[PATCH] D131087: [RISCV] Implement isUsedByReturnOnly TargetLowering hook in order to tailcall more libcalls

[Alex Bradbury via Phabricator via llvm-commits]

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Prior to this patch, libcalls inserted by the SelectionDAG legalizer
could never be tailcalled. The eligibility of libcalls for tail calling
is is partly determined by checking TargetLowering::isInTailCallPosition
and comparing the return type of the libcall and the calleer.
isInTailCallPosition in turn calls TargetLowering::isUsedByReturnOnly
(which always returns false if not implemented by the target).

This patch provides a minimal implementation of
TargetLowering::isUsedByReturnOnly - enough to support tail calling
libcalls on hard float ABIs. Soft-float ABIs are left for a follow on
patch. libcall-tail-calls.ll also shows missed opportunities to tail
call integer libcalls, but this is due to issues outside of
the isUsedByReturnOnly hook.

The isUsedByReturnOnly hook is essentially the same as for X86/Arm/AArch64. I've tried to clean it up a bit, but please do check I haven't copied across any bad practices / outdated API usage.


https://reviews.llvm.org/D131087

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.h
  llvm/test/CodeGen/RISCV/div.ll
  llvm/test/CodeGen/RISCV/double-frem.ll
  llvm/test/CodeGen/RISCV/double-intrinsics.ll
  llvm/test/CodeGen/RISCV/double-round-conv.ll
  llvm/test/CodeGen/RISCV/float-intrinsics.ll
  llvm/test/CodeGen/RISCV/float-round-conv.ll
  llvm/test/CodeGen/RISCV/fp16-promote.ll
  llvm/test/CodeGen/RISCV/libcall-tail-calls.ll
  llvm/test/CodeGen/RISCV/mul.ll
  llvm/test/CodeGen/RISCV/rem.ll
  llvm/test/CodeGen/RISCV/srem-lkk.ll
  llvm/test/CodeGen/RISCV/urem-lkk.ll

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