mips-elf-gcc -fno-delayed-branch problem

[Toshi Morita]

target: mips-elf
version: 4.4.1

There's a problem where compiling code with -fno-delayed-branch still fills 
branch delay slots.

[ubxju10]/home/jupiter/tmorita/dhrystone/test 1097 % cat minlib.c
#include <stdarg.h>

int printf(const char *format, ...)
{
        register int a0 asm ("a0");
        register int a1 asm ("a1");
        register int a2 asm ("a2");
        va_list ap;

        va_start(ap, format);

        a0 = 2;
        a1 = (int)format;
        a2 = (int)ap;

        __asm__ volatile ("nop; nop; syscall; nop" : : "r" (a0), "r" (a1), "r" 
(a2));

        va_end(ap);
}

[ubxju10]/home/jupiter/tmorita/dhrystone/test 1098 % mips-elf-gcc -O2 
-fno-delayed-branch -c minlib.c -o minlib.o
[ubxju10]/home/jupiter/tmorita/dhrystone/test 1099 % mips-elf-objdump 
--disassemble minlib.o

minlib.o:     file format elf32-bigmips

Disassembly of section .text:

00000000 <printf>:
   0:   27bdfff8        addiu   sp,sp,-8
   4:   27a2000c        addiu   v0,sp,12
   8:   afa5000c        sw      a1,12(sp)
   c:   afa60010        sw      a2,16(sp)
  10:   00802821        move    a1,a0
  14:   afa70014        sw      a3,20(sp)
  18:   afa20000        sw      v0,0(sp)
  1c:   00403021        move    a2,v0
  20:   24040002        li      a0,2
        ...
  2c:   0000000c        syscall
  30:   00000000        nop
  34:   03e00008        jr      ra
  38:   27bd0008        addiu   sp,sp,8                <- branch delay slot 
filled
[ubxju10]/home/jupiter/tmorita/dhrystone/test 1100 %

The problem appears to be caused with GNU AS. It now has the capability to 
reorder instructions, and there is no ".set noreorder" emitted by the compiler, 
so the assembler fills the branch delay slot.

[ubxju10]/home/jupiter/tmorita/dhrystone/test 1104 % cat test.s
        .text
        addiu $sp,$sp,8
        jr $ra
[ubxju10]/home/jupiter/tmorita/dhrystone/test 1105 % mips-elf-as test.s -o 
test.o
[ubxju10]/home/jupiter/tmorita/dhrystone/test 1106 % mips-elf-objdump 
--disassemble test.o

test.o:     file format elf32-bigmips

Disassembly of section .text:

00000000 <.text>:
   0:   03e00008        jr      ra
   4:   27bd0008        addiu   sp,sp,8
[ubxju10]/home/jupiter/tmorita/dhrystone/test 1107 %

So when the -fno-delayed-branch switch is passed to mips-elf-gcc, a ".set 
noreorder" directive should be emitted into the assembly file.

Toshi