Hello Andrew,
> -mcmodel=small'
> Generate code for the small code model: the program and its
> symbols must be linked in the lower 2 GB of the address space.
> Pointers are 64 bits. Programs can be statically or dynamically
> linked. This is the default code model.
You are right, the documentation I read, too. But as described even for
the small-model pointers are 64-bit and are treated beside the argument
passing via register correctly. The problem is that a function just set
the lower 32-bit of the register, but the method using this register as
argument uses the 64-bit register variant, which means the upper 32-bit of
the register have random values..
E.g: The c code
int foo(const char *h,int n) { n+=(int) *h; return n; }
int doo(int n) { return foo("abc ",n); }
Leads to the assembly on x86_64 (small)
.file "test.c"
.text
.globl _foo
.def _foo; .scl 2; .type 32; .endef
_foo:
LFB2:
pushq %rbp
LCFI0:
movq %rsp, %rbp
LCFI1:
movq %rcx, -8(%rbp)
movl %edx, -12(%rbp)
movq -8(%rbp), %rax
movzbl (%rax), %eax
movsbl %al,%eax
addl %eax, -12(%rbp)
movl -12(%rbp), %eax
leave
ret
LFE2:
.section .rdata,"dr"
LC0:
.ascii "abc \0"
.text
.globl _doo
.def _doo; .scl 2; .type 32; .endef
_doo:
LFB3:
pushq %rbp
LCFI2:
movq %rsp, %rbp
LCFI3:
subq $8, %rsp
LCFI4:
movl %ecx, -4(%rbp)
movl -4(%rbp), %edx
movl $LC0, %ecx
call _foo
leave
ret
...
You see, that function "doo" passes the symbolRef LC0 via ecx to function
foo. But function "foo" uses as input argument rcx for the pointer.
I think the definition of in i386.md is the reason for this incorrect
behaviour. I sent a small patch which seems to solve this problem quite
well without side-effect.
Regards,
i.A. Kai Tietz
--- gcc-4-2-20060930_org/gcc-4.2-20060930/gcc/config/i386/i386.md
2006-09-07 19:53:18.000000000 +0200
+++ gcc-4.2-20060930/gcc/config/i386/i386.md 2006-10-11
09:44:36.610866100 +0200
@@ -2008,9 +2008,7 @@
return "lea{q}\t{%a1, %0|%0, %a1}";
default:
gcc_assert (!flag_pic || LEGITIMATE_PIC_OPERAND_P (operands[1]));
- if (get_attr_mode (insn) == MODE_SI)
- return "mov{l}\t{%k1, %k0|%k0, %k1}";
- else if (which_alternative == 2)
+ if (which_alternative == 2)
return "movabs{q}\t{%1, %0|%0, %1}";
else
return "mov{q}\t{%1, %0|%0, %1}";
