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

Mon Mar 9 18:28:18 PDT 2015

> On Mar 5, 2015, at 1:01 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
>> 
>> On 2015 Feb 26, at 11:34, Eric Christopher <echristo at gmail.com> wrote:
>> 
>> 
>> 
>> 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.
>> 
> 
> I've done some more thinking about this.
> 
> 1. The module-level default attribute approach (what I started this
>    thread with) cleans up assembly files and provides a clean hook for
>    mutating the "defaults" from `llc`.  However, it doesn't cleanly fit
>    into the current semantic model for attributes, so you get weird
>    inconsistencies depending on how you query them.
> 2. Flipping a bit to indicate whether something was the "default"
>    solves the various technical problems.  However, overriding the
>    attributes requires walking all of the IR to rewrite them, and the
>    assembly syntax readability will get worse, not better.
> 3. Using a separate tool allows you to override the attributes
>    arbitrarily, but doesn't distinguish between "default" and explicit
>    attributes.  Furthermore, if the primary use for the tool is:
> 
>        llvm-attr-mutate -o - t.bc -attr target-cpu=proc123 | llc
> 
>    then I'm not sure we're gaining anything.  In particular, having
>    `llc` mutate the IR when you say:
> 
>        llc <t.bc -target-cpu=proc123
> 
>    is a far cleaner interface.  Not to say that a tool to mutate
>    attributes isn't a great idea -- I think it might be -- just that
>    it's not a great solution for *this* problem.
> 4. Having `llc` mutate the IR itself -- the obvious solution, which
>    Akira posted a patch for a few months ago -- does the job just as
>    well as `llvm-attr-mutate` but with a much cleaner interface.  It
>    fails to distinguish between target defaults and explicit
>    attributes, but when combined with `llvm-extract`, it gives you full
>    control over the codegen options for each function.
> 
> Remind me again why we don't just do #4?  It seems like the simplest way
> to keep `llc` viable in the short term.

llc should be able to override the default values for options without mutating anything that’s explicitly specified at a function (or module) level.

For example, if I have a .ll file with specialized functions with different CPUs specified, I want to be able to recompile that file with -mcpu= on the llc command line in such a way that those functions don’t get changed, but any functions that don’t have an explicit CPU on them do. Consider trying to test something like function multi versioning.

-Jim

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