[cfe-dev] [RFC] ASM Goto With Output Constraints

[Bill Wendling via cfe-dev]

On Fri, Jun 28, 2019 at 3:35 PM James Y Knight <jyknight at google.com> wrote:

> On Fri, Jun 28, 2019 at 5:53 PM Bill Wendling <isanbard at gmail.com> wrote:
>
>> On Fri, Jun 28, 2019 at 1:48 PM James Y Knight <jyknight at google.com>
>> wrote:
>>
>>> On Fri, Jun 28, 2019 at 3:00 PM Bill Wendling <isanbard at gmail.com>
>>> wrote:
>>>
>>>> On Thu, Jun 27, 2019 at 1:44 PM Bill Wendling <isanbard at gmail.com>
>>>> wrote:
>>>>
>>>>> On Thu, Jun 27, 2019 at 1:29 PM James Y Knight <jyknight at google.com>
>>>>> wrote:
>>>>>
>>>>>> I think this is fine, except that it stops at the point where things
>>>>>> actually start to get interesting and tricky.
>>>>>>
>>>>>> How will you actually handle the flow of values from the callbr into
>>>>>> the error blocks? A callbr can specify requirements on where its outputs
>>>>>> live. So, what if two callbr, in different branches of code, specify
>>>>>> _different_ constraints for the same output, and list the same block as a
>>>>>> possible error successor? How can the resulting phi be codegened?
>>>>>>
>>>>>> This is where I fall back on the statement about how "the programmer
>>>>> knows what they're doing". Perhaps I'm being too cavalier here? My concern,
>>>>> if you want to call it that, is that we don't be too restrictive on the new
>>>>> behavior. For example, the "asm goto" may set a register to an error value
>>>>> (made up on the spot; may not be a common use). But, if there's no real
>>>>> reason to have the value be valid on the abnormal path, then sure we can
>>>>> declare that it's not valid on the abnormal path.
>>>>>
>>>>> I think I should explain my "programmer knows what they're doing"
>>>> statement a bit better. I'm specifically referring to inline asm here. The
>>>> more general "callbr" case may still need to be considered (see Reid's
>>>> reply).
>>>>
>>>> When a programmer uses inline asm, they're implicitly telling the
>>>> compiler that they *do* know what they're doing  (I know this is common
>>>> knowledge, but I wanted to reiterate it.). In particular, either they need
>>>> to reference an instruction not readily available from the compiler (e.g.
>>>> "cpuid") or the compiler isn't able to give them the needed performance in
>>>> a critical section. I'm extending this sentiment to callbr with output
>>>> constraints. Let's take your example below and write it as "normal" asm
>>>> statements one on each branch of an if-then-else (please ignore any syntax
>>>> errors):
>>>>
>>>> if:
>>>>   br i1 %cmp, label %true, label %false
>>>>
>>>> true:
>>>>   %0 = call { i32, i32 } asm sideeffect "poetry $0, $1", "={r8},={r9}"
>>>> ()
>>>>   br label %end
>>>>
>>>> false:
>>>>   %1 = call { i32, i32 } asm sideeffect "poetry2 $0, $1",
>>>> "={r10},={r11}" ()
>>>>   br label %end
>>>>
>>>> end:
>>>>   %vals = phi { i32, i32 } [ %0, %true ], [ %1, %false ]
>>>>
>>>> How is this handled in codegen? Is it an error or does the back-end
>>>> handle it? Whatever's done today for "normal" inline asm is what I *think*
>>>> should be the behavior for the inline asm callbr variant. If this doesn't
>>>> seem sensible (and I realize that I may be thinking of an "in a perfect
>>>> world" scenario), then we'll need to come up with a more sensible solution
>>>> which may be to disallow the values on the error block until we can think
>>>> of a better way to handle them.
>>>>
>>>
>>> This example is no problem, because instructions can be emitted between
>>> what's emitted by "call asm" and the end of the block (be it a fallthrough,
>>> or a jump instruction. What gets emitted there is a move of the output
>>> register to another location -- either a register or to the stack. And
>>> therefore at the beginning of the "end" block, "%vals" is always in a
>>> consistent location, no matter how you got to that block.
>>>
>>> But in the callbr case, there is not a location at which those moves can
>>> be emitted, after the callbr, before the jump to "error".
>>>
>>
>> I see what you mean. Let's say we create a pseudo-instruction (similar to
>> landingpad, et al) that needs to be lowered by the backend in a reasonable
>> manner. The EH stuff has an external process/library that performs the
>> actual unwinding and which sets the values accordingly. We won't have this.
>>
>
>
>
>> What we could do instead is split the edges and insert the copy-to-<where
>> ever> statements there.
>>
>
> Exactly -- except that doing that is potentially an invalid transform,
> because the address is being used as a value, not simply a jump target. The
> label list is just a list of _possible_ jump targets, changing those won't
> actually affect anything. You'd instead need to change the blockaddress
> constant, but in the general case you don't know where that address came
> from -- (and it may therefore be required that you have the same address
> for two separate callbr instructions).
>
> I guess this kinda touches on some of the same issues as in the other
> discussion about the handling of the blockaddress in callbr and inlining,
> etc...
>
> I wonder if we could put some validity restrictions on the IR structure,
> rather than trying to fix things up after the fact by attempting to split
> blocks. E.g., we could state that it's invalid to have a phi which uses the
> value defined by a callbr, if it's conditioned on that same block as
> predecessor.  That is: it's valid to use _other_ values defined in the
> block ending in callbr, because they can be moved prior to the callbr. It's
> also valid to use the value defined by the callbr in a phi conditioned on
> some other intermediate block as predecessor, because then any required
> moves can happen in the intermediate block.
>
> I believe such an IR restriction should be sufficient to make it possible
> to emit valid code from the IR in all cases, but I'm a bit afraid of how
> badly adding such odd edge-cases might screw up the rest of the compiler
> and optimizer.
>

I want to revisit this. Here are the situations we're confronted with:


   1. The goto-target can be jumped to by 1 callbr instruction,
   2. The goto-target can be jumped to by N callbr instructions, which
   don't need a PHI node, and
   3. The goto-target can be jumped to by N callbr instructions, which *do*
   need a PHI node.

I'm going to plug the instruction I created out of thin air a few emails
back, but better explain (I'm using an instruction instead of an intrinsic
because we want that instruction to be right after all non-PHI instructions
in the goto-target block). I'm _not_ suggesting we need this instruction.
It's just for demonstration purposes.


Situations (1) and (2) don't encounter an problem. Any value used in the
goto-target can be handled by inserting the code to extract that value in
the goto-target block:


bb1:

  ...

  %x.bb1 = callbr i32 asm sideeffect "...", "=r,X"(i32 *%x*, i8*
blockaddress(@bar, %goto.target))

          to label %fallthrough1 [label %goto.target]


fallthrough1:

  ...


bb2:

  ...

  %y.bb2 = callbr i32 asm sideeffect "...", "=r,X"(i32 *%y*, i8*
blockaddress(@bar, %goto.target))

          to label %fallthrough2 [label %goto.target]


fallthrough2:

  ...


goto.target:

  %x.goto = <extract value from %x.bb1>

  %y.goto = <extract value from %y.bb2>

  ... <uses of %x.goto and %y.goto> ...


This leaves situation (3), which is far more complex as we've seen. To
reiterate, the issue here is that we need to extract the values returned by
callbr. This would typically be done by using a PHI node, but llvm may want
to split critical edges or push the calculation back to the predecessor
block, which won't work with the callbr asm, because it could branch out of
the asm at any point thus skipping the extraction. So we can't use PHI
nodes for these values. There are three classes of solutions to this:


   1. Don't allow the values to be used in goto-targets, or
   2. Allow them, but with significant restrictions, or
   3. Allow them without using PHI nodes.

Each has its benefits and drawbacks. As I've stated before, I think that
(1) is too restrictive, but if we can't come up with a good solution, it
may be our only option. Solution (2) could be a good compromise. However, I
want to propose a potential solution to (3).

The core of my proposal is to replace the PHI node with code that will
replicate its behavior without code lowering trying to modify the CFG (at
least not in ways that may invalidate the asm). Here is example code:


bb1:

  store i8* blockaddress(@bar, %bb1), i8** %src

  %x.bb1 = callbr i32 asm sideeffect "...", "=r,X"(i32 %x, i8*
blockaddress(@bar, %goto.target))

          to label %fallthrough1 [label %goto.target]


fallthrough1:

  ...


bb2:

  store i8* blockaddress(@bar, %bb2), i8** %src

  %x.bb2 = callbr i32 asm sideeffect "...", "=r,X"(i32 %x, i8*
blockaddress(@bar, %goto.target))

          to label %fallthrough2 [label %goto.target]


fallthrough2:

  ...


goto.target:

  %x1 = indirectval i8** %src, i32 [%x.bb1, %bb1], [%x.bb2, %bb2]

  <extract values from %x1>

  ...


This can be lowered to this:

bb1:

  store i8* blockaddress(@bar, %bb1), i8** %src

  %x.bb1 = callbr i32 asm sideeffect "...", "=r,X"(i32 %x, i8*
blockaddress(@bar, %error))

          to label %fallthrough1 [label %goto.target]


fallthrough1:

  ...


bb2:

  store i8* blockaddress(@bar, %bb2), i8** %src

  %x.bb2 = callbr i32 asm sideeffect "...", "=r,X"(i32 %x, i8*
blockaddress(@bar, %error))

          to label %fallthrough2 [label %goto.target]


fallthrough2:

  ...


goto.target:

  %src1 = load i8**, i8*** @src

  %src.bb = load i8*, i8** %src1

  switch i64 %src.bb, label %goto.target.body [ ; or if-then-else blocks

      i64 ptrtoint i8* blockaddress(@bar, %bb1) to i64, label
%goto.target.bb1

      i64 ptrtoint i8* blockaddress(@bar, %bb2) to i64, label
%goto.target.bb2

  ]


goto.target.bb1:

  %x1 = <extract value from %x.bb1>

  br label %goto.target.body


goto.target.bb2:

  %x2 = <extract value from %x.bb2>

  br label %goto.target.body


goto.target.body:

  %x.merge = phi i64 [%x1, label %goto.target.bb1], [%x1, label
%goto.target.bb2]

  ...


With this, we don't change any values used by the callbr instructions, and
the return values are extracted correctly. This has the unsavory issue of
using stores and loads, but this may be the price we need to pay.


Thoughts?


-bw
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