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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
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  • #14251
Closed
Open
Issue created Sep 19, 2017 by Moritz Angermann@angermanDeveloper

LLVM Code Gen messes up registers

Due to the way the LLVM Code Gen generates Function Singnatures, it is possible to end up mixed up registers.

A slightly adapted T8064

{-# LANGUAGE MagicHash, BangPatterns #-}
module Main where

import GHC.Exts

{-# NOINLINE f #-}
f :: (Int# -> Float# -> Double# -> Float# -> Double# -> String) -> String
f g = g 3# 4.0# 5.0## 6.0# 6.9## ++ " World!"

{-# NOINLINE p #-}
p :: Int# -> Float# -> Double# -> Float# -> Double# -> String
p i j k l m = "Hello"

{-# NOINLINE q #-}
q :: Int# -> Int# -> Float# -> Double# -> Float# -> Double# -> String
q _ i j k l m = "Hello " ++ show (F# l) ++ " " ++ show (D# m)

{-# NOINLINE r #-}
r :: Int# -> Float# -> Double# -> Float# -> Double# -> String
r i = let !(I# z) = length [I# 1# .. I# i] in \j k l m -> p z j k l m
  -- ghc won't eta-expand around the length, because it has unknown cost

main = do
  putStrLn (f p)    -- fast call
  putStrLn (f r)    -- slow call: function but wrong arity
  let g = last [q 1#]
  putStrLn (f g)    -- slow call: thunk

will produce the following results:

../inplace/bin/ghc-stage1 -fllvm -fforce-recomp T6084.hs -O2 -o T6084-llvm && ./T6084-llvm
[1 of 1] Compiling Main ( T6084.hs, T6084.o )
Linking T6084-llvm ...
Hello World!
Hello World!
Hello 4.0 5.0 World!

../inplace/bin/ghc-stage1 -fasm -fforce-recomp T6084.hs -O2 -o T6084-asm && ./T6084-asm
[1 of 1] Compiling Main ( T6084.hs, T6084.o )
Linking T6084-asm ...
Hello World!
Hello World!
Hello 6.0 6.9 World!

The underlying reason is that (at least for X86_64) the Float and Double registers alternate. The llvm code gen creates function signatures based on the live registers (instead of all).

For q only the last Float and Double register are live. However when calling q we pass f1: Float -> d1: Double -> f2: Float -> d2: Double. f2 and d2 are silently ignored, and in the function body, we now have f2 <- f1 and d2 <- d1.

Trac metadata
Trac field Value
Version 8.3
Type Bug
TypeOfFailure OtherFailure
Priority highest
Resolution Unresolved
Component Compiler (LLVM)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC bgamari, carter, simonmar
Operating system
Architecture
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