Bug with PPC 32-bit inline assembler [4.1.1 and 4.3.2]

[Andrew Haley]

Tom St Denis wrote:
> I ran into a bug with the PPC side of things with both 4.1.1 [came with
> FC5] and a freshly built 4.3.2, the snippet is this block
> 
> #if 0
> 
> #define SQRADDSC(i, j)            \
> asm(                              \
>   " mullw  %0,%6,%7        \n\t" \
>   " mulhwu %1,%6,%7        \n\t" \
>   " xor    %2,%2,%2        \n\t" \
> :"=r"(sc0), "=r"(sc1), "=r"(sc2):"0"(sc0), "1"(sc1), "2"(sc2),
> "r"(i),"r"(j) : "%cc");
> 
> #else
> 
> #define SQRADDSC(i, j)            \
> asm(                              \
>   " mullw  %0,%3,%4        \n\t" \
>   " mulhwu %1,%3,%4        \n\t" \
>   " xor    %2,%2,%2        \n\t" \
> :"=r"(sc0), "=r"(sc1), "=r"(sc2): "r"(i),"r"(j) : "%cc");
> 
> #endif
> 
> Where {sc0,sc1,sc2} are not previously initialized.  The first block
> (that is disabled) is my old code.  It would correctly warn about
> uninitialized variables.  The 2nd block was my fix which to me seems
> correct.  However, the compiler generates output like this
> 
> # 0 "" 2
> # 38 "fp_sqr_comba_6.i" 1
>         mullw  25,25,18               mulhwu 9,25,18              
> xor    0,0,0      
> Where r25 is both an input and output it seems.  Clearly r25 is not
> supposed to be overwritten here.

This isn't a bug: look for "earlyclobber" in the documentation.

Andrew.