset > > > > with y < x (because offset is huge). > > > > So if y < (x + offset) then y points to a different object by definition. > > Hence, the compiler is allowed to remove this test. > > > > Frank > > -- > > Dr.-Ing. Frank Mehnert, frank.mehn...@kernkonzept.com, +49-351-41 883 224 > > > > Kernkonzept GmbH. Sitz: Dresden. Amtsgericht Dresden, HRB 31129. > > Geschäftsführer: Dr.-Ing. Michael Hohmuth > > > > > > Correspondingly > y = x + offset > with y < x (Offset is small E.g 1). > > So if y < (x + offset) and offset is 1, then the Q+1 rule holds, and > the compiler is not allowed to remove this test as it is well defined. > > This cannot be assumed at compile time, thus the test needs to be done > to respect the > requirements of the C standards.
I think you mix things up: The error here is the pointer arithmetic, not the comparison. With offset=1 and x=0xffffffff'fffffff, y will 0, so y does not belong to the same object as x. There is no other value of x which could lead to a result y<x with offset=1! The comparison tests the wrap around and adapts end_ptr in that case. Again: If there was a wrap around, then the pointer arithmetic operation was wrong (undefined behavior). Hence, we can remove the test plus the code which is executed if the test succeeds. Frank -- Dr.-Ing. Frank Mehnert, frank.mehn...@kernkonzept.com, +49-351-41 883 224 Kernkonzept GmbH. Sitz: Dresden. Amtsgericht Dresden, HRB 31129. Geschäftsführer: Dr.-Ing. Michael Hohmuth _______________________________________________ devel mailing list -- devel@uclibc-ng.org To unsubscribe send an email to devel-le...@uclibc-ng.org
