Re: [C++ PATCH] Improve code generation for static_cast of pointers to reference types (PR c++/83555)

[Nathan Sidwell <nathan at acm dot org>]

On 01/02/2018 04:12 PM, Jakub Jelinek wrote:
> Hi!
>
> This patch improves code generated for:
> struct A { int a; };
> struct B { int b; };
> struct C : A, B { int c; };
> C *bar (B *b) { return &static_cast<C &>(*b); }
> Unlike return static_cast<C *>(b); where b can be validly NULL, the
> reference shouldn't bind to NULL, but we still emit
> b ? b - 4 : 0.  The following patch omits the non-NULL check except when
> -fsanitize=null (or undefined) and when sanitizing makes sure such bugs are
> diagnosed.

It's sad the optimizers don't know REFERENCE_TYPE (x) means x != NULL. 
(or perhaps that's just a C++ semantic of REFERENCE_TYPE?).  Do we 
manage to elide the check if we eventually dereference the pointer? 
(Not that that'd be an easy fix, but maybe worth a (new?) bug report.)

Your patch is fine, but could you add a test case to make sure the null 
check is not there in the output assembly -- it'd be $cpu-of-choice 
specific, of course.

nathan

--
Nathan Sidwell