"Julian Seward (Intl Vendor)"
| > char fooble ( ... ) | > { | > return 'z'; | > } | > | > on an x86, 'z' will be returned at the lowest 8 bits in %eax. What I
| > don't know is, is the C compiler obliged to clear the upper 24 bits of | > %eax, or does that onus fall on the callee? | | Yes, the callee is required to narrow <expr> in 'return | <expr>;' to fit that of the specified return type -- see 9.8 | of Harbison and Steele. |
...
So we don't need to worry about doing the masking on the Haskell side, right?
Yes, that's been my understanding. But, as Carl Witty rightly points out, a distinction can be made between narrowing down the result value and the transmission of the narrowed result back to the caller. Carl's example doesn't fully convince me though, since the masking done in the code for "g" is patently redundant ("f" does mask out the upper 3 bytes of %eax upon return). However, the example given in the original bug report can be made to emit code that doesn't mask %eax (via "gcc-2.96 -O" on an i686-pc-linux). Another data point is that MSVC also emits code that masks. So, indications are that you actually do need to mask on an x86, but I've got no supporting documentation. If anyone is able to dig up more info and/or know someone that could answer this one decisively, I'd interested to hear of their findings. --sigbjorn btw, the document specifying the x86 ABI, located via http://uwsg.iu.edu/hypermail/linux/kernel/0011.1/0596.html , doesn't cover this as far as I've been able to tell.