diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/SetLevels.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/SetLevels.hs | 10 |
1 files changed, 5 insertions, 5 deletions
diff --git a/compiler/GHC/Core/Opt/SetLevels.hs b/compiler/GHC/Core/Opt/SetLevels.hs index eab4d0ef4e..21ddfbda22 100644 --- a/compiler/GHC/Core/Opt/SetLevels.hs +++ b/compiler/GHC/Core/Opt/SetLevels.hs @@ -912,15 +912,15 @@ Note [Test cheapness with exprOkForSpeculation] We don't want to float very cheap expressions by boxing and unboxing. But we use exprOkForSpeculation for the test, not exprIsCheap. Why? Because it's important /not/ to transform - f (a /# 3) + let x = a /# 3 to - f (case bx of I# a -> a /# 3) -and float bx = I# (a /# 3), because the application of f no -longer obeys the let/app invariant. But (a /# 3) is ok-for-spec + let x = case bx of I# a -> a /# 3 +because the let binding no +longer obeys the let-can-float invariant. But (a /# 3) is ok-for-spec due to a special hack that says division operators can't fail when the denominator is definitely non-zero. And yet that same expression says False to exprIsCheap. Simplest way to -guarantee the let/app invariant is to use the same function! +guarantee the let-can-float invariant is to use the same function! If an expression is okay for speculation, we could also float it out *without* boxing and unboxing, since evaluating it early is okay. |