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

Kaylor, Andrew via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 14 11:39:37 PST 2019

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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