I agree that the register usage definition are not proper, as it has no destination register. something like
// asm( " instruction " : destination_regs : source_regs : clobber_regs )
asm(" umull %[yy],%[xx],%[xx],%[yy]" : [xx] "=r" (x) : [xx] (x) , [yy] "r" (y) ); seems more correct, but does not compile.
Actually not having a destination register did produce incorrect code in 1 out of 25 instances. But I guess without a destination register the compile thinks it returns void eg. void func(x,y).
consider this example.
ssmul_ls2_asm:
smull r1, r0, r0, r1
lsl r0,r0,#2
add r0,r0,r1,lsr #30
bx lr
or
static inline __attribute__((always_inline)) S32 ssmul_ls2(S32 x,S32 y)
{
S32 z;
asm(" smull %[yy],%[xx],%[xx],%[yy] \n" \
" lsl %[xx],%[xx],#2 \n" \
" add %[zz],%[xx],%[yy], LSR #30 \n" : [zz] "=r" (z) : [xx] "r" (x) , [yy] "r" (y));
return(z);
}
This works fine because the destination register is defined, the 'S32 z' variable will probably be optimized out.
Now consider this one that does not work
static inline __attribute__((always_inline))
S32 ssmul(S32 x,S32 y)
{
S32 z;
asm(" smull %[yy],%[zz],%[xx],%[yy]" : [zz] '=r" (z) : [xx] "r" (x) , [yy] "r" (y) );
return(z);
}
The only way I have found to make it work is forcing it to use a specific register in the clobber registers section....
static inline __attribute__((always_inline)) S32 ssmul(S32 x,S32 y)
{
S32 z;
asm(" smull r1,%[zz],%[xx],%[yy]" : [zz] "=r" (z) : [xx] "r" (x) , [yy] "r" (y) : "r1" );
return(z);
}
This syntax is insane, both for the inline assembly, and the hoops you have to jump though to inline a simple function.
