llvm dev - Oct 2015 - Break nested instructions?

On 16/10/15 14:09, mats petersson wrote:
> I think it's important to understand that this is not ALWAYS the case. 
> The operands can be either a constant expression or an instruction.
>
> For example:
>
>      char *a;
>      int x;
>
>      if(cond) a = "%d";
>      else a = "%x";
>      printf(a, x);
>
> In this case, the compiler may still be able to convert to constant 
> expression for `a`, depending on if it's able to deduce `cond`. But if 
> `cond` is not "constant", `a` will be a GEP instruction, not a 
> constant expression.
> (Of course, the compiler will probably also warn that passing a 
> variable to printf is a bad idea, but that's a different matter)
>
> --
> Mats
Does this mean that we can have a nested instruction into an 
instruction?! Wouldn't that lead having a LLVM IR language without 
terminals?


--istavrak

On 16 October 2015 at 12:21, Irini Stavrakantonaki <istavrak at
ics.forth.gr>
wrote:
>
>
> On 16/10/15 14:09, mats petersson wrote:
>
>> I think it's important to understand that this is not ALWAYS the
case.
>> The operands can be either a constant expression or an instruction.
>>
>> For example:
>>
>>      char *a;
>>      int x;
>>
>>      if(cond) a = "%d";
>>      else a = "%x";
>>      printf(a, x);
>>
>> In this case, the compiler may still be able to convert to constant
>> expression for `a`, depending on if it's able to deduce `cond`. But
if
>> `cond` is not "constant", `a` will be a GEP instruction, not
a constant
>> expression.
>> (Of course, the compiler will probably also warn that passing a
variable
>> to printf is a bad idea, but that's a different matter)
>>
>> --
>> Mats
>>
> Does this mean that we can have a nested instruction into an instruction?!
> Wouldn't that lead having a LLVM IR language without terminals?
>
Interesting point, I'm not sure - but the operand on an
"instruction" is a
Value, so I expect it can be any type that is within the Value class
hierarchy?

--
Mats
>
>
> --istavrak
>
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On 16/10/15 14:33, mats petersson wrote:
> Interesting point, I'm not sure - but the operand on an
"instruction"
> is a Value, so I expect it can be any type that is within the Value 
> class hierarchy?
I expected the same. That's why I was searching a way to "break"
the
inlined instructions, as nested instructions are not convenient for me.
In case the operands of instructions are always meant to be constant 
exprs, then it's simple to handle them differently by having different 
adaptor classes
as David proposed before.

--istavrak