[PATCH] D32980: Use the frame index side table for byval and inalloca arguments

[Reid Kleckner via Phabricator via llvm-commits]

rnk created this revision.

For inalloca functions, this is a very common code pattern:

  %argpack = type <{ i32, i32, i32 }>
  define void @f(%argpack* inalloca %args) {
  entry:
    %a = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 0
    %b = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 1
    %c = getelementptr inbounds %argpack, %argpack* %args, i32 0, i32 2
    tail call void @llvm.dbg.declare(metadata i32* %a, ... "a")
    tail call void @llvm.dbg.declare(metadata i32* %c, ... "b")
    tail call void @llvm.dbg.declare(metadata i32* %b, ... "c")

Even though these GEPs can be simplified to a constant offset from EBP
or RSP, we don't do that at -O0, and each GEP is computed into a
register. Registers used to compute argument addresses are typically
spilled and clobbered very quickly after the initial computation, so
live debug variable tracking loses information very quickly if we use
DBG_VALUE instructions.

This change moves processing of dbg.declare between argument lowering
and basic block isel, so that we can ask if an argument has a frame
index or not. If the argument lives in a register as is the case for
byval arguments on some targets, then we don't put it in the side table
and during ISel we emit DBG_VALUE instructions.


https://reviews.llvm.org/D32980

Files:
  llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
  llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
  llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  llvm/test/CodeGen/X86/2010-01-18-DbgValue.ll
  llvm/test/CodeGen/X86/2012-11-30-handlemove-dbg.ll
  llvm/test/CodeGen/X86/dbg-baseptr.ll
  llvm/test/DebugInfo/X86/dbg-declare-inalloca.ll

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