[LLVMdev] arithmetical operands signedness
Christophe de Dinechin
christophe at taodyne.com
Mon Sep 5 09:35:03 PDT 2011
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:
- 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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