[PATCH] D62833: [DAGCombine][Thumb] Use single CP entry for addressing GV with large offset

[Z. Zheng via Phabricator via llvm-commits]

zzheng created this revision.
zzheng added reviewers: efriedma, zinob, weimingz, t.p.northover.
Herald added a subscriber: javed.absar.
Herald added a project: LLVM.

  Original patch by Weiming Zhao.
  
  When addressing a global variable with large offsets, the offset may not fit in
  the load instruction and thus either needs rematerializaion or extra constant
  pool entry. This casues code size bloat, especially on Thumb1.
  
  This patch tries to encode the offset in the CP entry to save code size.
  
  For example:
    &g_big_struct.x_offset_300
  
  Before:                         After:
  movs r0, #75                    ldr r1, .LCPI0_0
  lsls r0, r0, #2                 .LCPI0_0: .long g_big_struct + 300
  ldr  r1, .LCPI0_0
  adds r0, r1
  .LCPI0_0: .long g_big_struct
  
  In best case, it saves 3 insts (and potentially reduces register pressure) as
  the example shows.
  
  In worst case, if the GV has multiple uses, the original CP entry for the GV
  base is still needed, and the member can be calculated from previous addressing,
  it will cost 4 extra bytes (for the CP entry).


Repository:
  rL LLVM

https://reviews.llvm.org/D62833

Files:
  lib/Target/ARM/ARMISelLowering.cpp
  test/CodeGen/Thumb/gv-large-offset.ll
  test/CodeGen/Thumb/single-gv-with-multiple-offsets.ll

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