[RISCV] Clear kill flags when forming FMA instructions in MachineCombiner.

[craig.topper]

Summary

If the operands to the mul have other uses we may be extending their
live range past a kill flag.

[wangpc]

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

1665

Wrong base line? only comments here.

[craig.topper]

Forgot to squash commits.

[asi-sc]

LGTM

[asb]

LGTM.

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

 //===-- RISCVInstrInfo.cpp - RISC-V Instruction Information -----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
static void combineFPFusedMultiply(MachineInstr &Root, MachineInstr &Prev,
   uint32_t IntersectedFlags = Root.getFlags() & Prev.getFlags();
   DebugLoc MergedLoc =
       DILocation::getMergedLocation(Root.getDebugLoc(), Prev.getDebugLoc());
 
   bool Mul1IsKill = Mul1.isKill();
   bool Mul2IsKill = Mul2.isKill();
   bool AddendIsKill = Addend.isKill();
 
+  // We need to clear kill flags since we may be extending the live range past
+  // a kill. If the mul had kill flags, we can preserve those since we know
+  // where the previous range stopped.
   MRI.clearKillFlags(Mul1.getReg());
   MRI.clearKillFlags(Mul2.getReg());
 
   MachineInstrBuilder MIB =
       BuildMI(*MF, MergedLoc, TII->get(FusedOpc), DstReg)
           .addReg(Mul1.getReg(), getKillRegState(Mul1IsKill))
           .addReg(Mul2.getReg(), getKillRegState(Mul2IsKill))
           .addReg(Addend.getReg(), getKillRegState(AddendIsKill))
           .setMIFlags(IntersectedFlags);

llvm/test/CodeGen/RISCV/machine-combiner.mir

 # NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 3
-# RUN: llc %s -mtriple=riscv64 -mattr=+f -run-pass=machine-combiner \
+# RUN: llc %s -o - -mtriple=riscv64 -mattr=+f -run-pass=machine-combiner \
 # RUN:   -verify-machineinstrs | FileCheck %s
 
 --- |
   source_filename = "test.ll"
   target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
   target triple = "riscv64"
 
   define void @foo(ptr %0, ptr %1, float %2, float %3, ptr %4, ptr %5) #0 {