[LLVMdev] [RFC] Storing default function attributes on the module

[Duncan P. N. Exon Smith]

> On 2015-Feb-24, at 13:25, Eric Christopher <echristo at gmail.com> wrote:
> 
> Hi Duncan,
> 
> Been thinking about this a bit and a few comments/questions. I may have misunderstood some things in your mail though so please feel free to explain at me :)
> 
> 
> Changing `clang` to store target defaults on the module will allow us to
> continue to override them when running `llc`.  The right precedence
> would be:
> 
>   1. Explicit attributes set on the function.
>   2. `llc` command-line options.
>   3. Default function attributes stored on the module.
> 
> (Outside of `llc`, skip step 2.)
> 
> In `lib/Linker` (i.e., `llvm-lto`, `llvm-link`, `libLTO.dylib`),
> defaults should be pushed down as explicit function attributes.
> 
> I think there are a few options/backend communications that aren't/haven't gone this way yet that are probably worth considering:
> 
> MCTargetOptions/TargetOptions: Basically a grab bag of functionality, some of which is duplicated via attributes and some that's not. I think at least some of these should be replaced by module flags, e.g. ABI.
> 
> Random backend flags: Anything for debugging.
> 
> I'm thinking things that are either set as Init(true/false) and affect things at a global level and not just the function level.

(I think my lib/Linker comment was unclear.  See below.)

Not at all trying to say that *everything* should be a function
attribute; global-level stuff should absolutely be module flags.

I'm just talking about infrastructure for the things that *are*
function-level.

> 
> They look like this in assembly:
> 
>     attributes default = { "no-frame-pointer-elim"="false" }
> 
> 
> So, how do you see module merging and defaults working? (Yes, there were testcases, but let's go with prose here. I found the testcases a bit hard to reason.)

This is where my lib/Linker comment applies:

>> In `lib/Linker` (i.e., `llvm-lto`, `llvm-link`, `libLTO.dylib`),
>> defaults should be pushed down as explicit function attributes.

^ This is how I see module merging and defaults working: push the
defaults down to explicit function attributes.  So there wouldn't
be any default function attributes in the output of `llvm-link`.
This means that `llc` will still have trouble overriding attributes
in the output of merged modules -- but at least it can handle the
output of `clang` without trouble.  In the future we could try to
be more intelligent about merged modules, and keep common options
in the default set.

>  
> Limitations
> ===========
> 
> There are a few limitations with this approach (at least, with my
> reference implementation).
> 
>   - `Function::getAttributes()` only reflects the explicitly specified
>     attributes, skipping those set as module defaults.
> 
> Ick. Pretty severe limitation? I.e. how would it work to test general attributes on a function during code gen?

As long as everyone calls `Function::hasFnAttribute()`, there's no
problem.  This proposal basically turns it into a bug to access
them directly; you need to go through `Function::hasFnAttribute()`
to get the right answer.  (Not sure if there's a better way?)

>  
>   - If an enum attribute is set as a default, there's no way for a
>     function-attribute to override it.  In practice, we could avoid the
>     feature for enum attributes.
> 
> Hrm. This seems like a pretty severe limitation? Anything come to mind in practice.

In the `Attribute` format, mutually exclusive attributes aren't
related at all (they're not inherently mutually exclusive).  To
make them overridable, we'd need a completely new design for
enum attributes.

As a result, this proposal only improves `llc` for string-based
attributes.  I don't see that as a problem... the string-based
attributes are more flexible anyway.  Maybe `Module` should only
allow `setDefaultFnAttribute()` for string attributes though?

(Some more context on why enum attributes can't really be
overridden.  This isn't just a problem for enum attributes that
are mutually exclusive.  Consider:

    attributes defaults = { noreturn }

Besides being somewhat insane, there's no `return` attribute,
so you can't really override it.  I suppose one idea would be to
explicitly mark a function `~noreturn` or something:

    define void @foo() ~noreturn { ; Ignore module default noreturn.

Not sure if this direction is a good one though.)

>  
>   - `CallSite` instructions store function-level attributes, but don't
>     forward to the module-level defaults.  There are places (like the
>     calls to `EmitUnaryFloatFnCall()` in `-simplify-libcalls`) where we
>     use the callee function attributes to set the call site attributes.
>     In practice, we could avoid the feature for attributes that are
>     meaningful for call sites.
> 
> Sure.
>  
>   - Intrinsics' attributes are independent of `CodeGenOptions`, and set
>     via `Instrinsic::getAttributes()`.  With this change they'd inherit
>     the default attributes like other functions.  Is this a problem?
>     If so, we can add a flag on `Function` that inhibits forwarding to
>     the defaults.
> 
> 
> Seems reasonable.
>  
> Thoughts?  Other ideas for solving the `llc` problem?
> 
> 
> I think this is a good start, I think I'd like to worry about some of the other issues in advance before we start incrementally changing things though. (Also, I really have no other ideas :)
> 


So Akira had an idea at the end of last week that I don't
think made it onto the list, and it's worth considering as an
alternative:

Add a bit to attributes indicating whether or not they're
overridable (i.e., they're overridable if they're target
defaults; they're not overridable if they've been explicitly
specified somehow).

Here's some straw-man syntax:

    attributes #0 = { noreturn ssp? "att1"="1" "att2"="2"? }

Where:

  - `noreturn` and `"att1"="1"` are required.
  - `ssp` and `"att2"="2"` can be overridden (e.g., by `llc`).

(Alternately, but equivalently:

    attributes #0 = { noreturn! ssp "att1"="1"! "att2"="2" }

I like this syntax better, but it would cause more churn, and
`!` is so far reserved for metadata.)

Whatever the syntax, the idea is: `llc` resets/deletes
attributes on every function to match what's specified on the
command-line.  In the above example, functions with attribute
set #0 could have `ssp` and `"att2"` overridden via the `llc`
command-line, but not `noreturn` and `"att1"`.

To compare it to my proposal:

  - Storing a default attribute set (my proposal) makes it
    easier to look at and set the defaults.  Applying `llc`
    command-line options is easy, too -- just override the
    default attribute set on the module -- although it doesn't
    really work on the output of `lib/Linker`.
  - Storing a bit on each attribute (Akira's proposal) handles
    more cases.  Nothing needs to be done in `lib/Linker`
    (`llc` is able to override the output of `llvm-link`),
    and it doesn't have a disconnect between `hasFnAttribute()`
    and `getAttributes().hasAttribute(FunctionIndex)`.

Awkwardly, having written that out, I'm kind of leaning toward
it myself right now (maybe I'm fickle?) -- it seems to have
fewer limitations.  The main thing I prefer about my proposal
is that it's easier to change the default attributes when
modifying an assembly file by hand, but I suppose we could
write a tool for that?