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Date: Tue, 02 Aug 2011 06:47:00 -0000
From: Georg-Johann Lay <avr@gjlay.de>
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To: Michael Walle <michael@walle.cc>
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Subject: Re: libgcc: strange optimization
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Michael Walle wrote:
> Hi,
> 
> That was quick :)
> 
>> Your asm has no output operands and no side effects, with more
>> aggressive optimization the whole ask would disappear.
> Sorry, that was just a small test file, the original code has output operands.
> 
> The new test code:
>  static int inline f1(int arg)
>  {
>    register int ret asm("r1");
>    register int a1 asm("r8") = 10;
>    register int a2 asm("r1") = arg;
> 
>    asm volatile ("scall" : "=r"(ret) : "r"(a1), "r"(a2) : "memory");
> 
>    return ret;
>  }
> 
>  int f2(int arg1, int arg2)
>  {
>    return f1(arg1 >> 10);
>  }
> 
> translates to the same assembly:
> f2:
>         addi     sp, sp, -4
>         sw       (sp+4), ra
>         addi     r2, r0, 10
>         calli    __ashrsi3
>         scall
>         lw       ra, (sp+4)
>         addi     sp, sp, 4
>         b        ra
> 
> PS. R1 is the return register in the target architecture ABI.

I'd guess you ran into

http://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html#Local-Reg-Vars

A common pitfall is to initialize multiple call-clobbered registers
with arbitrary expressions,  where a function call or  library call
for an arithmetic operator will overwrite a register value from a
previous assignment, for example r0 below:

     register int *p1 asm ("r0") = ...;
     register int *p2 asm ("r1") = ...;

In those cases, a solution is to use a temporary variable for each
arbitrary expression.

So I'd try to rewrite it as

static int inline f1 (int arg0)
{
    int arg = arg0;
    register int ret asm("r1");
    register int a1 asm("r8") = 10;
    register int a2 asm("r1") = arg;

    asm volatile ("scall" : "=r"(ret) : "r"(a1), "r"(a2) : "memory");

    return ret;
}

and if that does not help the rather hackish

static int inline f1 (int arg0)
{
    int arg = arg0;
    register int ret asm("r1");
    register int a1 asm("r8");
    register int a2 asm("r1");

    asm ("" : "+r" (arg));

    a1 = 10;
    a2 = arg;

    asm volatile ("scall" : "=r"(ret) : "r"(a1), "r"(a2) : "memory");

    return ret;
}