[RISCV][GlobalISel] Add lowerReturn for calling conv

[nitinjohnraj]

Summary

Add minimal support to lower return, and introduce an OutgoingValueHandler and an OutgoingValueAssigner for returns.

Supports return values with integer, pointer and aggregate types.

(Update of D69808 - avoiding commandeering that revision)

[arsenm]

llvm/lib/CodeGen/TargetRegisterInfo.cpp

237–239 ↗

(On Diff #399993)

This needs its own patch and justification

llvm/lib/Target/RISCV/RISCVCallLowering.cpp

80–84 ↗

(On Diff #399993)

This looks like you're copying the broken form of splitToValueTypes from x86. I think trying to handle all of the argument marshalling in simple looking callbacks like this is unworkable. If you call the generic splitToValueTypes in CallLowering, it should try to handle everything for you

132–159 ↗

(On Diff #399993)

This is reinventing generic infrastructure

161 ↗

(On Diff #399993)

Ditto, this is a less complete copy of the generic infrastructure

203 ↗

(On Diff #399993)

Ditto

[lewis-revill]

Don't reinvent existing generic functions; instead introduce an OutgoingValueAssigner to allow us to call determineAndHandleAssignments.

[arsenm]

LGTM

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/ret.ll

145

Probably should have some vectors

[nitinjohnraj]

Rebased

[eopXD]

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp

83

This looks redundant.

llvm/lib/Target/RISCV/RISCVISelLowering.h

749

Can we avoid this change?

878

Why do we need to promote the static functions into the header here?

[nitinjohnraj]

Addressing comments

[nitinjohnraj]

This has been stagnant for a while, and there were a few open comments, so could I reaffirm the approval of this patch?

llvm/lib/Target/RISCV/RISCVISelLowering.h

749

This change is in order to use RISCVCCAssignFn in RISCVOutgoingValueAssigner above. We could maybe avoid that by inlining the type instead of changing the access specifier, but should we?

878

In order to use them on line 111 in llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp. Should we maybe move this outside the RISCV namespace?

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/ret.ll

145

If I understand correctly, this patch doesn't handle vectors.

[arsenm]

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/ret.ll

103–104

New tests must use opaque pointers

[nitinjohnraj]

Used opaque pointer in test

[nitinjohnraj]

Fixed test

[craig.topper]

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp

123

Drop curly braces

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.h

24

alphabetize the forward declarations

[nitinjohnraj]

Addressed comments

[nitinjohnraj]

Added lewis as co-author in commit message