Machine Description for Retargetable Compiler Backend

1
Machine Description for Retargetable Compiler
Backend

• Wei Qin
• Nov, 2000

2
Outline

• Motivation
• Necessary information for compiler backend
• Machine view
• Instruction view
• Machine Description
• HMDES
• Machine
• Instruction
• Conclusion

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Motivation Compiler Backend
4
Motivation- Compiler Backend

• Traditional compiler backends are mostly hand
  written.
• Mescal design flow requires that the compiler
  retarget itself.
• The architect should provide enough information
  to drive the backend.
• A formal language between the architect and the
  compiler not English.

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Motivation Compiler Backend

• Mescal uni-processor compiler backend
• Framework all in Xcode
• Xcode IR input from front end
• Xcode output to Liberty simulator
• Have been done
• HMDES, for scheduler
• To do
• Direct mapping code generator
• Register allocation
• Code optimization

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Machine Description Instruction

• Compilers view of instructions
• Instruction format, i.e. and type of src, dest,
  predicate, constraints
• Instruction function what the instruction does
• For control/dataflow analysis (category of the
  instruction)
• For functional simulator
• Instruction behavior
• Resource usage spatial and temporal

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Machine Description-Machine

• Compilers view of machine
• General architecture information
• Bus Widths
• FU mix
• register file number and size, overlapping
• Specialty
• Special registers
• Delay slots

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Machine Description- Part 1

• High level machine description language
• Resources register banks, issue slots, FUs
• Operations format, resource usage, latency
• Flags for compiler, mainly indicating category
  of instructions (control flow, memory operation)
• Ref J. Gyllenhaal, W. Hwu, B. Rau, HMDES Version
  2.0 Specification IMPACT Technical Report, UIUC,
  IL 1996.

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Machine Description Part 2

• Machine Information
• Macro registers SP, IP, OP, LV, ST
• Data memory alignment, data type sizes,
  endianness (mspec file in IMPACT)
• Number of delay slots, nullification
• Function calling convention
• Instruction sizes

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Machine Description Part 3

• HMDES provides a instruction list and the
  instruction formats and categories (by compiler
  flag), but little about functionality
• Machine code description
• One one mapping no need to specify
• Most should fall in here since X code space is
  already big
• One Many mapping
• E.g. compare and branch
• Optimization
• One one mapping, e.g. move zero and xor
• Many one mapping, e.g. Complex ALU, CISC memory
  op
• Special instructions e.g. multiple load/store
  instruction

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Machine Description Part 3 (contd)

• Operand constraint
• 3 address or 2 address
• E.g. Thumb instruction set of Strong Arm
• Binding to macro register
• Illegal field
• Functionality

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Conclusion

• Architect needs to provide machine information
  from various views
• HMDES needs to be accompanied by the more
  detailed instruction view and machine view
• XML can be used as a media
• XML version of HMDES is there
• Need semantics
• Information complete?
• Do the best we can