[llvm-dev] Manipulating DAGs in TableGen

Paul C. Anagnostopoulos via llvm-dev
Mon, 12 Oct 2020 18:01:02 -0000

I understood that the name is a matching tag for the operand and not its name
(as in named macro or function arguments). However, I was assuming that the
names in any one DAG node had to be unique and so could serve as selectors for
operands. But a quick investigation shows that I was wrong: names can be
duplicated in the same node.

So DAG indexes are integers only.


At 10/12/2020 01:46 PM, Nicolai HÃ¤hnle wrote:
>On Mon, Oct 12, 2020 at 7:37 PM Paul C. Anagnostopoulos
><paul at windfall.com> wrote:
>> I included the ability to get/set an operand by name because I thought
it would be easier to copy+modify an existing DAG by specifying the name of the
operand you want to replace rather than having to remember its position. For
example, if you want to replace the first source, isn't it easier to specify
$src than remember it's the second operand?
>
>My point is precisely that the $names don't work that way. Your
>reasoning would be valid if the $names were function/operator argument
>names, like in programming languages where you can pass function
>arguments based on their order but also by naming them (e.g.
>"functionName(argName=x, otherArgName=y)"). However, this is _not_
how
>$names work!
>
>Their most prominent application is for instruction selection pattern
>matching, e.g. taken at random from AMDGPU/SOPInstructions.td:
>
>def : GCNPat <
>  (i32 (smax i32:$x, (i32 (ineg i32:$x)))),
>  (S_ABS_I32 SReg_32:$x)
>>;
>
>The $x is _not_ the name of the argument to smax, ineg, or S_ABS_I32.
>For example, if you look at how S_ABS_I32 is defined, you'll see that
>its input operand is called $src0.
>
>Instead, the name allows us to tie three locations in the DAG together
>for purposes of pattern matching. The name is only meaningful in the
>context of this pattern. You could substitute $x by $y or $whatever
>without changing the meaning of the DAG.
>
>That the name is the name of an operator argument is an understandable
>misunderstanding, but it _is_ a misunderstanding. If you were to add
>that particular feature, you would encourage this misunderstanding
>even more.


----------------------------------------------------------------
Windfall               Paul C. Anagnostopoulos
      ----------------------------------------------------------
   Software            978 369-0839
                             www.windfall.com
----------------------------------------------------------------
My life has been filled with calamities,
some of which actually happened.
---Mark Twain

Guga 'mzimba, sala 'nhliziyo