haskell - 为什么用 -fllvm 编译这个 Haskell 程序会产生不同的结果？

Question

编译以下程序:

main = print (sqrt (-7))

在 OSX Yosemite、LLVM 版本 3.4.2、目标 x86_64-apple-darwin14.0.0、GHC 7.8.4 上会产生两种不同的结果，具体取决于 llvm 的使用:

apple1$ ghc -fforce-recomp a.hs -o a; ./a
[1 of 1] Compiling Main             ( a.hs, a.o )
Linking a ...
NaN

apple1$ ghc -fforce-recomp -fllvm a.hs -o a; ./a
[1 of 1] Compiling Main             ( a.hs, a.o )
Linking a ...
0.0

为什么会发生这种情况？

Answer 1

(在评论中回答了问题。转换为社区 wiki 答案。见 Question with no answers, but issue solved in the comments (or extended in chat))

@leftaroundabout 写道:

I would consider it a bug, though it might well have been acceptable according to some specification. LLVM evidently fired some optimisation here which assumes non-negative arguments, making √(-7) undefined behaviour.

@Ed'ka 写道:

Probably related: LLVM bug 21048