[LLVMdev] arithmetical operands signedness (original) (raw)

Duncan Sands baldrick at free.fr
Mon Sep 5 08:48:05 PDT 2011

Hi Jonas,

my target handles operands of multiplying instructions differently based on signedness.

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).

I wonder then how I would do instruction selection based on the operands signs? The mul instruction sets a nsw for signed,

No it doesn't. nsw doesn't mean signed, it means that if the multiplication would overflow as a signed multiplication then the result is undefined which is completely different.

but when i try unsigned ops, there is

no wrap flag at all. I'm not sure this is enough information to work with, or?

No information is needed, see above.

Ciao, Duncan.

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