[LLVMdev] sext..to instruction

[Nick Lewycky]

Le Anh Quang wrote:
> Hi,
> I have a question about the "sext..to" instruction. In the document, I found
> two examples:
> %x = sext i8 -1 to i16
> It means:
> i8 -1   = 1111 1111 --> 1111 1111 1111 1111 = i16
> how can it determinate, that the i16 value %x positive is (65535)?

i16 65536 = i16 -1. They're both 1111 1111 1111 1111.

> And the second example:
> 
> %y = sext i1 true to i32
> 1 --> 1111 1111 1111 1111 1111 1111 1111 1111
> In this example, %y is -1
> 
> I'm not sure about it, when sext to results a positve/negative value?

Since they're the same bits, it doesn't matter at all until you perform 
you I/O, at which time your code makes the decision as to whether the 
top bit should be treated as a sign bit or not.

The LangRef should probably be updated, as LLVM's AsmPrinter will never 
print i16 65536, preferring to show all values as signed.

Nick Lewycky

> Thank for any advice
> Quang
> 
> 
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