Created attachment 24386 [details] Contains the object-file and the executable I hope it is no missunderstanding, but as it seems, the code generation is wrong when using global variables (I'm wondering because it is such a basic thing). For example, take the following C file: ---- struct foo { int x,y; }; static struct foo f; static int x; static void func1(int *i) { } static void func2(struct foo *pf) { } int main(void) { int a; f.x = 0; f.y = 1; x = 2; func1(&x); func1(&a); func2(&f); return 0; } ---- I'm using the following commands to build the executable: $ mmix-elf-escape-gcc -o test.o -c test.c $ mmix-elf-escape-gcc -nostdlib -o test test.o The disassembly for the main-function is: ---- 0000000000000040 <main>: 40: 27fefe10 subu $254,$254,16 44: affdfe08 stou $253,$254,8 48: 23fdfe10 addu $253,$254,16 4c: fe000004 get $0,rJ 50: e3010000 setl $1,0x0 54: ab01fe00 sttu $1,$254,0 58: e3010001 setl $1,0x1 5c: ab01fe04 sttu $1,$254,4 60: e3010002 setl $1,0x2 64: ab01fe08 sttu $1,$254,8 68: 2303fe08 addu $3,$254,8 6c: f302ffe5 pushj $2,0 <_start.> 70: f6040000 put rJ,$0 74: 2701fd0c subu $1,$253,12 78: c1030100 set $3,$1 7c: f302ffe1 pushj $2,0 <_start.> 80: f6040000 put rJ,$0 84: 2303fe00 addu $3,$254,0 88: f302ffe6 pushj $2,20 <func2> 8c: f6040000 put rJ,$0 90: e3000000 setl $0,0x0 94: 8dfdfe08 ldo $253,$254,8 98: 23fefe10 addu $254,$254,16 9c: f8010000 pop 1,0 ---- That means, the code expects, that f is located at $254, offset 0. But $254 is the frame-pointer according to http://www.bitrange.com/mmix/mmixfest-2001/mmixabi.html. The example shows a few other usages, that are wrong for the same reason. Note that it is correct for the local variable. For the case that it helps, I've attached the object file and the executable. I hope that the fix is easy. It would be really nice to know shortly how to resolve that, because before that, I think one can't do anything useful with gcc for MMIX :/
With -nostdlib, you're basically disabling the startup code that is supposed to setup everything. If it still doesn't work without -nostdlib, then I'll have a look.
See gcc/config/mmix/crtn.asm, where the global registers used by the ABI are allocated.
Thanks for your reply! Perhaps I miss something, but I've no idea how that should work. Regardless of whether $254 is initialized previously or not (in my case, it's a bootloader, so there is no stdlib and no crt*, but I have to do that myself), using the stack-pointer to access global variables can't work, right? Lets suppose, that the offset in main is correct. That would mean, that x is located at $254 + 8. But $254 is changed (of course) at the beginning and end of a function (and nowhere else). If func2 accesses x as well, gcc generates code to access it at $254 + 8. So, it expects it to be always at offset 8 from $254, but of course that can't work, because $254 changes.
(In reply to comment #3) > Perhaps I miss something, but I've no idea how that should work. Writing a bootloader, you really should. > Regardless of > whether $254 is initialized previously or not (in my case, it's a bootloader, > so there is no stdlib and no crt*, but I have to do that myself), using the > stack-pointer to access global variables can't work, right? The key words are "setup" and "allocated" not "initialized". Again, I suggest you have a look at crtn.asm, in particular these lines: % This must be the last file on the link-line, allocating global registers % from the top. % Register $254 is the stack-pointer. sp GREG So, if you leave that out, the global-register-allocation machinery in the linker will allocate registers starting from the topmost, which is $254 ($255 being a scratch register).
> Again, I suggest you have a look at crtn.asm, in particular these lines: > > % This must be the last file on the link-line, allocating global registers > % from the top. > > % Register $254 is the stack-pointer. > sp GREG > > So, if you leave that out, the global-register-allocation machinery in the > linker will allocate registers starting from the topmost, which is $254 ($255 > being a scratch register). Oh, ok. I thought in a different direction :/ Then it's perfectly clear. Thanks!