On Fri, 2009-05-01 at 09:28 -0400, John Fine wrote: > > An examination of the assembler produced always shows it passes the buffer > > pointer in %edi and not on the stack. > > > ?? %edi not %rdi ?? > > That would mean you are compiling 32 bit code. I assume that is not > what you intended. > > But possibly with a statically allocated buffer, the compiler can deduce > that the buffer will be in the first 4GB at run time and it can put the > correct address into rdi more efficiently by moving it to edi (with > implicit zeroing of the high 32 bits) than by moving it to rdi. Yes, apparently the compiler does this. It will, for example, put constants like text messages in a "small memory" area and pass the address in the 32-bit portion of the register. Because changing the 32-bit portion of the register always zeros the high-order 32 bits, this results in a correct 64-bit address. The main saving here is that the instruction to access edi does not require a REX prefix byte, but rdi does. Aside: changing 8 or 16 bits in a register does NOT affect the other portions of the register. Bob
