[llvm-dev] [CodeGen] CodeSize - TailMerging and BlockPlacement

[via llvm-dev]

Hi everyone,

The code layout that TailMerging (inside BranchFolding) works on is not 
the final layout optimized based on the branch probability.  Generally, 
after BlockPlacement, many new merging opportunities emerge. I did an 
experiment of adding additional BranchFolding and BlockPlacement after 
the existing BlockPlacement (i.e., -block-placement  -branch-folder 
-block-placement) targeting an AARch64 backend. Thousands of 
instructions can be removed in spec2006 benchmarks as shown below. I 
checked the binaries and did not find any increase of unwanted 
instructions. The change does not hurt any benchmark with noticeable 
regression and sometimes results in small improvement (1%-3%).

473.astar        -7
401.bzip2        -110
403.gcc          -13,006
445.gobmk        -1,716
464.h264ref      -684
456.hmmer        -391
462.libquantum   -4
429.mcf          -4
471.omnetpp      -1,980
400.perlbench    -4,176
458.sjeng        -338
450.soplex       -395
483.xalancbmk    -4,183
447.dealII       -186
433.milc         -34
444.namd         -104
453.povray       -1,785
482.sphinx3      -112

I propose to factor out the relevant code from BranchFolding into a 
utility, and call it from BlockPlacement whenever the layout is changed. 
  It is similar to D18226 and D18411 which factor tail duplication into a 
utility and call it from BlockPlacement.  Any thoughts, advice, or 
comments?

Best,

Haicheng