[GHC] #14251: LLVM Code Gen messes up registers

19 Sep 2017

      #14251: LLVM Code Gen messes up registers
-------------------------------------+-------------------------------------
           Reporter:  angerman       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.3
  (LLVM)                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Due to the way the LLVM Code Gen generates Function Singnatures, it is
 possible to end up mixed up registers.

 A slightly adapted T8064
 {{{#!hs
 {-# 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`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14251
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