Re: Question about strange register calling truncates to SImode on x86_64

[Andrew Haley <aph at redhat dot com>]

Kai Tietz writes:

 > I am currently on to build the gcc target support for x86_64-pc-mingw64. 
 > While this porting I found a strange register truncation, I do not believe 
 > it is valid. For the c code:
 > 
 >         int foo(char *,...);
 >         int doo(char *h) { return foo("abc ",h); }
 > 
 > compiled result ->
 > 
 >         .file   ""
 >  doo    .section .rdata,"dr"
 > LC0:
 >         .ascii "abc \0"
 >         .text
 > .globl _doo
 >         .def    _doo;   .scl    2;      .type   32;     .endef
 > _doo:
 > LFB2:
 >         pushq   %rbp
 > LCFI0:
 >         movq    %rsp, %rbp
 > LCFI1:
 >         subq    $16, %rsp
 > LCFI2:
 >         movq    %rcx, -8(%rbp)
 >         movq    -8(%rbp), %rdx
 >         movl    $LC0, %ecx
 >         movl    $0, %eax
 >         call    _foo
 > ...
 > 
 > It leads to a symbol ref to "abc", which is passed to function foo on 
 > calling via a 32 bit register. This behaviour seems to be wrong, because 
 > if the base adress of the code segment gets out-side of 2^32 the 
 > relocation will be incorrect for sure. May somebody can help if my 
 > thoughts are wrong or not ...

There's nothing strange about it, it's the default with the small code
model.  Here's the doc:

`-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.

`-mcmodel=kernel'
     Generate code for the kernel code model.  The kernel runs in the
     negative 2 GB of the address space.  This model has to be used for
     Linux kernel code.

`-mcmodel=medium'
     Generate code for the medium model: The program is linked in the
     lower 2 GB of the address space but symbols can be located
     anywhere in the address space.  Programs can be statically or
     dynamically linked, but building of shared libraries are not
     supported with the medium model.

`-mcmodel=large'
     Generate code for the large model: This model makes no assumptions
     about addresses and sizes of sections.  Currently GCC does not
     implement this model.

Andrew.