[LLVMdev] Simple question on sign

[James Molloy]

Hi Sam,

Whereas most languages track signedness on the variable/value level, LLVM IR
takes a more machine-like approach of having the sign apply to the
instruction rather than the value.

It is therefore the frontend (or whatever is initially producing the LLVM
IR) that should know whether an operation should be signed or unsigned.

Hopefully that makes sense,

Cheers,

James

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of sam parker
Sent: 23 February 2012 09:45
To: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Simple question on sign


how does llvm decide when to use unsigned instructions then? such as
unsigned
adds and loads? I'm trying to describe some multiply shift ops and getting a
bit stuck differentiating between signed and unsigned.

sam
 

Eli Friedman-2 wrote:
> 
> On Wed, Feb 22, 2012 at 4:28 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
> On Wed, Feb 22, 2012 at 4:28 PM, Ryan Taylor <ryta1203 at gmail.com> wrote:
>> How do you determine if a shift is signed or not?
>>
>> ashr = always signed?
> 
> Essentially, yes.
> 
>> lshr = always unsigned?
> 
> Essentially, yes.
> 
>> shl = always signed?
> 
> Signed left shift and unsigned left shift are both shl.
> 
> http://llvm.org/docs/LangRef.html#i_shl describes the semantics of shifts.
> 
>> The CmpInst has the "isSigned()" function, but it appears that every
>> other
>> Instruction I've looked at doesn't seem to have this.
> 
> There isn't an isSigned() function because the query doesn't really
> make sense.  LLVM IR doesn't in general track whether a value is
> signed or unsigned.
> 
> -Eli
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> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> 
> 

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