[LLVMdev] arithmetical operands signedness
baldrick at free.fr
Mon Sep 5 09:41:35 PDT 2011
On 05/09/11 18:35, Christophe de Dinechin wrote:
> On 5 sept. 2011, at 17:48, Duncan Sands wrote:
>> since the result of a multiply doesn't depend on the signedness, I find it
>> strange that your target differentiates between them. What I'm saying is
>> that if you have (say) two i32 numbers a and b and you do a signed multiply:
>> c = a *s b
>> and an unsigned multiply
>> d = a *u b
>> then c and d are the same number (exactly the same bits set).
> At least two architectures I know about have size-extending multiplication, for which your statement is not true:
yup, and that's why LLVM codegen has the SMUL_LOHI/UMUL_LOHI etc nodes. However
that's not relevant to ordinary multiplication (codegen MUL node) which is what
I understand the question to be about.
> - Motorola MC68K has i16 x 16 -> i32 instructions in signed and unsigned forms
> - Itanium has signed and unsigned multiplications with i64 x i64 -> i64 where you can take the high or low part of the resulting i128. While xma.lu is a pseudo-op since it's the same as xma.lu, xma.hu and xma.h (unsigned and signed) are distinct.
> I'm pretty sure there are other similar architectures.
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