[all-commits] [llvm/llvm-project] 1c21d5: [GlobalISel] Remove GI known bits cache (#157352)

[David Green via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 1c21d5cb9b8e48ab928919a6f358eba8ffd3b49c
      https://github.com/llvm/llvm-project/commit/1c21d5cb9b8e48ab928919a6f358eba8ffd3b49c
  Author: David Green <david.green at arm.com>
  Date:   2025-09-15 (Mon, 15 Sep 2025)

  Changed paths:
    M llvm/include/llvm/CodeGen/GlobalISel/GISelValueTracking.h
    M llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
    M llvm/test/CodeGen/AArch64/rem-by-const.ll
    M llvm/test/CodeGen/AMDGPU/GlobalISel/sdiv.i64.ll
    M llvm/test/CodeGen/AMDGPU/GlobalISel/srem.i64.ll
    M llvm/test/CodeGen/RISCV/GlobalISel/div-by-constant.ll
    M llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
    M llvm/unittests/CodeGen/GlobalISel/KnownBitsVectorTest.cpp

  Log Message:
  -----------
  [GlobalISel] Remove GI known bits cache (#157352)

There is a cache on the known-bit computed by global-isel. It only works
inside a single query to computeKnownBits, which limits its usefulness,
and according to the tests can sometimes limit the effectiveness of
known-bits queries. (Although some AMD tests look longer). Keeping the
cache valid and clearing it at the correct times can also require being
careful about the functions called inside known-bits queries.

I measured compile-time of removing it and came up with:
```
7zip      2.06405E+11     2.06436E+11     0.015018992
Bullet    1.01298E+11     1.01186E+11     -0.110236169
ClamAV    57942466667     57848066667     -0.16292023
SPASS     45444466667     45402966667     -0.091320249
consumer  35432466667     35381233333     -0.144594317
kimwitu++ 40858833333     40927933333     0.169118877
lencod    70022366667     69950633333     -0.102443457
mafft     38439900000     38413233333     -0.069372362
sqlite3   35822266667     35770033333     -0.145812474
tramp3d   82083133333     82045600000     -0.045726
Average                                   -0.068828739
```
The last column is % difference between with / without the cache. So in
total it seems to be costing slightly more to keep the current
known-bits cache than if it was removed. (Measured in instruction count,
similar to llvm-compile-time-tracker).

The hit rate wasn't terrible - higher than I expected. In the
llvm-test-suite+external projects it was hit 4791030 times out of
91107008 queries, slightly more than 5%.

Note that as globalisel increases in complexity, more known bits calls
might be made and the numbers might shift. If that is the case it might
be better to have a cache that works across calls, providing it doesn't
make effectiveness worse.



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