http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58744
--- Comment #3 from Marco Vanotti <marcovanotti15+gcc at gmail dot com> --- (In reply to Richard Biener from comment #2) > Confirmed. On i?86 we properly do a 16byte and a 8byte access (but we copy > to stack anyway). > Yes, if the value is passed on the stack, it gets copied right. (For example, if it is the seventh parameter of a function, it will be passed on the stack and will be copied right). The thing is that in the x86_64 calling convention it has to be passed on registers while the are available (rdi, rsi, rdx, rcx, r8 and r9). Reading the source code, precisely the gcc/calls.c file: http://gcc.gnu.org/viewcvs/gcc/trunk/gcc/calls.c?revision=203967&view=markup --- The params that are passed on the stack are handled in line 3027, which says: /* Now store (and compute if necessary) all non-register parms. These come before register parms, since they can require block-moves, which could clobber the registers used for register parms. Parms which have partial registers are not stored here, but we do preallocate space here if they want that. */ Assuming that the registers may not require block-moves. It uses the function "store_one_arg" to store the arg in the stack (it doesn't work with a non-partial register). --- After a while, the function "load_register_parameters" (line 1860) is called, in this function, it falls in the case: move_block_to_reg (REGNO (reg), mem, nregs, args[i].mode); where nregs == 1. So a whole register is copied. --- I don't know how this issue should be fixed, should we copy the register into pseudos before the "load_register_parameters" ? Or should we change the move_block_to_reg function to make the right size of move instructions, for x86_64 we don't need "backup-registers", but maybe this bug is also in another arch. size 3: mov di, [rax] sal rdi, 16 mov dil, [rax] --- size 5: mov edi, [rax] sal rdi, 8 mov dil, [rax] --- size 6: mov edi, [rax] sal rdi, 16 mov di, [rax] --- size 7: mov edi, [rax] ;move 4 sal rdi, 24 mov di, [rax] ;move 3 sal rdi, 16 mov dil, [rax] ----------------------------- I would gladly submit a patch if I can get some advice on how this should be fixed :)
