llvm dev - Oct 2014 - [LLVMdev] on type annotations of LLVM IR

Hi,

 I am curious to know the design decision on why operands of LLVM IR
instructions are type annotated.

Why wasn't the C style of explicit declaration of variables (or values in
LLVM context) followed by their uses model not adopted here?

or may be even annotate the value (result of the Instruction) with type
rather than annotating each operands on the Instruction?

Just felt that by annotating each operand with types may make the
Instruction 'complex' to understand. May be I am wrong.

Thanks for any explanation,
-- 
Rekha
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On 27 Oct 2014, at 12:46, Rekha R <rekharamapai at nitc.ac.in> wrote:
> Why wasn't the C style of explicit declaration of variables (or values
in LLVM context) followed by their uses model not adopted here?
For the simple reason that there are no variables in LLVM IR.  The SSA form
means that every value is declared and initialised exactly once.  Some values,
such as constant integers, are created (in the text serialisation) inline in
their use so there is no point where they are declared.
> or may be even annotate the value (result of the Instruction) with type
rather than annotating each operands on the Instruction?
> 
> Just felt that by annotating each operand with types may make the
Instruction 'complex' to understand. May be I am wrong.
You will find it much easier to read when you are debugging.  The IR is not
intended to be read or written by humans, the text serialisation is there as a
debugging aid when something has gone wrong.  It is *very* useful to be able to
look at a single instruction and see all of the types, without having to chase
through a load of other parts of the code to find where things were declared.

David

Hi,

Thank you David.
My question was from a machine independent code analyzer/optimizer
perspective where types are of less importance. Hence I felt difficult to
understand the IR with the types.

Rekha

On Mon, Oct 27, 2014 at 6:44 PM, David Chisnall <David.Chisnall at
cl.cam.ac.uk
> wrote:
> On 27 Oct 2014, at 12:46, Rekha R <rekharamapai at nitc.ac.in> wrote:
>
> > Why wasn't the C style of explicit declaration of variables (or
values
> in LLVM context) followed by their uses model not adopted here?
>
> For the simple reason that there are no variables in LLVM IR.  The SSA
> form means that every value is declared and initialised exactly once.  Some
> values, such as constant integers, are created (in the text serialisation)
> inline in their use so there is no point where they are declared.
>
> > or may be even annotate the value (result of the Instruction) with
type
> rather than annotating each operands on the Instruction?
> >
> > Just felt that by annotating each operand with types may make the
> Instruction 'complex' to understand. May be I am wrong.
>
> You will find it much easier to read when you are debugging.  The IR is
> not intended to be read or written by humans, the text serialisation is
> there as a debugging aid when something has gone wrong.  It is *very*
> useful to be able to look at a single instruction and see all of the types,
> without having to chase through a load of other parts of the code to find
> where things were declared.
>
> David
>
>

-- 
Rekha
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