expensive loop invariant expression is not hoisted out of recursive function

Summary

Expensive loop invariant expression is not hoisted out of a recursive function.

Steps to reproduce

Compile and run the attached code: loopInvariant.hs

> ghc -O2 loopInvariant.hs
Loaded package environment from /Users/gcolpitts/.ghc/x86_64-darwin-9.8.2/environments/default
[1 of 2] Compiling Main             ( loopInvariant.hs, loopInvariant.o )
[2 of 2] Linking loopInvariant
ld: warning: ignoring duplicate libraries: '-lm'
ld: warning: search path '/opt/local/lib/' not found
gcolpitts@Georges-Mini haskell % ./loopInvariant +RTS -s
39001250856960000
   1,920,053,000 bytes allocated in the heap
          16,488 bytes copied during GC
       8,028,224 bytes maximum residency (2 sample(s))
         393,152 bytes maximum slop
              22 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0       239 colls,     0 par    0.001s   0.002s     0.0000s    0.0000s
  Gen  1         2 colls,     0 par    0.000s   0.002s     0.0009s    0.0017s

  INIT    time    0.003s  (  0.003s elapsed)
  MUT     time    3.141s  (  3.138s elapsed)
  GC      time    0.001s  (  0.004s elapsed)
  EXIT    time    0.001s  (  0.006s elapsed)
  Total   time    3.146s  (  3.151s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    611,361,327 bytes per MUT second

  Productivity  99.8% of total user, 99.6% of total elapsed

Comment out the line:

    let p = smallest maxN ! m

Uncomment the lines:

      -- let p = sm ! m
    -- sm = smallest maxN

Compile and run the changed code:

> ghc -O2 loopInvariant.hs
Loaded package environment from /Users/gcolpitts/.ghc/x86_64-darwin-9.8.2/environments/default
[1 of 2] Compiling Main             ( loopInvariant.hs, loopInvariant.o ) [Source file changed]
[2 of 2] Linking loopInvariant [Objects changed]
ld: warning: ignoring duplicate libraries: '-lm'
ld: warning: search path '/opt/local/lib/' not found
gcolpitts@Georges-Mini haskell % ./loopInvariant +RTS -s
39001250856960000
      16,051,152 bytes allocated in the heap
          10,040 bytes copied during GC
          44,328 bytes maximum residency (1 sample(s))
          29,400 bytes maximum slop
              22 MiB total memory in use (0 MiB lost due to fragmentation)

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0         2 colls,     0 par    0.000s   0.000s     0.0000s    0.0000s
  Gen  1         1 colls,     0 par    0.000s   0.001s     0.0012s    0.0012s

  INIT    time    0.003s  (  0.003s elapsed)
  MUT     time    0.018s  (  0.018s elapsed)
  GC      time    0.000s  (  0.001s elapsed)
  EXIT    time    0.001s  (  0.012s elapsed)
  Total   time    0.022s  (  0.034s elapsed)

  %GC     time       0.0%  (0.0% elapsed)

  Alloc rate    889,408,322 bytes per MUT second

  Productivity  80.9% of total user, 53.0% of total elapsed

Allocation has been reduced from 2 billion bytes to 16 million. Runtime has been reduced from 3 seconds to 0.02 seconds.

Expected behavior

I expect the compiler to do the optimization given above

Environment

• GHC version used: 9.8.2 (reproducible on 9.10.1 also)

Optional:

• Operating System: MacOS Sonoma 14.5
• System Architecture: x86_64 (w 9.8.2) and Apple Silicon (w 9.10.1)loopInvariant.hs