https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66960
Bug ID: 66960
Summary: Add a builtin to get the address of the current stack
frame
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: hjl.tools at gmail dot com
Target Milestone: ---
The initial i386 stack layout is:
environ 4*argc + 4 + %esp
0 4 bytes
envp 8 + 4*argc + %esp N * 4 bytes
0 4 + 4*argc + %esp 4 bytes
argv %esp + 4 4*argc bytes
argc %esp 4 bytes
To get argc, argv and environ in C, we can use
---
extern char **environ;
extern void exit (int status);
extern int main (int argc, char **argv, char **envp);
void
_start (void)
{
void *argc_p = __builtin_frame_address (0) + 4;
char **argv = (char **) (argc_p + 4);
int argc = *(int *) argc_p;
int status;
environ = argv + argc + 1;
status = main (argc, argv, environ);
exit (status);
}
---
With -O2 -m32 -miamcu, we generate
_start:
pushl %ebp
movl %esp, %ebp
movl 4(%ebp), %eax
leal 8(%ebp), %edx
leal 4(%edx,%eax,4), %ecx
movl %ecx, environ
call main
call exit
.size _start, .-_start
%ebp is used since __builtin_frame_address always keeps the frame
pointer. With a new builtin, __builtin_current_frame, which returns
the top of the stack frame, we can use
---
extern char **environ;
extern void exit (int status);
extern int main (int argc, char **argv, char **envp);
void
_start (void)
{
void *argc_p = __builtin_current_frame ();
char **argv = (char **) (argc_p + 4);
int argc = *(int *) argc_p;
int status;
environ = argv + argc + 1;
status = main (argc, argv, environ);
exit (status);
}
---
and generate
_start:
movl (%esp), %eax
leal 4(%esp), %edx
leal 8(%esp,%eax,4), %ecx
movl %ecx, environ
call main
call exit
.size _start, .-_start
We can avoid using %ebp.
__builtin_current_frame may not be the best name. We can use
__builtin_stack_top, __builtin_top_of_stack, which gives us the
stack address when the function is called.
