[PATCH v3 02/13] loader: Remove GCC <=4.x EBX register spilling workaround for i386.

[Jinoh Kang]

Arbitrarily manipulating the stack pointer in inline assembly interferes
with stack unwinding and debugging experience.  Note that it's
impossible to reliably specify unwinding information in inline assembly,
other than adjusting CFA offset.

The workaround appears to be due to "buggy" register allocation that
manifests in GCC <= 4.x when emitting position-independent code.
This is not an issue, since the preloader isn't a position-independent
executable or a shared library.

Fix this by getting rid of extra spilling and value transfer of the EBX
register, and telling the compiler to allocate EBX directly.

Signed-off-by: Jinoh Kang <jinoh.kang.kr at gmail.com>
---
 loader/preloader.c | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/loader/preloader.c b/loader/preloader.c
index 937843eb9c5..9675dc3f8eb 100644
--- a/loader/preloader.c
+++ b/loader/preloader.c
@@ -241,32 +241,32 @@ __ASM_GLOBAL_FUNC(_start,
 static inline __attribute__((noreturn)) void wld_exit( int code )
 {
     for (;;)  /* avoid warning */
-        __asm__ __volatile__( "pushl %%ebx; movl %1,%%ebx; int $0x80; popl %%ebx"
-                              : : "a" (1 /* SYS_exit */), "r" (code) );
+        __asm__ __volatile__( "int $0x80"
+                              : : "a" (1 /* SYS_exit */), "b" (code) );
 }
 
 static inline int wld_open( const char *name, int flags )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
-                          : "=a" (ret) : "0" (5 /* SYS_open */), "r" (name), "c" (flags) );
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (5 /* SYS_open */), "b" (name), "c" (flags) );
     return SYSCALL_RET(ret);
 }
 
 static inline int wld_close( int fd )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
-                          : "=a" (ret) : "0" (6 /* SYS_close */), "r" (fd) );
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (6 /* SYS_close */), "b" (fd) );
     return SYSCALL_RET(ret);
 }
 
 static inline ssize_t wld_read( int fd, void *buffer, size_t len )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
+    __asm__ __volatile__( "int $0x80"
                           : "=a" (ret)
-                          : "0" (3 /* SYS_read */), "r" (fd), "c" (buffer), "d" (len)
+                          : "0" (3 /* SYS_read */), "b" (fd), "c" (buffer), "d" (len)
                           : "memory" );
     return SYSCALL_RET(ret);
 }
@@ -274,16 +274,16 @@ static inline ssize_t wld_read( int fd, void *buffer, size_t len )
 static inline ssize_t wld_write( int fd, const void *buffer, size_t len )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
-                          : "=a" (ret) : "0" (4 /* SYS_write */), "r" (fd), "c" (buffer), "d" (len) );
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (4 /* SYS_write */), "b" (fd), "c" (buffer), "d" (len) );
     return SYSCALL_RET(ret);
 }
 
 static inline int wld_mprotect( const void *addr, size_t len, int prot )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
-                          : "=a" (ret) : "0" (125 /* SYS_mprotect */), "r" (addr), "c" (len), "d" (prot) );
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (125 /* SYS_mprotect */), "b" (addr), "c" (len), "d" (prot) );
     return SYSCALL_RET(ret);
 }
 
@@ -329,8 +329,8 @@ __ASM_GLOBAL_FUNC(wld_mmap,
 static inline int wld_prctl( int code, long arg )
 {
     long ret;
-    __asm__ __volatile__( "pushl %%ebx; movl %2,%%ebx; int $0x80; popl %%ebx"
-                          : "=a" (ret) : "0" (172 /* SYS_prctl */), "r" (code), "c" (arg) );
+    __asm__ __volatile__( "int $0x80"
+                          : "=a" (ret) : "0" (172 /* SYS_prctl */), "b" (code), "c" (arg) );
     return SYSCALL_RET(ret);
 }
 
-- 
2.31.1