On Thu, Feb 17, 2005 at 01:52:58PM -0800, Till Straumann wrote: > David Querbach wrote: > >The ss555 BSP uses -mcpu=505, because as far as I can tell, it's the most > >appropriate gcc cpu variant for the MPC555. From what I can see in the > >source for gcc (up to 3.4.1), there is no other -mcpu=5xx variant > >available. > > > >If a 555 variant has been added in newer versions of gcc, I'd like a chance > >to test it before the m505 multilib disappears. > > > Why do you think you need mcpu=xxx other than powerpc ? Working by analogy from other ports, especially the mbx8xx ports. Upon closer examination, it's clear that the mbx8xx ports are not good examples, since they need soft-float, whereas the mpc555 does not. The mpc555 is thus much closer to the PowerPC mainstream, and we might expect that -mcpu=powerpc would work. I just tried rebuilding the ss555 port using -mcpu=powerpc. The code seems to work properly, but it's not identical to the -mcpu=505 code at the object code level. The same functionality appears in both, but re-arranged. I've looked through the compiler specs and through gcc/config/rs6000/* in the compiler source, but I can't for the life of me see where the -mcpu=505 switch is converted to "-mno-power, -mpowerpc, -mnew-mnemonics", as gcc.info claims. > It's amazing: until now, nobody has come up with a good reason for > creating any > -mcpu=xxx variant and yet there are still way too many... If they truly generate identical code (or even functionally equivalent code), then they serve only to muddy the waters. > Note that a #ifdef __mpcXXX in a BSP is *not* a valid reason as the > respective compiler switch could go into bsp_specs. A run-time check for > CPU type is even better. There aren't any of these sorts of tests in the ss555 port. > The only (legacy) reason I can think of are #ifdef __mpcXXX in cpukit > and/or libcpu - however, these should be cleaned up. There are still a few of these, but only one in .../new-exceptions/cpu.h refers to "mpc555". (It's used to add a CPU clock speed field to the rtems_cpu_table structure.) Again, I'm working by analogy with other ports, and I welcome suggestions for better ways of doing this. As long as the code still works, I see no reason not to use -mcpu=powerpc in the ss555 port. But since it generates different object code, I'd like to either understand why, or at least test it a bit first. I'm in the middle of an ss555 project, so I'll switch over to -mcpu=powerpc now to put a few miles on it. Regards, David Querbach Real-Time Systems Inc.
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