GCC cutting of higher 32 bits of qword in dlmalloc port

Qeroq · **Posted:** Fri Feb 11, 2011 8:17 am

Hello,
porting dlmalloc (2.8.4) to my OS, I've encountered a very strange error: My sbrk function returns a void*, which is 8 bytes in size (I checked on the type almost 20 times), but the higher dword is stripped away in the dlmalloc code. To check what caused the error I compiled the following C code in both my main.c, where everything works correctly, and directly at the top of dlmalloc.c (after the required imports for types and console):

Code:

uintptr_t result = heap_sbrk(0x2000);
console_print_hex(result);

In my main.c this generates the following byte code, which seems perfectly fine to me:

Code:

  5a:   bf 00 20 00 00          mov    $0x2000,%edi
  5f:   48 b8 00 00 00 00 00    movabs $0x0,%rax
  66:   00 00 00 
  69:   ff d0                   callq  *%rax
  6b:   48 89 45 f8             mov    %rax,-0x8(%rbp)
  6f:   48 8b 45 f8             mov    -0x8(%rbp),%rax
  73:   48 89 c7                mov    %rax,%rdi
  76:   48 b8 00 00 00 00 00    movabs $0x0,%rax
  7d:   00 00 00 
  80:   ff d0                   callq  *%rax

But in dlmalloc.c this byte code is generated:

Code:

      12:   bf 00 20 00 00          mov    $0x2000,%edi
      17:   b8 00 00 00 00          mov    $0x0,%eax
      1c:   48 ba 00 00 00 00 00    movabs $0x0,%rdx
      23:   00 00 00 
      26:   ff d2                   callq  *%rdx
      28:   48 98                   cltq   
      2a:   48 89 45 f8             mov    %rax,-0x8(%rbp)
      2e:   48 8b 45 f8             mov    -0x8(%rbp),%rax
      32:   48 89 c7                mov    %rax,%rdi
      35:   48 b8 00 00 00 00 00    movabs $0x0,%rax
      3c:   00 00 00 
      3f:   ff d0                   callq  *%rax

For some reason there is this strange cltq that copies only the half of sbrk's result.

Both files are compiled with the same flags:

Code:

    -m64 \
    -nostdlib -nostdinc -fno-builtin -fno-stack-protector \
    -D__AMD64__ \
    -D__DEBUG__ \
    -x c \
    -mcmodel=large \
    -I./src/

Does anyone of you have an idea what could have caused this?

qw · **Joined:** Mon Jan 26, 2009 2:48 am **Posts:** 792

My guess is that "heap_sbrk" is not properly declared in the latter case, and the compiler assumes it is returning an int (32 bits) instead of an uintptr_t (64 bits). Check the inclusion of the header file.

Qeroq · **Posted:** Fri Feb 11, 2011 10:09 am

Oh, yeah, pretty obvious: I refactored some of the heap code in earlier debugging moving heap_sbrk into another header, which i did not include... #-o

Is there a way to configure gcc to print a warning if a function is neither defined or extern?

Solar · **Posted:** Fri Feb 11, 2011 10:24 am

My project Makefile uses, among others:

Code:

-Wall -Wextra -pedantic -Wshadow -Wpointer-arith -Wcast-align -Wwrite-strings -Wmissing-prototypes -Wmissing-declarations -Wredundant-decls -Wnested-externs -Winline -Wuninitialized

Between -Wmissing-prototypes, -Wmissing-declarations and -Wredundant-decls, most of your header woes should be detected at compile time.

qw · **Joined:** Mon Jan 26, 2009 2:48 am **Posts:** 792

I call this one solved. Glad I could help.

Roel

Combuster · **Posted:** Fri Feb 11, 2011 4:44 pm

berkus wrote:

add -Werror

Amen. It will make you a much better developer.

OSDev.org

GCC cutting of higher 32 bits of qword in dlmalloc port

Who is online