[PATCH] D124517: [DebugInfo][InstrRef] Use a cache to avoid creating redundant DBG_PHIs

[Jeremy Morse via Phabricator via llvm-commits]

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In SelectionDAG, DBG_PHI instructions are created to "read" physreg values and give them an instruction number, when they can't be traced back to a defining instruction. The most common scenario is arguments to a function. Unfortunately, if you have 100 inlined methods, each of which has the same "this" pointer, then the 100 dbg.value instructions become 100 DBG_INSTR_REFs and 100 DBG_PHIs, where only one DBG_PHI would suffice.

This patch adds a vreg cache for MachineFunction::salvageCopySSA, if we've already traced a value back to the start of a block and create a DBG_PHI then it allows us to re-use the DBG_PHI, as well as reducing work.

I've added a test with an implicit check-not for DBG_PHIs, which I think is the best way of making sure they aren't over-created in the future, and deserves its own test file.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D124517

Files:
  llvm/include/llvm/CodeGen/MachineFunction.h
  llvm/lib/CodeGen/MachineFunction.cpp
  llvm/test/DebugInfo/X86/dbg-value-funcarg.ll
  llvm/test/DebugInfo/X86/dbg-value-funcarg4.ll
  llvm/test/DebugInfo/X86/instr-ref-selectiondag.ll

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