[llvm-dev] RFC: [GlobalISel] propagating int/float type information
Amara Emerson via llvm-dev
llvm-dev at lists.llvm.org
Fri May 1 10:12:55 PDT 2020
GlobalISel currently drops all type information relating to the integer/FP distinction during the IR translation pass, as the LLT types only represent whether a value is a scalar/vector/pointer and it’s size/shape. To compensate, later passes use the FP operations on those values to guess what kind of value is being stored within that virtual register.
This means that i32/float loads get translated into the same thing, and only when that value is used, say by an fadd, then will we know that it was an FP value. The regbankselect pass on AArch64 currently tries to walk uses/defs in order to guess what kind fo regbank to assign to vregs. This however doesn’t work all the time, and most commonly, it doesn’t work when a load of an FP value is used in a loop. In that case, the FP users are obscured by PHIs which make it difficult (although not strictly impossible) to guess what regbank to assign. This has drastic consequences for performance on FP workloads.
But this isn’t the first time we’ve had this kind of issue, and it probably won’t be the last . propose that we have some form of type hint propagation done at the IRTranslator stage in order to make this whole situation easier (and faster in compile-time).
Option 1) We use some form of metadata on the MIR instructions like G_LOADs to signify that the vreg defined likely has an FP IR type. IIUC the current Metadata MachineOperand type is only intended for debug info. This approach is probably the cheapest in compile time/complexity and is the least invasive, but we’d need to find somewhere in MachineInstr to store this extra information.
Option 2) Store the type hints in an analysis. In its simplest form at translation time we could keep a set of all the vregs that we know have FP types and then try to maintain that as new vregs are created to replace those throughout the pipeline. Keeping it updated might turn out to be expensive during passes like the legalizer.
 Currently we have a workaround for the specific case in https://reviews.llvm.org/D79207 <https://reviews.llvm.org/D79207>, but as Matt correctly points out, this isn’t viable in the long term because using the IR value type from the MachineMemOperand won’t work when opaque pointers finally land.
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