#5165: GHC doesn't optimize FP excess precision properly
-----------------------+----------------------------------------------------
Reporter: ezyang | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.1 | Keywords:
Testcase: | Blockedby:
Os: Linux | Blocking:
Architecture: x86 | Failure: Incorrect result at runtime
-----------------------+----------------------------------------------------
jmcarthur helped in constructing the test case. Consider the following
program:
{{{
module Main where
{-# NOINLINE x #-}
{-# NOINLINE y #-}
x, y :: Double
x = 3
y = 7
main = print $ x/y == (3/7 :: Double)
}}}
When run on 32-bit Linux, we get the following results:
{{{
ezyang@javelin:~$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.0.3
ezyang@javelin:~$ ghc --make Bar.hs -o Bar -O0 -fforce-recomp && ./Bar
[1 of 1] Compiling Main ( Bar.hs, Bar.o )
Linking Bar ...
True
ezyang@javelin:~$ ghc --make Bar.hs -o Bar -O -fforce-recomp && ./Bar
[1 of 1] Compiling Main ( Bar.hs, Bar.o )
Linking Bar ...
False
}}}
The second result should be True (and only false if `-fexcess-precision`
is specified). I also tested on a devel2 build of GHC HEAD and got similar
results.
This appears to be a regression from 6.12.1:
{{{
ezyang@javelin:~$ /usr/bin/ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.1
ezyang@javelin:~$ /usr/bin/ghc --make Bar.hs -o Bar -O && ./Bar
True
}}}
I also kind of suspect `-fexcess-precision` doesn't do anything for the
native codegen (based on source code surfing), but it's hard to tell with
this current bug.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5165>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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