[PATCH] D34640: Create a PHI value when merging with a known undef live-in

[Krzysztof Parzyszek via Phabricator via llvm-commits]

kparzysz created this revision.
Herald added a subscriber: qcolombet.

The following snippet illustrates the problem.

During register allocation, the value defined at `868` for `vreg154:isub_hi` (lane mask 0x0001) is extended to the use at `1564` via a single segment.  That segment spans over the entry to `BB#6`, which is reachable from two places: the end of `BB#5` (dominated by `868`), and the end of `BB#6`.  The value of `vreg154:isub_hi` is undefined when coming from the end of `BB#6`, which causes the verifier to complain.

To fix this, create a PHI when a defined value is merged with an undef value at the entry to a block.  Currently, undef values are simply ignored when determining the need for PHIs, which leads to a problem when there is a unique defined value.

  704B    BB#5: derived from LLVM BB %b17
              Predecessors according to CFG: BB#2 BB#4
  868B            %vreg154<def> = COPY %vreg148; DoubleRegs:%vreg154,%vreg148
              Successors according to CFG: BB#6(?%)
  
  1172B   BB#6: derived from LLVM BB %b30
              Predecessors according to CFG: BB#5 BB#6
  1212B           %vreg66<def> = C2_cmpgt %vreg152, %vreg154:isub_lo; PredRegs:%vreg66 IntRegs:%vreg152 DoubleRegs:%vreg154
  1564B           %vreg153<def> = COPY %vreg154; DoubleRegs:%vreg153,%vreg154
  1572B           %vreg154<def> = COPY %vreg153; DoubleRegs:%vreg154,%vreg153
  1856B           %vreg154:isub_lo<def,read-undef> = A2_addi %vreg154:isub_lo, 1; DoubleRegs:%vreg154
  1984B           J2_jump <BB#6>, %PC<imp-def,dead>
              Successors according to CFG: BB#6(?%)
  
  # End machine code for function fred.
  
  *** Bad machine code: Register not marked live out of predecessor ***
  - function:    fred
  - basic block: BB#6 b30 (0x50bb8b8) [1172B;1992B)
  - liverange:   [868r,1564r:1)[1572r,1572d:0)  0 at 1572r 1 at 868r
  - v. register: %vreg154
  - lanemask:    00000001
  - ValNo:       1 (def 868r)
   live into BB#6 at 1172B, not live before 1992B
  LLVM ERROR: Found 1 machine code errors.


Repository:
  rL LLVM

https://reviews.llvm.org/D34640

Files:
  lib/CodeGen/LiveRangeCalc.cpp
  test/CodeGen/Hexagon/regalloc-liveout-undef.ll

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