llvm dev - Jan 2014 - [LLVMdev] Question About the LLVM IR unnamed values!

Hi.

I found that, in all the LLVM IR I get from the source C file.  the unnamed
values all created by the "load" instruction. eg:



; Function Attrs: nounwind uwtable
define i32 @add(i32 %x, i32 %y) #0 {
entry:
  %x.addr = alloca i32, align 4
  %y.addr = alloca i32, align 4
  %c = alloca i32, align 4
  store i32 %x, i32* %x.addr, align 4
  store i32 %y, i32* %y.addr, align 4
*  %0 = load i32* %x.addr, align 4*
*  %1 = load i32* %y.addr, align 4*
  %add = add nsw i32 %0, %1
  store i32 %add, i32* %c, align 4
  %2 = load i32* %c, align 4
  ret i32 %2
}

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  %a = alloca i32, align 4
  %b = alloca i32, align 4
  %d = alloca i32, align 4
  %f = alloca i32, align 4
  %g = alloca i32, align 4
  %e = alloca i32, align 4
  store i32 0, i32* %retval
  store i32 8, i32* %a, align 4
  store i32 9, i32* %b, align 4
 * %0 = load i32* %a, align 4*
*  %1 = load i32* %b, align 4*
  %add = add nsw i32 %0, %1
  store i32 %add, i32* %d, align 4
*  %2 = load i32* %a, align 4*
*  %3 = load i32* %d, align 4*
  %add1 = add nsw i32 %2, %3
  store i32 %add1, i32* %f, align 4
*  %4 = load i32* %f, align 4*
*  %5 = load i32* %b, align 4*
  %add2 = add nsw i32 %4, %5
  store i32 %add2, i32* %g, align 4
*  %6 = load i32* %d, align 4*
*  %7 = load i32* %b, align 4*
  %call = call i32 @add(i32 %6, i32 %7)
  store i32 %call, i32* %e, align 4
  %call3 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str, i32 0, i32 0), i32* %a)
  %call4 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str1, i32 0, i32 0), i32* %b)
  %call5 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str2, i32 0, i32 0), i32* %e)
  %call6 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str, i32 0, i32 0), i32* %d)
  %call7 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str, i32 0, i32 0), i32* %g)
  %call8 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x
i8]* @.str, i32 0, i32 0), i32* %f)
  ret i32 0
}
​
the "add" instructions all have names like "%add",
"%add1",etc.But the
"load" instruction all use "%n". ​

​is that means the unnamed instructions like "%0 %1" all created by
the
"load" instructions? ​is there any other type instructions can create
unnamed values?

Thanks!


-- 
Kind Regards!
-Jin Huang
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Hi Jin,
> is that means the unnamed instructions like "%0 %1" all created
by the
> "load" instructions? is there any other type instructions can
create unnamed
> values?
The names are chosen in clang rather than LLVM. When it's creating an
instruction there's an optional "Name" field, which seems to
mostly be
filled in. So to answer that question, you'd have to look at all
places in Clang that create instructions.

I strongly suspect there are other cases that don't give a name, but
the only ones I can think of at the moment are expansions of certain
compiler builtins (e.g. SIMD intrinsics, probably va_arg and similar).
Also, in release builds, the name is completely ignored and all
instructions are anonymous.

So you certainly shouldn't be relying on the fact that everything
except loads are named. But I can't see any particular reason we'd
turn down patches that added useful names in more cases.

Cheers.

Tim.

On Jan 14, 2014, at 12:41 AM, Tim Northover <t.p.northover at gmail.com>
wrote:
> Hi Jin,
> 
>> is that means the unnamed instructions like "%0 %1" all
created by the
>> "load" instructions? is there any other type instructions can
create unnamed
>> values?
> 
> The names are chosen in clang rather than LLVM. When it's creating an
> instruction there's an optional "Name" field, which seems to
mostly be
> filled in. So to answer that question, you'd have to look at all
> places in Clang that create instructions.
> 
> I strongly suspect there are other cases that don't give a name, but
> the only ones I can think of at the moment are expansions of certain
> compiler builtins (e.g. SIMD intrinsics, probably va_arg and similar).
> Also, in release builds, the name is completely ignored and all
> instructions are anonymous.
> 
> So you certainly shouldn't be relying on the fact that everything
> except loads are named. But I can't see any particular reason we'd
> turn down patches that added useful names in more cases.
> 
Also, you can get names on the rest by running the instuction-namer pass (“opt
-instnamer”) on the IR.
> Cheers.
> 
> Tim.
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