Hi Nicolas! I tried to reproduce the difference between 7.0.4 and 7.6.2 on the exp3_8, wheel-sieve1, and primes and couldn't get the same percent difference as you. We need to reconcile these differences somehow. Lets start with more exact machine specs. I have a: $ cat /proc/cpuinfo processor : 7 vendor_id : GenuineIntel cpu family : 6 model : 58 model name : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz stepping : 9 microcode : 0x12 cpu MHz : 1600.000 cache size : 8192 KB ... $ uname -a Linux johantibell.com 3.2.0-29-generic #46-Ubuntu SMP Fri Jul 27 17:03:23 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux And GHC versions: $ ghc-7.0.4 --info [("Project name","The Glorious Glasgow Haskell Compilation System") ,("Project version","7.0.4") ,("Booter version","6.12.1") ,("Stage","2") ,("Build platform","x86_64-unknown-linux") ,("Host platform","x86_64-unknown-linux") ,("Target platform","x86_64-unknown-linux") ,("Have interpreter","YES") ,("Object splitting","YES") ,("Have native code generator","YES") ,("Have llvm code generator","YES") ,("Support SMP","YES") ,("Unregisterised","NO") ,("Tables next to code","YES") ,("RTS ways","l debug thr thr_debug thr_l thr_p dyn debug_dyn thr_dyn thr_debug_dyn") ,("Leading underscore","NO") ,("Debug on","False") ,("LibDir","/usr/local/lib/ghc-7.0.4") ,("Global Package DB","/usr/local/lib/ghc-7.0.4/package.conf.d") ,("C compiler flags","[\"-fno-stack-protector\"]") ,("Gcc Linker flags","[]") ,("Ld Linker flags","[]") ] $ ghc-7.6.2 --info [("Project name","The Glorious Glasgow Haskell Compilation System") ,("GCC extra via C opts"," -fwrapv") ,("C compiler command","/usr/bin/gcc") ,("C compiler flags"," -fno-stack-protector ") ,("ar command","/usr/bin/ar") ,("ar flags","q") ,("ar supports at file","@ArSupportsAtFile@") ,("touch command","touch") ,("dllwrap command","/bin/false") ,("windres command","/bin/false") ,("perl command","/usr/bin/perl") ,("target os","OSLinux") ,("target arch","ArchX86_64") ,("target word size","8") ,("target has GNU nonexec stack","True") ,("target has .ident directive","True") ,("target has subsections via symbols","False") ,("LLVM llc command","llc") ,("LLVM opt command","opt") ,("Project version","7.6.2") ,("Booter version","7.4.1") ,("Stage","2") ,("Build platform","x86_64-unknown-linux") ,("Host platform","x86_64-unknown-linux") ,("Target platform","x86_64-unknown-linux") ,("Have interpreter","YES") ,("Object splitting supported","YES") ,("Have native code generator","YES") ,("Support SMP","YES") ,("Unregisterised","NO") ,("Tables next to code","YES") ,("RTS ways","l debug thr thr_debug thr_l thr_p dyn debug_dyn thr_dyn thr_debug_dyn") ,("Leading underscore","NO") ,("Debug on","False") ,("LibDir","/usr/local/lib/ghc-7.6.2") ,("Global Package DB","/usr/local/lib/ghc-7.6.2/package.conf.d") ,("Gcc Linker flags","[\"-Wl,--hash-size=31\",\"-Wl,--reduce-memory-overheads\"]") ,("Ld Linker flags","[\"--hash-size=31\",\"--reduce-memory-overheads\"]") ] I ran the benchmarks by running e.g.: $ cd nofib/imaginary/sieve-wheel1 $ make clean && make boot WithNofibHc=ghc-${VERSION} && make WithNofibHc=ghc-${VERSION} Could you please try to run the "imaginary" benchmarks using exactly these commands and report the difference you see between 7.0.4 and 7.6.2. Here's what I see. 7.0.4 vs 7.6.2: -------------------------------------------------------------------------------- Program Size Allocs Runtime Elapsed TotalMem -------------------------------------------------------------------------------- bernouilli +3.3% +0.2% 0.12 0.13 +0.0% exp3_8 +1.1% +53.7% 0.14 0.14 +300.0% gen_regexps +18.7% +3.9% 0.00 0.00 +0.0% integrate -0.1% +39.0% 0.21 0.23 +0.0% kahan +1.7% +98.6% +9.9% +7.3% +0.0% paraffins +1.3% -1.2% 0.06 0.08 +0.0% primes +1.4% +64.7% 0.04 0.05 +50.0% queens +0.8% -0.5% 0.02 0.02 +0.0% rfib +1.7% +42.8% 0.02 0.02 +0.0% tak +0.9% +12.0% 0.01 0.01 +0.0% wheel-sieve1 +0.8% +66.6% -4.6% -5.8% -12.5% wheel-sieve2 +0.9% +0.0% 0.12 0.13 +0.0% x2n1 +10.3% +87.3% 0.00 0.01 +200.0% -------------------------------------------------------------------------------- Min -0.1% -1.2% -4.6% -5.8% -12.5% Max +18.7% +98.6% +9.9% +7.3% +300.0% Geometric Mean +3.2% +31.7% +2.4% +0.5% +23.6% -- Johan