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

[Eric Christopher]

On Tue, Feb 24, 2015 at 3:18 PM Sean Silva <chisophugis at gmail.com> wrote:

> On Tue, Feb 24, 2015 at 2:53 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
>
>>
>> > 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?
>>
>
> I think the tool approach deserves a bit of attention for the original
> usecase of trying different target attributes to see if they tickle a bug.
> Would it be feasible to have a purpose-built tool `llvm-attr-mutate`
> (bikeshed) so you can do `cat foo.bc | llvm-attr-mutate .... | llc`. The
> arguments to llvm-attr-mutate could then be made as convenient/powerful as
> needed for this debugging task.
>
>>
>>
FWIW the tool approach is what we were coming up with in the first place :)

Which would obviate the need for handling the command line options at all
of course.

-eric
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