llvm dev - Dec 2009 - [LLVMdev] Question to use inline assemble in X86

Hi everyone,

I try to add an instruction to x86. The instruction is a
multiply-add instruction
MULADD A, B, C; //A = A + B * C.
I use the instruction by inline assemble as below

int x, y, z;
.....  ....
x = 0;
asm("MULADD %0, %1, %2":"=r"(x):"0"(x),
"r"(y), "r"(z));
..... ....

The backend does allocate registers %edx, %edi, %esi for x,y, z
respectively,
but its assemble output is
        MULADD %edx, %edx, %esi
I expects it could output:
        MULADD %edx, %edi, %esi

What's  matter with my constraints of inline asm?

Thanks.
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On Dec 29, 2009, at 3:09 AM, Heyu Zhu wrote:
> Hi everyone,
>  
> I try to add an instruction to x86. The instruction is a multiply-add
instruction
> MULADD A, B, C; //A = A + B * C.
> I use the instruction by inline assemble as below
>  
> int x, y, z;
> .....  ....
> x = 0;
> asm("MULADD %0, %1, %2":"=r"(x):"0"(x),
"r"(y), "r"(z));
> ..... ....
>  
> The backend does allocate registers %edx, %edi, %esi for x,y, z
respectively,
> but its assemble output is
>         MULADD %edx, %edx, %esi
> I expects it could output:
>         MULADD %edx, %edi, %esi
>  
> What's  matter with my constraints of inline asm?
>  
The constraint tells the compiler what registers are valid to use. 
"r" means that it can use any register.  You need to tell the compiler
what registers need to be used, because it doesn't look inside the asm
string.  Please take a look at the GCC documentation for more details:
http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Extended-Asm
http://gcc.gnu.org/onlinedocs/gcc/Constraints.html#Constraints

-Chris

After look at documents of inline assemble,  i have known its reason.

Thanks!

2009/12/30, Chris Lattner <clattner at apple.com>:
>
>
> On Dec 29, 2009, at 3:09 AM, Heyu Zhu wrote:
>
> > Hi everyone,
> >
> > I try to add an instruction to x86. The instruction is a multiply-add
> instruction
> > MULADD A, B, C; //A = A + B * C.
> > I use the instruction by inline assemble as below
> >
> > int x, y, z;
> > .....  ....
> > x = 0;
> > asm("MULADD %0, %1, %2":"=r"(x):"0"(x),
"r"(y), "r"(z));
> > ..... ....
> >
> > The backend does allocate registers %edx, %edi, %esi for x,y, z
> respectively,
> > but its assemble output is
> >         MULADD %edx, %edx, %esi
> > I expects it could output:
> >         MULADD %edx, %edi, %esi
> >
> > What's  matter with my constraints of inline asm?
> >
>
> The constraint tells the compiler what registers are valid to use. 
"r"
> means that it can use any register.  You need to tell the compiler what
> registers need to be used, because it doesn't look inside the asm
> string.  Please take a look at the GCC documentation for more details:
> http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Extended-Asm
> http://gcc.gnu.org/onlinedocs/gcc/Constraints.html#Constraints
>
> -Chris
>
>
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