[PATCH] D91719: [RISCV] Add support for using -msave-restore with tailcalls

[Edward Jones via Phabricator via llvm-commits]

edward-jones added a comment.

On the RISC-V sync-up call I said I would try and provide some code size numbers for these changes. First I mentioned that I saw a code size regression when compiled Embench, here's per-benchmark numbers:

`-Oz -msave-restore` (no tailcall version)

   Benchmark            size
  ---------            ----
  aha-mont64          1,964
  crc32               1,140
  cubic              55,942
  edn                 2,364
  huffbench           2,456
  matmult-int         1,420
  minver             11,160
  nbody              10,748
  nettle-aes          3,852
  nettle-sha256       8,552
  nsichneu           14,268
  picojpeg            9,476
  qrduino             7,256
  sglib-combined      3,528
  slre                3,652
  st                 11,056
  statemate           4,908
  ud                  1,620
  wikisort           15,060
  ---------           -----
  Geometric mean      5,347
  Geometric SD            2.65
  Geometric range  12,143.345300622888

`-Oz -msave-restore` (with tailcall version)

  Benchmark            size
  ---------            ----
  aha-mont64          1,968
  crc32               1,340
  cubic              55,942
  edn                 2,372
  huffbench           2,656
  matmult-int         1,476
  minver             11,164
  nbody              10,752
  nettle-aes          3,896
  nettle-sha256       8,588
  nsichneu           14,272
  picojpeg            9,476
  qrduino             7,428
  sglib-combined      6,512
  slre                3,656
  st                 11,116
  statemate           4,912
  ud                  1,620
  wikisort           15,060
  ---------           -----
  Geometric mean      5,620
  Geometric SD            2.59
  Geometric range  12,365.236826034434

The Embench benchmarks are very small, so the constant cost of the additional `__riscv_restore_tailcall` entry points exceeds the benefit of the optimization.

To try something larger I also grabbed sqlite 3.35.5 and tried building that (again `-Oz -msave-restore`). Below are results for that experiment.

    text    data     bss     dec     hex filename
  652816    5604     903  659323   a0f7b sqlite3-save-restore.o
  645328    5604     903  651835   9f23b sqlite3-save-restore-tailcall.o

So for this larger codebase where the constant cost of the additional functions is minor, we see approximately a 1% reduction in code size (note that this only compiling to an object file, so it doesn't include the the cost of the save-restore compiler-rt functions)


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https://reviews.llvm.org/D91719