[LLVMdev] Multiply i8 operands promotes to i32

[Duncan Sands]

Hi Pedro, what is the "front-end" you refer to?  Clang?

Ciao, Duncan.


> I am trying to complete the hardware multiplier option for MSP430 backend.
>
> As the hardware multiplier in most of the MSP430 devices is for i8 and
> i16 operands, with i16 and i32 result, I am lowering MUL_i8 and MUL_I16.
> However, the front-end promotes the i8 argument to i32, executes 32-bit
> multiplier and truncates to 16-bit, so I never lower MUL_I8 nor MUL_I16
> but MUL_I32, wchich is lowered to an external libcall (__mulsi3) that I
> don´t have.
>
> What should I do in order to prevent the front-end from promote to
> 32-bit multiplier?
> If that is not possible, how can I detect when lowering that it is
> actually a MUL_I8 or MUL_I16 in order to do the correct lowering?
>
> Thanks in advance,
> Pedro
>
> P.S: I add C code and corresponding LLVM code.
>
> C code:
> void
>   (const u_int16_t in_data, u_int16_t* out)
> {
>    u_int8_t kk = in_data&0xFF;
>    u_int16_t kk16 = kk * kk;
>    *out = kk16;
> }
>
>
> LLVM:
>    %1 = load i8* %kk, align 1
>    %conv2 = zext i8 %1 to i32
>    %2 = load i8* %kk, align 1
>    %conv3 = zext i8 %2 to i32
>    %mul = mul nsw i32 %conv2, %conv3
>    %conv4 = trunc i32 %mul to i16
>    store i16 %conv4, i16* %kk16, align 2
>