GHC inlining primitive string literals can affect program output
First noted in #11292 (closed), this program, when compiled with -O1 or higher:
{-# LANGUAGE MagicHash #-}
module Main (main) where
import GHC.Exts (Addr#, isTrue#)
import GHC.Prim (eqAddr#)
data A = A { runA :: Addr# }
a :: A
a = A "a"#
main :: IO ()
main = print (isTrue# (eqAddr# (runA a) (runA a)))will result in the following after inlining:
Main.main2 =
case GHC.Prim.tagToEnum#
@ GHC.Types.Bool (GHC.Prim.eqAddr# "a"# "a"#)
of _ [Occ=Dead] {
GHC.Types.False -> GHC.Show.shows26;
GHC.Types.True -> GHC.Show.shows24
}As a result, there are two of the same string constant with different addresses, which causes eqAddr# to return False. If compiled without optimizations, "a"# is not inlined, and as a result, eqAddr# returns True.
Two questions:
- Is this okay semantics-wise? Or is this a necessary risk when working with primitive string literals, and should programmers judiciously use
{-# NOINLINE #-}with them? - Is this okay from a code duplication standpoint? As Reid Barton noted in #11292 (closed),
"a"#is duplicated due to inlining. In this example, not much is duplicated, but if it were a longer string constant, that could result in a noticeable increase in the object file size.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.10.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | #11292 (closed) |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |