#7684: cgrun071 segfaults ----------------------------------+---------------------------------- Reporter: tibbe | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 (amd64) Type of failure: Runtime crash | Difficulty: Unknown Test Case: cgrun071 | Blocked By: Blocking: | Related Tickets: ----------------------------------+---------------------------------- Comment (by rwbarton): Well, which compiler is doing the wrong thing? According to the x86_64 ABI, gcc 4.2 is doing the wrong thing, because when it generates code for `hs_popcnt8`, it assumes that the high 7 bytes of the argument register `%edi` are 0. The caller is not supposed to have to guarantee that, and gcc 4.8 does the right thing by masking off the high 7 bytes inside `hs_popcnt8`. ghc sees a primop `PopCnt8Op` of type `Word# -> Word#`, and given that, it is doing the right thing by taking the whole `Word#` value extracted from the argument to `popcnt8` (in `cgrun071`) and putting it in `%edi` before calling `hs_popcnt8`. I don't think ghc looks at the C prototype of functions that implement primops, does it? It just prepares the arguments for the call in registers or on the stack according to their Cmm sizes, and does a call to the symbol of the right name. Now, the combination of ghc passing a whole `Word#` and `hs_popcnt8` treating the argument as an `unsigned char` works out according to the x86_64 ABI since the intent is to compute the popcnt of the lowest byte. It doesn't work out if `hs_popcnt8` expects its argument to be zero- extended from 1 byte to 8 bytes like under gcc 4.2. Changing the declared argument type of `hs_popcnt8` to `StgWord` will make `popcnt.c` agree with the list of primops. Given that we can modify ghc but not gcc 4.2 this seems like the best solution for now. If `Word8#` becomes a thing, we can change the signature of `hs_popcnt8` back and also change the declared type of `PopCnt8Op` to `Word8# -> Word#`. As I understand things, the key is that those two declarations agree. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7684#comment:35 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler