From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ralf Guetlein To: gcc-help@gcc.gnu.org Subject: [Fwd: Re: initializing ints with mem refs (ANSI C??)] Date: Sun, 05 Mar 2000 09:29:00 -0000 Message-id: <38C299F4.EDC79545@aranea.de> X-SW-Source: 2000-03/msg00021.html Moogla, thank you for your reply > > Ralf Gütlein wrote: > > > const int object = 0; > > int a[2] = {1, (const int)&object;}; Moogla wrote: > Is there any problem with: > const int object = 0; > int a[2]; > a[0] = 1; > a[1] = (const int) &object; I should have written: enum {item1, item2, item3, item4}; const int menu1[] = {item1, item2, item3}; const int menu2[] = {item4, (const int)menu1}; What I intended to do is generating a linked list in ROM to implement a hierachical menu structure. In gcc this is no problem, as I stated earlier. > > > What does the ANSI standard demand here? > > ANSI only requires it to minimally be a constant expression, one that > can be evaluated at compile time. Of course, we know the address of > object at compile time, but strictly, it's not a constant. The difference between 'item1' and '(const int)menu1' is that the latter is evaluated at *link* time. So you are right in that it is not a *compile* time constant. This is no problem for gcc. But IMO this should be generally no problem because typedef void* PVOID; enum {item1, item2, item3, item4}; const PVOID menu1[] = {(PVOID)item1, (PVOID)item2, (PVOID)item3}; const PVOID menu2[] = {(PVOID)item4, (PVOID)menu1}; works with both compilers, although '(PVOID)menu1' again can be calculated only at link time. (The assembler listing issued by gcc is exactly the same for both of the above contructs, and the IAR assembler listing for the latter one is functionally identical to those made by gcc.) > a subexpression elimination pass which rejects address references, and > this pass is used to verify aggregate initializers. Also, IAR may feel > it wants to move addresses of variables around in the data segment if > it's feeling particularly optimizationy or whatever. GCC, oth, only > resolves the initialization until it knows what the final value will be. > I don't think you can count on that in future releases of EGCS, though. > I guess the compiler behaviour in these cases depends in no way on any kind of optimization. It's merely a restriction implemented by the compiler writers (with or without intent). > moogla Thanks again. Regards, Ralf From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ralf Guetlein To: gcc-help@gcc.gnu.org Subject: [Fwd: Re: initializing ints with mem refs (ANSI C??)] Date: Sat, 01 Apr 2000 00:00:00 -0000 Message-ID: <38C299F4.EDC79545@aranea.de> X-SW-Source: 2000-q1/msg00331.html Message-ID: <20000401000000.QE4rPPcvsItREERIAARzBGfgNWdx6PsIUqJ-S0hjkqs@z> Moogla, thank you for your reply > > Ralf Gütlein wrote: > > > const int object = 0; > > int a[2] = {1, (const int)&object;}; Moogla wrote: > Is there any problem with: > const int object = 0; > int a[2]; > a[0] = 1; > a[1] = (const int) &object; I should have written: enum {item1, item2, item3, item4}; const int menu1[] = {item1, item2, item3}; const int menu2[] = {item4, (const int)menu1}; What I intended to do is generating a linked list in ROM to implement a hierachical menu structure. In gcc this is no problem, as I stated earlier. > > > What does the ANSI standard demand here? > > ANSI only requires it to minimally be a constant expression, one that > can be evaluated at compile time. Of course, we know the address of > object at compile time, but strictly, it's not a constant. The difference between 'item1' and '(const int)menu1' is that the latter is evaluated at *link* time. So you are right in that it is not a *compile* time constant. This is no problem for gcc. But IMO this should be generally no problem because typedef void* PVOID; enum {item1, item2, item3, item4}; const PVOID menu1[] = {(PVOID)item1, (PVOID)item2, (PVOID)item3}; const PVOID menu2[] = {(PVOID)item4, (PVOID)menu1}; works with both compilers, although '(PVOID)menu1' again can be calculated only at link time. (The assembler listing issued by gcc is exactly the same for both of the above contructs, and the IAR assembler listing for the latter one is functionally identical to those made by gcc.) > a subexpression elimination pass which rejects address references, and > this pass is used to verify aggregate initializers. Also, IAR may feel > it wants to move addresses of variables around in the data segment if > it's feeling particularly optimizationy or whatever. GCC, oth, only > resolves the initialization until it knows what the final value will be. > I don't think you can count on that in future releases of EGCS, though. > I guess the compiler behaviour in these cases depends in no way on any kind of optimization. It's merely a restriction implemented by the compiler writers (with or without intent). > moogla Thanks again. Regards, Ralf