register declared variable for no optimization

register declared variable for no optimization

[Long Li]

Hi,

I have a question on the GCC optimization. I am using
GCC3.0.4 MIPS crosscompiler on Redhat 7.1. In a
function I did the following:

init()
{
  register unsigned int temp;
      .
      .
  temp = devict.dev1.devtc
      .
} 

I compiled the code with optimization level 0 and
found something weird about the register variable
temp. This is shows in the disassembly file:

for temp = devict.dev1.devtc
    lui $3, 0xb801
    lw  $3, 28($3)

This is right since the CPU loads the
devict.dev1.devtc value from memory to the register 3.
However, after the above instructions, I found:

     nop
     sw $3, 0($30)
     lw $2, 0($30)

now the compiler asked the CPU to store the register
in the stack. However, I declared the variable as a
register, how come it still needs to go to the stack?
I compiled the same code with gcc-2.8.1, optimization
level 0, and did not find the same issue there. 

Why the gcc-3.0.4 did the weird stuff? Do I have to
use at least level 1 to make the register declared
work for it?

Thanks a lot!


Long

 


__________________________________________________
Do you Yahoo!?
Yahoo! Mail Plus - Powerful. Affordable. Sign up now.
http://mailplus.yahoo.com

Re: register declared variable for no optimization

[Ralf Baechle]

On Thu, Jan 30, 2003 at 03:11:19PM -0800, Long Li wrote:

> Why the gcc-3.0.4 did the weird stuff? Do I have to
> use at least level 1 to make the register declared
> work for it?

Most modern C compilers simply ignore the register specifier because in
practice it would result in worse code and as for code quality the
result of -O0 are not of interest anyway.

Linux code must be optmized at least -O1 or it probably won't build or
work properly.

  Ralf