Gcc generates poor codes for this:
[...@gnu-6 tmp]$ cat x.i
typedef unsigned int fpu_control_t __attribute__ ((__mode__ (__HI__)));
extern fpu_control_t __fpu_control;
struct rtld_global_ro
{
fpu_control_t _dl_fpu_control;
};
extern const struct rtld_global_ro _rtld_global_ro;
extern void __setfpucw (fpu_control_t);
int __libc_multiple_libcs __attribute__ ((visibility ("hidden"))) = 1;
int __libc_argc __attribute__ ((visibility ("hidden")));
char **__libc_argv __attribute__ ((visibility ("hidden")));
extern char **__environ;
extern void __init_misc (int, char **, char **);
extern void __libc_global_ctors (void);
extern int _dl_starting_up;
#pragma weak _dl_starting_up
void
__attribute__ ((visibility ("hidden")))
_init (int argc, char **argv, char **envp)
{
__libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;
if (!__libc_multiple_libcs
&& __fpu_control != _rtld_global_ro._dl_fpu_control)
__setfpucw (__fpu_control);
__libc_argc = argc;
__libc_argv = argv;
__environ = envp;
__init_misc (argc, argv, envp);
__libc_global_ctors ();
}
[...@gnu-6 tmp]$ /usr/gcc-4.5/bin/gcc -m32 -march=i686 -fPIC -O2 x.i -c
-fomit-frame-pointer
[...@gnu-6 tmp]$ objdump -dw x.o
x.o: file format elf32-i386
Disassembly of section .text:
00000000 <_init>:
0: 83 ec 2c sub $0x2c,%esp
3: 89 5c 24 1c mov %ebx,0x1c(%esp)
7: e8 fc ff ff ff call 8 <_init+0x8>
c: 81 c3 02 00 00 00 add $0x2,%ebx
12: 89 74 24 20 mov %esi,0x20(%esp)
16: 8b 74 24 30 mov 0x30(%esp),%esi
1a: 89 7c 24 24 mov %edi,0x24(%esp)
1e: 8b 7c 24 34 mov 0x34(%esp),%edi
22: 89 6c 24 28 mov %ebp,0x28(%esp)
26: 8b 6c 24 38 mov 0x38(%esp),%ebp
2a: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
30: 85 c0 test %eax,%eax
32: 74 54 je 88 <_init+0x88>
34: 8b 00 mov (%eax),%eax
36: 85 c0 test %eax,%eax
38: 75 4e jne 88 <_init+0x88>
3a: c7 83 00 00 00 00 01 00 00 00 movl $0x1,0x0(%ebx)
44: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
4a: 89 b3 00 00 00 00 mov %esi,0x0(%ebx)
50: 89 bb 00 00 00 00 mov %edi,0x0(%ebx)
56: 89 28 mov %ebp,(%eax)
58: 89 6c 24 08 mov %ebp,0x8(%esp)
5c: 89 7c 24 04 mov %edi,0x4(%esp)
60: 89 34 24 mov %esi,(%esp)
63: e8 fc ff ff ff call 64 <_init+0x64>
68: e8 fc ff ff ff call 69 <_init+0x69>
6d: 8b 5c 24 1c mov 0x1c(%esp),%ebx
71: 8b 74 24 20 mov 0x20(%esp),%esi
75: 8b 7c 24 24 mov 0x24(%esp),%edi
79: 8b 6c 24 28 mov 0x28(%esp),%ebp
7d: 83 c4 2c add $0x2c,%esp
80: c3 ret
81: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
88: 8b 83 00 00 00 00 mov 0x0(%ebx),%eax
8e: 8b 93 00 00 00 00 mov 0x0(%ebx),%edx
94: c7 83 00 00 00 00 00 00 00 00 movl $0x0,0x0(%ebx)
9e: 0f b7 00 movzwl (%eax),%eax
a1: 66 39 02 cmp %ax,(%edx)
a4: 74 9e je 44 <_init+0x44>
a6: 0f b7 c0 movzwl %ax,%eax
a9: 89 04 24 mov %eax,(%esp)
ac: e8 fc ff ff ff call ad <_init+0xad>
b1: eb 91 jmp 44 <_init+0x44>
Why does gcc choose EBP over ECX/EDX?
--
Summary: Poor register choice on x86
Product: gcc
Version: 4.5.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: hjl dot tools at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44958
