#8195: Different floating point results with -msse2 on 32bit Linux ------------------------------------------------+-------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.7 Resolution: | Keywords: Operating System: Linux | Architecture: x86 Type of failure: None/Unknown | Difficulty: Test Case: perf/should_run/Conversions | Unknown Blocking: | Blocked By: | Related Tickets: ------------------------------------------------+-------------------------- Description changed by jstolarek: Old description:

I noticed strange behaviour of `Conversions` test on 32bit machines. Here's the code of this test:

{{{ {-# LANGUAGE BangPatterns #-}

-- | Tests that conversions between various primitive types (e.g. -- Word, Double, etc) doesn't allocate. module Main (main) where

import Data.Word

-- Repeatedly convert Words to Doubles loop :: Floating a => Word -> a loop n = go 0 0.0 where go i !acc | i < n = go (i+1) (acc + fromIntegral i) | otherwise = acc {-# SPECIALISE loop :: Word -> Float #-} {-# SPECIALISE loop :: Word -> Double #-}

main :: IO () main = do print (loop 1000000 :: Float) print (loop 1000000 :: Double) }}} This test is expected to produce: {{{ [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate] inplace/bin/ghc- stage2 -O -fforce-recomp testsuite/tests/perf/should_run/Conversions.hs [1 of 1] Compiling Main ( testsuite/tests/perf/should_run/Conversions.hs, testsuite/tests/perf/should_run/Conversions.o ) Linking testsuite/tests/perf/should_run/Conversions ... [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate] ./testsuite/tests/perf/should_run/Conversions 4.9994036e11 4.999995e11 }}} My optimization of self-recursive tail loops (d61c3ac186c94021c851f7a2a6d20631e35fc1ba) broke that and now both `Float` and `Double` results are identical: {{{ 4.999995e11 4.999995e11 }}} I believe my optimisation shouldn't affect this, but clearly it does.

Now here's the interesting part: if I generate SSE assembly by adding `-msse2` flag, then I get expected results: {{{ 4.9994036e11 4.999995e11 }}} I'm not sure if we should get different results for different instruction sets. I'm putting it up as a ticket so perhaps we could investigate this more one day if we consider it relevant.

New description: I noticed strange behaviour of `Conversions` test on 32bit machines. Here's a slightly reduced version of that test: {{{ {-# LANGUAGE BangPatterns #-} -- | Tests that conversions between various primitive types (e.g. -- Word, Double, etc) doesn't allocate. module Main (main) where import Data.Word -- Repeatedly convert Words to Doubles loop :: Word -> Float loop n = go 0 0.0 where go i !acc | i < n = go (i+1) (acc + fromIntegral i) | otherwise = acc main :: IO () main = do print (loop 1000000 :: Float) }}} This test is expected to produce: {{{ [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate] inplace/bin/ghc- stage2 -O -fforce-recomp testsuite/tests/perf/should_run/Conversions.hs [1 of 1] Compiling Main ( testsuite/tests/perf/should_run/Conversions.hs, testsuite/tests/perf/should_run/Conversions.o ) Linking testsuite/tests/perf/should_run/Conversions ... [t-jastol@cam-05-unx : /5playpen/t-jastol/ghc-validate] ./testsuite/tests/perf/should_run/Conversions 4.9994036e11 }}} My optimization of self-recursive tail loops (d61c3ac186c94021c851f7a2a6d20631e35fc1ba) change results of that program to be slightly different: {{{ 4.999995e11 }}} I believe my optimisation shouldn't affect this, but clearly it does. Now here's the interesting part: if I generate SSE assembly by adding `-msse2` flag, then I get expected results: {{{ 4.9994036e11 }}} I'm not sure if we should get different results for different instruction sets. I'm putting it up as a ticket so perhaps we could investigate this more one day if we consider it relevant. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8195#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8195: Different floating point results with -msse2 on 32bit Linux ------------------------------------------------+-------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.7 Resolution: | Keywords: Operating System: Linux | Architecture: x86 Type of failure: None/Unknown | Difficulty: Test Case: perf/should_run/Conversions | Unknown Blocking: | Blocked By: | Related Tickets: ------------------------------------------------+-------------------------- Comment (by rwbarton): Yeah, the fact that `-msse2` affects the output is expected for the reason carter stated. This issue is discussed generally at http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html#bugs-ghc (last bullet point of 14.2.1). The reason your tail loop optimization affected the output is probably that with the optimization the code compiles to a loop in which `acc` lives in an 80-bit x87 register for the entire duration, while before the optimization `acc` was pushed on the stack as a 32-bit value for the recursive call. (Or possibly it was spilled to memory for some other reason.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8195#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler