Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach!
On Wed, May 20, 2015 at 11:03:00AM +0200, Richard Biener wrote: > On Wed, May 20, 2015 at 9:34 AM, Jens Maurer wrote: > > On 05/20/2015 04:34 AM, Paul E. McKenney wrote: > >> On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote: > > > >>> - the "you can add/subtract integral values" still opens you up to > >>> language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the > >>> dependency, which it clearly doesn't. But language-lawyering it does, > >>> since all those operations (cast to pointer, cast to integer, > >>> subtracting an integer) claim to be dependency-preserving operations. > > > > [...] > > > >> There are some stranger examples, such as "(char *)ptr - > >> ((intptr_t)ptr)/7", > >> but in that case, if the resulting pointer happens by chance to reference > >> valid memory, I believe a dependency would still be carried. > > [...] > > > > From a language lawyer standpoint, pointer arithmetic is only valid > > within an array. These examples seem to go beyond the bounds of the > > array and therefore have undefined behavior. > > > > C++ standard section 5.7 paragraph 4 > > "If both the pointer operand and the result point to elements of the > > same array object, or one past the last element of the array object, > > the evaluation shall not produce an overflow; otherwise, the behavior > > is undefined." > > > > C99 and C11 > > identical phrasing in 6.5.6 paragraph 8 > > Of course you can try to circumvent that by doing > (char*)((intptr_t)ptr - (intptr_t)ptr + (intptr_t)ptr) > (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 for extra fun). > > Which (IMHO) gets you into the standard language that only makes conversion of > the exact same integer back to a pointer well-defined(?) I am feeling good about leaving the restriction and calling out the two paragraphs in a footnote, then. ;-) Thanx, Paul
Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach!
On Wed, May 20, 2015 at 9:34 AM, Jens Maurer wrote: > On 05/20/2015 04:34 AM, Paul E. McKenney wrote: >> On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote: > >>> - the "you can add/subtract integral values" still opens you up to >>> language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the >>> dependency, which it clearly doesn't. But language-lawyering it does, >>> since all those operations (cast to pointer, cast to integer, >>> subtracting an integer) claim to be dependency-preserving operations. > > [...] > >> There are some stranger examples, such as "(char *)ptr - ((intptr_t)ptr)/7", >> but in that case, if the resulting pointer happens by chance to reference >> valid memory, I believe a dependency would still be carried. > [...] > > From a language lawyer standpoint, pointer arithmetic is only valid > within an array. These examples seem to go beyond the bounds of the > array and therefore have undefined behavior. > > C++ standard section 5.7 paragraph 4 > "If both the pointer operand and the result point to elements of the > same array object, or one past the last element of the array object, > the evaluation shall not produce an overflow; otherwise, the behavior > is undefined." > > C99 and C11 > identical phrasing in 6.5.6 paragraph 8 Of course you can try to circumvent that by doing (char*)((intptr_t)ptr - (intptr_t)ptr + (intptr_t)ptr) (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752 for extra fun). Which (IMHO) gets you into the standard language that only makes conversion of the exact same integer back to a pointer well-defined(?) Richard. > Jens
Re: [c++std-parallel-1614] Re: Compilers and RCU readers: Once more unto the breach!
On 05/20/2015 04:34 AM, Paul E. McKenney wrote: > On Tue, May 19, 2015 at 06:57:02PM -0700, Linus Torvalds wrote: >> - the "you can add/subtract integral values" still opens you up to >> language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the >> dependency, which it clearly doesn't. But language-lawyering it does, >> since all those operations (cast to pointer, cast to integer, >> subtracting an integer) claim to be dependency-preserving operations. [...] > There are some stranger examples, such as "(char *)ptr - ((intptr_t)ptr)/7", > but in that case, if the resulting pointer happens by chance to reference > valid memory, I believe a dependency would still be carried. [...] >From a language lawyer standpoint, pointer arithmetic is only valid within an array. These examples seem to go beyond the bounds of the array and therefore have undefined behavior. C++ standard section 5.7 paragraph 4 "If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined." C99 and C11 identical phrasing in 6.5.6 paragraph 8 Jens
