diff options
Diffstat (limited to 'compiler/prelude/PrelRules.hs')
| -rw-r--r-- | compiler/prelude/PrelRules.hs | 25 |
1 files changed, 22 insertions, 3 deletions
diff --git a/compiler/prelude/PrelRules.hs b/compiler/prelude/PrelRules.hs index b0ad91db99..57b1cb8228 100644 --- a/compiler/prelude/PrelRules.hs +++ b/compiler/prelude/PrelRules.hs @@ -475,8 +475,7 @@ shiftRule shift_op -> return e1 -- See Note [Guarding against silly shifts] | shift_len < 0 || shift_len > wordSizeInBits dflags - -> return $ mkRuntimeErrorApp rUNTIME_ERROR_ID wordPrimTy - ("Bad shift length " ++ show shift_len) + -> return $ Lit $ mkLitNumberWrap dflags LitNumInt 0 (exprType e1) -- Do the shift at type Integer, but shift length is Int Lit (LitNumber nt x t) @@ -701,7 +700,27 @@ can't constant fold it, but if it gets to the assember we get Error: operand type mismatch for `shl' So the best thing to do is to rewrite the shift with a call to error, -when the second arg is stupid. +when the second arg is large. However, in general we cannot do this; consider +this case + + let x = I# (uncheckedIShiftL# n 80) + in ... + +Here x contains an invalid shift and consequently we would like to rewrite it +as follows: + + let x = I# (error "invalid shift) + in ... + +This was originally done in the fix to #16449 but this breaks the let/app +invariant (see Note [CoreSyn let/app invariant] in CoreSyn) as noted in #16742. +For the reasons discussed in Note [Checking versus non-checking primops] (in +the PrimOp module) there is no safe way rewrite the argument of I# such that +it bottoms. + +Consequently we instead take advantage of the fact that large shifts are +undefined behavior (see associated documentation in primops.txt.pp) and +transform the invalid shift into an "obviously incorrect" value. There are two cases: |