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llvm-project/llvm/lib/Target/MSP430
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Anton Korobeynikov 064dbac212 `MSP430InstrInfo::loadRegFromStackSlot` forgets to set register def. 
Summary:
For instance, compiling the below results in a panic:

```
llc: ../lib/CodeGen/InlineSpiller.cpp:1140: bool (anonymous namespace)::InlineSpiller::foldMemoryOperand(ArrayRef<std::pair<MachineInstr *, unsigned int> >, llvm::MachineInstr *): Assertion `MO->isDead() && "Cannot fold physreg def"' failed.
#0 0x00007f50fbcf353e llvm::sys::PrintStackTrace(llvm::raw_ostream&) /home/h/3rd/llvm/build/../lib/Support/Unix/Signals.inc:321:15
#1 0x00007f50fbcf3929 PrintStackTraceSignalHandler(void*) /home/h/3rd/llvm/build/../lib/Support/Unix/Signals.inc:380:1
#2 0x00007f50fbcf22a3 llvm::sys::RunSignalHandlers() /home/h/3rd/llvm/build/../lib/Support/Signals.cpp:45:5
#3 0x00007f50fbcf3bb4 SignalHandler(int) /home/h/3rd/llvm/build/../lib/Support/Unix/Signals.inc:210:1
#4 0x00007f50fa87a180 (/lib/x86_64-linux-gnu/libc.so.6+0x35180)
#5 0x00007f50fa87a107 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x35107)
#6 0x00007f50fa87b4e8 abort (/lib/x86_64-linux-gnu/libc.so.6+0x364e8)
#7 0x00007f50fa873226 (/lib/x86_64-linux-gnu/libc.so.6+0x2e226)
#8 0x00007f50fa8732d2 (/lib/x86_64-linux-gnu/libc.so.6+0x2e2d2)
#9 0x00007f50fddd9287 (anonymous namespace)::InlineSpiller::foldMemoryOperand(llvm::ArrayRef<std::pair<llvm::MachineInstr*, unsigned int> >, llvm::MachineInstr*) /home/h/3rd/llvm/build/../lib/CodeGen/InlineSpiller.cpp:1141:21
#10 0x00007f50fddd9ee9 (anonymous namespace)::InlineSpiller::spillAroundUses(unsigned int) /home/h/3rd/llvm/build/../lib/CodeGen/InlineSpiller.cpp:1286:9
#11 0x00007f50fddd388b (anonymous namespace)::InlineSpiller::spillAll() /home/h/3rd/llvm/build/../lib/CodeGen/InlineSpiller.cpp:1338:21
#12 0x00007f50fddd221d (anonymous namespace)::InlineSpiller::spill(llvm::LiveRangeEdit&) /home/h/3rd/llvm/build/../lib/CodeGen/InlineSpiller.cpp:1391:3
#13 0x00007f50fdfd921b (anonymous namespace)::RAGreedy::selectOrSplitImpl(llvm::LiveInterval&, llvm::SmallVectorImpl<unsigned int>&, llvm::SmallSet<unsigned int, 16u, std::less<unsigned int> >&, unsigned int) /home/h/3rd/llvm/build/../lib/CodeGen/RegAllocGreedy.cpp:2555:5
#14 0x00007f50fdfd647b (anonymous namespace)::RAGreedy::selectOrSplit(llvm::LiveInterval&, llvm::SmallVectorImpl<unsigned int>&) /home/h/3rd/llvm/build/../lib/CodeGen/RegAllocGreedy.cpp:2221:12
#15 0x00007f50fdfc89f9 llvm::RegAllocBase::allocatePhysRegs() /home/h/3rd/llvm/build/../lib/CodeGen/RegAllocBase.cpp:110:14
#16 0x00007f50fdfd6337 (anonymous namespace)::RAGreedy::runOnMachineFunction(llvm::MachineFunction&) /home/h/3rd/llvm/build/../lib/CodeGen/RegAllocGreedy.cpp:2611:3
#17 0x00007f50fded33ee llvm::MachineFunctionPass::runOnFunction(llvm::Function&) /home/h/3rd/llvm/build/../lib/CodeGen/MachineFunctionPass.cpp:43:3
#18 0x00007f50fd6cdc6f llvm::FPPassManager::runOnFunction(llvm::Function&) /home/h/3rd/llvm/build/../lib/IR/LegacyPassManager.cpp:1550:23
#19 0x00007f50fd6cdf85 llvm::FPPassManager::runOnModule(llvm::Module&) /home/h/3rd/llvm/build/../lib/IR/LegacyPassManager.cpp:1571:16
#20 0x00007f50fd6ce71a (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&) /home/h/3rd/llvm/build/../lib/IR/LegacyPassManager.cpp:1627:23
#21 0x00007f50fd6ce246 llvm::legacy::PassManagerImpl::run(llvm::Module&) /home/h/3rd/llvm/build/../lib/IR/LegacyPassManager.cpp:1730:16
#22 0x00007f50fd6cec31 llvm::legacy::PassManager::run(llvm::Module&) /home/h/3rd/llvm/build/../lib/IR/LegacyPassManager.cpp:1761:3
#23 0x0000000000415bdc compileModule(char**, llvm::LLVMContext&) /home/h/3rd/llvm/build/../tools/llc/llc.cpp:405:5
#24 0x0000000000414571 main /home/h/3rd/llvm/build/../tools/llc/llc.cpp:211:13
#25 0x00007f50fa866b45 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b45)
#26 0x0000000000414296 _start (/home/h/3rd/llvm/build/bin/llc+0x414296)
Stack dump:
0.	Program arguments: ./bin/llc -mtriple msp430 loadstore.ll 
1.	Running pass 'Function Pass Manager' on module 'loadstore.ll'.
2.	Running pass 'Greedy Register Allocator' on function '@inc'
```

Original IR:

```llvm
%struct.VeryLarge = type { i8, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32, i32 }

; Function Attrs: norecurse nounwind
define void @inc(%struct.VeryLarge* noalias nocapture sret %agg.result, %struct.VeryLarge* byval align 1 %s) #0 {
entry:
  %p0 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 0
  %0 = load i8, i8* %p0, align 1, !tbaa !1
  %p1 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 1
  %1 = load i32, i32* %p1, align 1, !tbaa !6
  %p2 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 2
  %2 = load i32, i32* %p2, align 1, !tbaa !7
  %p3 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 3
  %3 = load i32, i32* %p3, align 1, !tbaa !8
  %p4 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 4
  %4 = load i32, i32* %p4, align 1, !tbaa !9
  %p5 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 5
  %5 = load i32, i32* %p5, align 1, !tbaa !10
  %p6 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 6
  %6 = load i32, i32* %p6, align 1, !tbaa !11
  %p7 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 7
  %7 = load i32, i32* %p7, align 1, !tbaa !12
  %p8 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 8
  %8 = load i32, i32* %p8, align 1, !tbaa !13
  %p9 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 9
  %9 = load i32, i32* %p9, align 1, !tbaa !14
  %p10 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 10
  %10 = load i32, i32* %p10, align 1, !tbaa !15
  %p11 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 11
  %11 = load i32, i32* %p11, align 1, !tbaa !16
  %p12 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 12
  %12 = load i32, i32* %p12, align 1, !tbaa !17
  %p13 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 13
  %13 = load i32, i32* %p13, align 1, !tbaa !18
  %p14 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 14
  %14 = load i32, i32* %p14, align 1, !tbaa !19
  %p15 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 15
  %15 = load i32, i32* %p15, align 1, !tbaa !20
  %p16 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 16
  %16 = load i32, i32* %p16, align 1, !tbaa !21
  %p17 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 17
  %17 = load i32, i32* %p17, align 1, !tbaa !22
  %p18 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 18
  %18 = load i32, i32* %p18, align 1, !tbaa !23
  %p19 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 19
  %19 = load i32, i32* %p19, align 1, !tbaa !24
  %p20 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 20
  %20 = load i32, i32* %p20, align 1, !tbaa !25
  %p21 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 21
  %21 = load i32, i32* %p21, align 1, !tbaa !26
  %p22 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 22
  %22 = load i32, i32* %p22, align 1, !tbaa !27
  %p23 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 23
  %23 = load i32, i32* %p23, align 1, !tbaa !28
  %p24 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 24
  %24 = load i32, i32* %p24, align 1, !tbaa !29
  %p25 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 25
  %25 = load i32, i32* %p25, align 1, !tbaa !30
  %p26 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 26
  %26 = load i32, i32* %p26, align 1, !tbaa !31
  %p27 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 27
  %27 = load i32, i32* %p27, align 1, !tbaa !32
  %p28 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 28
  %28 = load i32, i32* %p28, align 1, !tbaa !33
  %p29 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 29
  %29 = load i32, i32* %p29, align 1, !tbaa !34
  %p30 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 30
  %30 = load i32, i32* %p30, align 1, !tbaa !35
  %p31 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 31
  %31 = load i32, i32* %p31, align 1, !tbaa !36
  %p32 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %s, i32 0, i32 32
  %32 = load i32, i32* %p32, align 1, !tbaa !37
  %add = add i8 %0, 1
  store i8 %add, i8* %p0, align 1, !tbaa !1
  %add2 = add i32 %1, 2
  store i32 %add2, i32* %p1, align 1, !tbaa !6
  %add3 = add i32 %2, 3
  store i32 %add3, i32* %p2, align 1, !tbaa !7
  %add4 = add i32 %3, 4
  store i32 %add4, i32* %p3, align 1, !tbaa !8
  %add5 = add i32 %4, 5
  store i32 %add5, i32* %p4, align 1, !tbaa !9
  %add6 = add i32 %5, 6
  store i32 %add6, i32* %p5, align 1, !tbaa !10
  %add7 = add i32 %6, 7
  store i32 %add7, i32* %p6, align 1, !tbaa !11
  %add8 = add i32 %7, 8
  store i32 %add8, i32* %p7, align 1, !tbaa !12
  %add9 = add i32 %8, 9
  store i32 %add9, i32* %p8, align 1, !tbaa !13
  %add10 = add i32 %9, 10
  store i32 %add10, i32* %p9, align 1, !tbaa !14
  %add11 = add i32 %10, 11
  store i32 %add11, i32* %p10, align 1, !tbaa !15
  %add12 = add i32 %11, 12
  store i32 %add12, i32* %p11, align 1, !tbaa !16
  %add13 = add i32 %12, 13
  store i32 %add13, i32* %p12, align 1, !tbaa !17
  %add14 = add i32 %13, 14
  store i32 %add14, i32* %p13, align 1, !tbaa !18
  %add15 = add i32 %14, 15
  store i32 %add15, i32* %p14, align 1, !tbaa !19
  %add16 = add i32 %15, 16
  store i32 %add16, i32* %p15, align 1, !tbaa !20
  %add17 = add i32 %16, 17
  store i32 %add17, i32* %p16, align 1, !tbaa !21
  %add18 = add i32 %17, 18
  store i32 %add18, i32* %p17, align 1, !tbaa !22
  %add19 = add i32 %18, 19
  store i32 %add19, i32* %p18, align 1, !tbaa !23
  %add20 = add i32 %19, 20
  store i32 %add20, i32* %p19, align 1, !tbaa !24
  %add21 = add i32 %20, 21
  store i32 %add21, i32* %p20, align 1, !tbaa !25
  %add22 = add i32 %21, 22
  store i32 %add22, i32* %p21, align 1, !tbaa !26
  %add23 = add i32 %22, 23
  store i32 %add23, i32* %p22, align 1, !tbaa !27
  %add24 = add i32 %23, 24
  store i32 %add24, i32* %p23, align 1, !tbaa !28
  %add25 = add i32 %24, 25
  store i32 %add25, i32* %p24, align 1, !tbaa !29
  %add26 = add i32 %25, 26
  store i32 %add26, i32* %p25, align 1, !tbaa !30
  %add27 = add i32 %26, 27
  store i32 %add27, i32* %p26, align 1, !tbaa !31
  %add28 = add i32 %27, 28
  store i32 %add28, i32* %p27, align 1, !tbaa !32
  %add29 = add i32 %28, 29
  store i32 %add29, i32* %p28, align 1, !tbaa !33
  %add30 = add i32 %29, 30
  store i32 %add30, i32* %p29, align 1, !tbaa !34
  %add31 = add i32 %30, 31
  store i32 %add31, i32* %p30, align 1, !tbaa !35
  %add32 = add i32 %31, 32
  store i32 %add32, i32* %p31, align 1, !tbaa !36
  %add33 = add i32 %32, 33
  store i32 %add33, i32* %p32, align 1, !tbaa !37
  %33 = getelementptr inbounds %struct.VeryLarge, %struct.VeryLarge* %agg.result, i32 0, i32 0
  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %33, i8* %p0, i32 129, i32 1, i1 false), !tbaa.struct !38
  ret void
}

; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i32, i1) #1

attributes #0 = { norecurse nounwind "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { argmemonly nounwind }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.8.0 (git://github.com/llvm-mirror/clang 40ef2b7531472c41212c4719a9294aeb7bddebbc) (git://github.com/llvm-mirror/llvm c601eaf55606dfb9ad372b514b77aa00d1409be1)"}
!1 = !{!2, !3, i64 0}
!2 = !{!"", !3, i64 0, !5, i64 1, !5, i64 5, !5, i64 9, !5, i64 13, !5, i64 17, !5, i64 21, !5, i64 25, !5, i64 29, !5, i64 33, !5, i64 37, !5, i64 41, !5, i64 45, !5, i64 49, !5, i64 53, !5, i64 57, !5, i64 61, !5, i64 65, !5, i64 69, !5, i64 73, !5, i64 77, !5, i64 81, !5, i64 85, !5, i64 89, !5, i64 93, !5, i64 97, !5, i64 101, !5, i64 105, !5, i64 109, !5, i64 113, !5, i64 117, !5, i64 121, !5, i64 125}
!3 = !{!"omnipotent char", !4, i64 0}
!4 = !{!"Simple C/C++ TBAA"}
!5 = !{!"int", !3, i64 0}
!6 = !{!2, !5, i64 1}
!7 = !{!2, !5, i64 5}
!8 = !{!2, !5, i64 9}
!9 = !{!2, !5, i64 13}
!10 = !{!2, !5, i64 17}
!11 = !{!2, !5, i64 21}
!12 = !{!2, !5, i64 25}
!13 = !{!2, !5, i64 29}
!14 = !{!2, !5, i64 33}
!15 = !{!2, !5, i64 37}
!16 = !{!2, !5, i64 41}
!17 = !{!2, !5, i64 45}
!18 = !{!2, !5, i64 49}
!19 = !{!2, !5, i64 53}
!20 = !{!2, !5, i64 57}
!21 = !{!2, !5, i64 61}
!22 = !{!2, !5, i64 65}
!23 = !{!2, !5, i64 69}
!24 = !{!2, !5, i64 73}
!25 = !{!2, !5, i64 77}
!26 = !{!2, !5, i64 81}
!27 = !{!2, !5, i64 85}
!28 = !{!2, !5, i64 89}
!29 = !{!2, !5, i64 93}
!30 = !{!2, !5, i64 97}
!31 = !{!2, !5, i64 101}
!32 = !{!2, !5, i64 105}
!33 = !{!2, !5, i64 109}
!34 = !{!2, !5, i64 113}
!35 = !{!2, !5, i64 117}
!36 = !{!2, !5, i64 121}
!37 = !{!2, !5, i64 125}
!38 = !{i64 0, i64 1, !39, i64 1, i64 4, !40, i64 5, i64 4, !40, i64 9, i64 4, !40, i64 13, i64 4, !40, i64 17, i64 4, !40, i64 21, i64 4, !40, i64 25, i64 4, !40, i64 29, i64 4, !40, i64 33, i64 4, !40, i64 37, i64 4, !40, i64 41, i64 4, !40, i64 45, i64 4, !40, i64 49, i64 4, !40, i64 53, i64 4, !40, i64 57, i64 4, !40, i64 61, i64 4, !40, i64 65, i64 4, !40, i64 69, i64 4, !40, i64 73, i64 4, !40, i64 77, i64 4, !40, i64 81, i64 4, !40, i64 85, i64 4, !40, i64 89, i64 4, !40, i64 93, i64 4, !40, i64 97, i64 4, !40, i64 101, i64 4, !40, i64 105, i64 4, !40, i64 109, i64 4, !40, i64 113, i64 4, !40, i64 117, i64 4, !40, i64 121, i64 4, !40, i64 125, i64 4, !40}
!39 = !{!3, !3, i64 0}
!40 = !{!5, !5, i64 0}
```



Reviewers: asl

Subscribers: qcolombet

Differential Revision: http://reviews.llvm.org/D17441

llvm-svn: 261746
 		2016-02-24 15:15:02 +00:00
..
InstPrinter Remove autoconf support 2016-01-26 21:29:08 +00:00
MCTargetDesc Remove autoconf support 2016-01-26 21:29:08 +00:00
TargetInfo Remove autoconf support 2016-01-26 21:29:08 +00:00
CMakeLists.txt Remove getDataLayout() from TargetSelectionDAGInfo (had no users) 2015-07-09 02:10:08 +00:00
LLVMBuild.txt
…
MSP430.h Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 2015-06-23 09:49:53 +00:00
MSP430.td
…
MSP430AsmPrinter.cpp MC: Add target hook to control symbol quoting 2015-06-09 00:31:39 +00:00
MSP430BranchSelector.cpp MSP430: Remove implicit ilist iterator conversions, NFC 2015-10-20 01:18:39 +00:00
MSP430CallingConv.td
…
MSP430FrameLowering.cpp [ShrinkWrap] Add (a simplified version) of shrink-wrapping. 2015-05-05 17:38:16 +00:00
MSP430FrameLowering.h Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 2015-06-23 09:49:53 +00:00
MSP430ISelDAGToDAG.cpp Make TargetLowering::getPointerTy() taking DataLayout as an argument 2015-07-09 02:09:04 +00:00
MSP430ISelLowering.cpp MSP430: Remove implicit ilist iterator conversions, NFC 2015-10-20 01:18:39 +00:00
MSP430ISelLowering.h Re-instate the EVT parameter to getScalarShiftAmountTy() for OOT user 2015-07-09 15:12:23 +00:00
MSP430InstrFormats.td
…
MSP430InstrInfo.cpp `MSP430InstrInfo::loadRegFromStackSlot` forgets to set register def. 2016-02-24 15:15:02 +00:00
MSP430InstrInfo.h CodeGen: TII: Take MachineInstr& in predicate API, NFC 2016-02-23 02:46:52 +00:00
MSP430InstrInfo.td
…
MSP430MCInstLower.cpp Move most user of TargetMachine::getDataLayout to the Module one 2015-07-16 06:11:10 +00:00
MSP430MCInstLower.h Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 2015-06-23 09:49:53 +00:00
MSP430MachineFunctionInfo.cpp Fix some comment typos. 2015-08-08 18:27:36 +00:00
MSP430MachineFunctionInfo.h Fix typo "fuction" noticed in comments in AssumptionCache.h, and also all the other files that have the same typo. All comments, no functionality change! (Merely a "fuctionality" change.) 2015-07-29 22:32:47 +00:00
MSP430RegisterInfo.cpp Target RegisterInfo: devirtualize TargetFrameLowering 2015-07-10 18:13:17 +00:00
MSP430RegisterInfo.h Have getCalleeSavedRegs take a non-null MachineFunction all the 2015-03-11 21:41:28 +00:00
MSP430RegisterInfo.td
…
MSP430Subtarget.cpp Revert r247692: Replace Triple with a new TargetTuple in MCTargetDesc/* and related. NFC. 2015-09-15 16:17:27 +00:00
MSP430Subtarget.h Rename TargetSelectionDAGInfo into SelectionDAGTargetInfo and move it to CodeGen/ 2016-01-27 16:32:26 +00:00
MSP430TargetMachine.cpp Replace string GNU Triples with llvm::Triple in TargetMachine. NFC. 2015-06-11 19:41:26 +00:00
MSP430TargetMachine.h Replace string GNU Triples with llvm::Triple in TargetMachine. NFC. 2015-06-11 19:41:26 +00:00
README.txt Fix typo in README 2015-10-04 00:46:16 +00:00

README.txt 

//===---------------------------------------------------------------------===//
// MSP430 backend.
//===---------------------------------------------------------------------===//

DISCLAIMER: This backend should be considered as highly experimental. I never
seen nor worked with this MCU, all information was gathered from datasheet
only. The original intention of making this backend was to write documentation
of form "How to write backend for dummies" :) Thes notes hopefully will be
available pretty soon.

Some things are incomplete / not implemented yet (this list surely is not
complete as well):

1. Verify, how stuff is handling implicit zext with 8 bit operands (this might
be modelled currently in improper way - should we need to mark the superreg as
def for every 8 bit instruction?).

2. Libcalls: multiplication, division, remainder. Note, that calling convention
for libcalls is incomptible with calling convention of libcalls of msp430-gcc
(these cannot be used though due to license restriction).

3. Implement multiplication / division by constant (dag combiner hook?).

4. Implement non-constant shifts.

5. Implement varargs stuff.

6. Verify and fix (if needed) how's stuff playing with i32 / i64.

7. Implement floating point stuff (softfp?)

8. Implement instruction encoding for (possible) direct code emission in the
future.

9. Since almost all instructions set flags - implement brcond / select in better
way (currently they emit explicit comparison).

10. Handle imm in comparisons in better way (see comment in MSP430InstrInfo.td)

11. Implement hooks for better memory op folding, etc.