Re: [Bug inline-asm/37562] New: [4.2] -funroll-loops destroys inline asm code von powerpc

[Andrew Thomas Pinski <pinskia@gmail.com>]

Sent from my iPhone

On Sep 17, 2008, at 8:42 AM, "rbuergel at web dot de" <gcc-bugzilla@gcc.gnu.org 
 > wrote:

> typedef unsigned int UInt32;
> typedef unsigned char UInt8;
>
> struct Data
> {
>  UInt8 data[16];
>
>  const UInt8* getData() const
>  {
>    return data + 4;
>  }
> };
>
> struct Value {
>  UInt32 value;
>  static void update (Value& signal, const Data& source, UInt32  
> startBit);
> };
>
> void Value::update(Value& signal, const Data& source, UInt32  
> startByte)
> {
>  UInt32& value = signal.value;
>  const UInt8* data = source.getData();
>  asm( "lwbrx %0,%1,%2;\n":"=r"(value): "r"(data), "r"(startByte),  
> "m" (*(data
> + startByte)) );
> }

The second constraint should be using "b" instead of "r" as b says  
don't use r0.


>
>
> generated for gcc-4.2.4 on powerpc:
> 00000000 <_ZN5Value6updateERS_RK4Dataj>:
>   0:   38 04 00 04     addi    r0,r4,4
>   4:   7c 80 2c 2c     lwbrx   r4,0,r5
>   8:   90 83 00 00     stw     r4,0(r3)
>   c:   4e 80 00 20     blr
>
> the correct version without -funroll-loops reads:
> 00000000 <_ZN5Value6updateERS_RK4Dataj>:
>   0:   38 84 00 04     addi    r4,r4,4
>   4:   7c 84 2c 2c     lwbrx   r4,r4,r5
>   8:   90 83 00 00     stw     r4,0(r3)
>   c:   4e 80 00 20     blr
>
>
> lwbrx sums the contents of r4 and r5 and accesses this address,  
> swaps it's
> value and stores it again in r4. The incorrect version accesses an  
> invalid
> address, which leads to a segmentation fault.
>
> this is corrected at least with gcc-4.3.1, there i couldn't  
> reproduce the
> behaviour.
>
>
> -- 
>           Summary: [4.2] -funroll-loops destroys inline asm code von
>                    powerpc
>           Product: gcc
>           Version: 4.2.4
>            Status: UNCONFIRMED
>          Severity: normal
>          Priority: P3
>         Component: inline-asm
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: rbuergel at web dot de
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37562
>