[all-commits] [llvm/llvm-project] b1b2c6: [DebugInstrRef] Handle DBG_INSTR_REFs use-before-d...

[Jeremy Morse via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: b1b2c6ab667d50e68a6f945f6b64ce0351c2739c
      https://github.com/llvm/llvm-project/commit/b1b2c6ab667d50e68a6f945f6b64ce0351c2739c
  Author: Jeremy Morse <jeremy.morse at sony.com>
  Date:   2020-10-23 (Fri, 23 Oct 2020)

  Changed paths:
    M llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
    M llvm/test/DebugInfo/MIR/InstrRef/livedebugvalues_instrref_tolocs.mir

  Log Message:
  -----------
  [DebugInstrRef] Handle DBG_INSTR_REFs use-before-defs in LiveDebugValues

Deciding where to place debugging instructions when normal instructions
sink between blocks is difficult -- see PR44117. Dealing with this with
instruction-referencing variable locations is simple: we just tolerate
DBG_INSTR_REFs referring to values that haven't been computed yet. This
patch adds support into InstrRefBasedLDV to record when a variable value
appears in the middle of a block, and should have a DBG_VALUE added when it
appears (a debug use before def).

While described simply, this relies heavily on the value-propagation
algorithm in InstrRefBasedLDV. The implementation doesn't attempt to verify
the location of a value unless something non-trivial occurs to merge
variable values in vlocJoin. This means that a variable with a value that
has no location can retain it across all control flow (including loops).
It's only when another debug instruction specifies a different variable
value that we have to check, and find there's no location.

This property means that if a machine value is defined in a block dominated
by a DBG_INSTR_REF that refers to it, all the successor blocks can
automatically find a location for that value (if it's not clobbered). Thus
in a sense, InstrRefBasedLDV is already supporting and implementing
use-before-defs. This patch allows us to specify a variable location in the
block where it's defined.

When loading live-in variable locations, TransferTracker currently discards
those where it can't find a location for the variable value. However, we
can tell from the machine value number whether the value is defined in this
block. If it is, add it to a set of use-before-def records. Then, once the
relevant instruction has been processed, emit a DBG_VALUE immediately after
it.

Differential Revision: https://reviews.llvm.org/D85775