[llvm] a38cf85 - [MachineLICM] Let targets decide if copy instructions are cheap (#146599)

[via llvm-commits]

Author: Guy David
Date: 2025-07-05T14:06:33+03:00
New Revision: a38cf8573890103c8a26227bb9c395fd00102273

URL: https://github.com/llvm/llvm-project/commit/a38cf8573890103c8a26227bb9c395fd00102273
DIFF: https://github.com/llvm/llvm-project/commit/a38cf8573890103c8a26227bb9c395fd00102273.diff

LOG: [MachineLICM] Let targets decide if copy instructions are cheap (#146599)

When checking whether it is profitable to hoist an instruction, the pass
may override a target's ruling because it assumes that all COPY
instructions are cheap, and that may not be the case for all
micro-architectures (especially for when copying between different
register classes).

On AArch64 there's 0% difference in performance in LLVM's test-suite
with this change. Additionally, very few tests were affected which shows
how it is not so useful to keep it.

x86 performance is slightly better (but maybe that's just noise) for an
A/B comparison consisting of five iterations on LLVM's test suite (Ryzen
5950X on Ubuntu):
```
$ ./utils/compare.py build-a/results* vs build-b/results* --lhs-name base --rhs-name patch --absolute-diff
Tests: 3341
Metric: exec_time

Program                                       exec_time                 
                                              base      patch     diff  
LoopVector...meChecks4PointersDBeforeA/1000   824613.68 825394.06 780.38
LoopVector...timeChecks4PointersDBeforeA/32    18763.60  19486.02 722.42
LCALS/Subs...test:BM_MAT_X_MAT_LAMBDA/44217    37109.92  37572.52 462.60
LoopVector...ntimeChecks4PointersDAfterA/32    14211.35  14562.14 350.79
LoopVector...timeChecks4PointersDEqualsA/32    14221.44  14562.85 341.40
LoopVector...intersAllDisjointIncreasing/32    14222.73  14562.20 339.47
LoopVector...intersAllDisjointDecreasing/32    14223.85  14563.17 339.32
LoopVector...nLoopFrom_uint32_t_To_uint8_t_      739.60    807.45  67.86
harris/har...est:BENCHMARK_HARRIS/2048/2048    15953.77  15998.94  45.17
LoopVector...nLoopFrom_uint8_t_To_uint16_t_      301.94    331.21  29.27
LCALS/Subs...Raw.test:BM_DISC_ORD_RAW/44217      616.35    637.13  20.78
LCALS/Subs...Raw.test:BM_MAT_X_MAT_RAW/5001     3814.95   3833.70  18.75
LCALS/Subs...Raw.test:BM_HYDRO_2D_RAW/44217      812.98    830.64  17.66
LCALS/Subs...test:BM_IMP_HYDRO_2D_RAW/44217      811.26    828.13  16.87
ImageProce...ENCHMARK_BILATERAL_FILTER/64/4      714.77    726.23  11.46
           exec_time                            
l/r             base          patch         diff
count  3341.000000    3341.000000    3341.000000
mean   903.866450     899.732349    -4.134101   
std    20635.900959   20565.289417   115.346928 
min    0.000000       0.000000      -3380.455787
25%    0.000000       0.000000       0.000000   
50%    0.000000       0.000000       0.000000   
75%    1.806500       1.836397       0.000100   
max    824613.680801  825394.062500  780.381699
```

Added: 
    

Modified: 
    llvm/lib/CodeGen/MachineLICM.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index 699d7ab175568..f1811c47e5ad4 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -1219,7 +1219,7 @@ bool MachineLICMImpl::HasHighOperandLatency(MachineInstr &MI, unsigned DefIdx,
 /// Return true if the instruction is marked "cheap" or the operand latency
 /// between its def and a use is one or less.
 bool MachineLICMImpl::IsCheapInstruction(MachineInstr &MI) const {
-  if (TII->isAsCheapAsAMove(MI) || MI.isCopyLike())
+  if (TII->isAsCheapAsAMove(MI) || MI.isSubregToReg())
     return true;
 
   bool isCheap = false;