[llvm-dev] RFC: token arguments and operand bundles

Kaylor, Andrew via llvm-dev llvm-dev at lists.llvm.org
Fri Nov 15 12:16:57 PST 2019

We really have been trying to keep in mind that LLVM needs to support multiple front ends, which may be implementing different language standards. As much as possible, I’ve been trying to let the IEEE 754 spec drive my thinking about this, though I’ll admit that on a few points I’ve use the C99 spec as a sort of reference interpretation of IEEE 754.

LLVM’s IRBuilder has been recently updated to provide an abstraction layer between front ends and the optimizer. So, if you’re using IRBuilder, you set need to call setIsFPConstrained() then, optionally, IRBuilder::setDefaultConstraiedExcept() and/or setDefaultConstrainedRounding(). After that, calls to something like IRBuilder::CreateFAdd() will automatically create the constrained intrinsic with the appropriate constraints, regardless of how we end up representing them. If your front end isn’t using IRBuilder, I will admit it gets a bit more complicated.

I wouldn’t be opposed to a solution that involved a custom printer for these arguments, but I don’t think it really adds anything that we wouldn’t get from using tokens as I have proposed. Likewise with the named constant idea. On the other hand, if I’m misusing tokens then maybe what constants would add is a way to avoid that misuse.

Regarding the question of what is exposed to users and how, that’s mostly up to the front end. I would like to clarify how we intend for this to work, in general. Simon touched on this briefly, but I’d like to be a bit more verbose to make sure we’re all on the same page.

There are effectively two distinct modes of source code translation to IR with respect to floating point operations -- one where the user is allowed to modify the floating point environment and one where they are not. This may not have been clear to everyone, but by default LLVM IR carries with it the assumption that the runtime rounding mode is “to nearest” and that floating point operations do not have side effects. This was only documented recently, but this is the way the optimizer has always behaved. In this default mode, the IR *shouldn’t* change the floating point environment. I would encourage front ends to document this more specifically saying that the user is not permitted to change the FP environment.

This leads to the necessity of a second state in which the optimizer does not assume the default rounding mode and does not assume that floating point operations have no side effects. Proscribing these assumptions limits optimization, so we want to continue allowing the assumptions by default. The state where the assumptions are not made is accomplished through the use of constrained intrinsics. However, we do not wish to completely eliminate optimizations in all cases, so we want a way to communicate to the optimizer what it can assume. That is the purpose of the fpround and fpexcept arguments. These are not intended to control the rounding mode or exception reporting. They only tell the compiler what it can assume.

Understanding this, front ends can control these in any way they see fit. For instance, the front end might have a global setting the changes the rounding mode to “toward zero.” In that case, it would create constrained intrinsics for all FP operations and set the rounding mode argument (however we end up representing in) to rmTowardZero (a constant currently defined by LLVM corresponding to the “fpround.towardzero” metadata argument).  Then the optimizer can use this information to perform optimizations like constant folding.

Runtime changes to the rounding mode are a separate matter. As I said above, I think front ends should define clear circumstances under which such changes are permitted, but the mechanism for making such changes is independent of the constrained FP intrinsics. For example, consider the following C function.

double foo(double A, double B, double C) {
  int OrigRM = fegetround();
  double tmp = A + B;
  return tmp + C;

Assuming the compiler was in a state where it knew fenv access was enabled, I would expect that to get translated to something like this (after SROA cleanup):

define double @foo(double %A, double %B, double %C) {
  %orig.rm = call i32 @fegetround()
  %ignored = call i32 @fesetround(i32 3072)
  %tmp = call double @llvm.experimental.constrained.fadd(double %A, double %B) [ “fpround”(token rmDynamic), “fpexcept”(token rmStrict) ]
  %ignored = call i32 @fesetround(i32 %orig.rm)
  %result = call double @llvm.experimental.constrained.fadd(double %tmp, double %C) [ “fpround”(token rmDynamic), “fpexcept”(token rmStrict) ]

Notice here the literal constant that C defines is still used for the call to fesetround(FE_TOWARDZERO), and the variable is used for the call that restores the rounding mode. Also notice that in both fadd operations, the rounding mode is declared as rmDynamic. I have an idea that we ought to have a pass that recognizes the fesetround library call an uses the information it finds there to change the rounding mode operand in the first fadd to rmTowardZero, but the front end won’t be expected to do that. We’ll probably want an intrinsic to change the rounding mode so that we don’t need to recognize all manner of language-specific libcalls, but that’s a problem for later.

I hope this has been more helpful than tedious. Also, I feel like I should reiterate that I am still seeking all opinions about the use of tokens and operand bundles or any other means of representing the fp constraints. I just want to make sure that we all have the same understanding of what the information I’m trying to represent in IR means.


From: Jameson Nash <vtjnash at gmail.com>
Sent: Thursday, November 14, 2019 8:00 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] RFC: token arguments and operand bundles

I understand that, but I think you missed my point. Not all front-ends are clang, and non-C frontends are also interested in this work. And even C might want to eventually want to be able to use these more generally. For example, the current C standard (afaik) doesn’t define what must happen if this pragma tried to use a non-literal constant, such as a template attribute as the arguments. But it’s not obvious to me why LLVM should inherit that limitation. Currently it seems to be implemented in a way that requires special handling in any front-end, influenced strong by the special handling it’s now getting in C. For other languages, it’s doable to expose this to users regardless, but if you’re already considering changing it, my vote would be to use a normal representation with first-class values.

However, I  really appreciate the specifics on the concern you brought up, because that’s a good point. If it’s just about better IR printing, perhaps we can just address that directly?

Most simply, perhaps these calls could customize the printing to append a comment? Some places already do that, for example to show Function Attributes.

Similarly, but more major, LLVM could perhaps define a new “named constant” syntax for the parser format (either with special tokens like your current PR and/or that get defined elsewhere like existing global constants). Certain instructions (such as these) could then use the option to customize the printing of their arguments to use the named constant (after parsing, they’d just be a normal Constant—only printing would optionally use them to show the information better to the reader).

Thu, Nov 14, 2019 at 15:58 Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
Let me clarify. These aren’t intended to be exposed to the user. The user code that leads to the generation of these intrinsics will be normal floating point operations combined with either pragmas (such as “STDC FENV_ACCESS ON”) or command line options (such as the recently introduced “-fp-model=strict”).
The reason I’ve been avoiding normal constant values is that it provides no information when you’re reading the IR. For example:

%sum = call double @llvm.experimental.constrained.fadd(double %x, double %y, i32 1, i32 2)
What does that mean? You’d need to consult an external reference to have any idea.


From: Jameson Nash <vtjnash at gmail.com<mailto:vtjnash at gmail.com>>
Sent: Thursday, November 14, 2019 12:27 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>
Cc: LLVM Developers Mailing List <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] RFC: token arguments and operand bundles

From a front-end perspective, I think it'd be preferable if these either got encoded in the function name or were normal enum value arguments. It's a bit awkward to expose things to the user that must be constant or of a special type or in a special metadata slot, since we now need more special support for it. If the optimization passes couldn't identify a constant value for one of the arguments, these seem like they can fallback to assuming the most conservative semantics (of round.dynamic and fpexcept.strict--e.g. don't optimize) without loss of precision or generality.


On Thu, Nov 14, 2019 at 2:40 PM Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello everyone,

I’ve just uploaded a patch (https://reviews.llvm.org/D70261) to introduce a could of new token types to be used with constrained floating point intrinsics and, optionally, vector predicated intrinsics. These intrinsics may not be of interest to many of you, but I have a more general question.

I would like some general feedback on the way I am proposing to use token arguments and operand bundles. I have an incomplete understanding of how these are intended to be used, and I want to make sure what I have in mind is consistent with the philosophy behind them.

Currently, the constrained floating point intrinsics require string metadata arguments to describe the rounding mode and exception semantics. These “arguments” are really providing information to the optimizer about what it can and cannot assume when acting on these intrinsics. The rounding mode argument potentially overrides the default optimizer assumption that the “to nearest” rounding mode is in use, and the exception behavior argument overrides the default optimizer assumption that floating point operations have no side effects. I’ve never liked the use of strings here, and the fact that these arguments are not actually inputs to the operation represented by the intrinsic seems vaguely wrong.

A typical call to a current intrinsic looks like this:

%sum = call double @llvm.experimental.constrained.fadd(double %x,
                                                       double %y,
                                                       Metadata “fpround.dynamic”,
                                                       Metadata “fpexcept.strict”)

The idea I am pursuing in my patch is to replace these metadata arguments with optional operand bundles, “fpround” and “fpexcept”. If the operand bundles are present, they would mean what the arguments currently mean. If not, the default assumption is allowed. A typical call to a constrained intrinsic would look like this:

%sum = call double @llvm.experimental2.constrained.fadd(double %x,
                                                        double %y) [ “fpround”(token rmDynamic),
                                                                     “fpexcept”(token ebStrict) ]

Does that seem like a valid use of tokens and operand bundles? Does it seem better than the current approach?

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