Re: [PATCH 2/6] Add returns_zero_on_success/failure attributes
From: David Malcolm <hidden>
Date: 2021-11-18 23:45:47
On Thu, 2021-11-18 at 14:08 -0600, Segher Boessenkool wrote:
On Wed, Nov 17, 2021 at 10:43:58PM +0000, Joseph Myers wrote:quoted
On Wed, 17 Nov 2021, Prathamesh Kulkarni via Gcc-patches wrote:quoted
More generally, would it be a good idea to provide attributes for mod/ref anaylsis ? So sth like: void foo(void) __attribute__((modifies(errno))); which would state that foo modifies errno, but neither reads nor modifies any other global var. and void bar(void) __attribute__((reads(errno))) which would state that bar only reads errno, and doesn't modify or read any other global var.Many math.h functions are const except for possibly setting errno, possibly raising floating-point exceptions (which might have other effects when using alternate exception handling) and possibly reading the rounding mode. To represent that, it might be useful for such attributes to be able to describe state (such as the floating-point environment) that doesn't correspond to a C identifier. (errno tends to be a macro, so referring to it as such in an attribute may be awkward as well.)We need some way to describe these things in Gimple and RTL as well, and not just on function calls: also on other expressions. Adding attributes that allow to describe this (partially, only per function) in C source code does not bring us closer to where we need to be.
Right, but those IR concerns are orthogonal to the needs of the patch kit, which is a way to express certain *other* things per-function in the C frontend. As noted in my other replies, this thread seems to be turning into something of a scope-creep pile-on, when I have some specific things I need for the rest of the patch kit, and they're unrelated to the problems of errno or floating-point handling. Dave