Messages in this thread | | | Date | Mon, 11 Jul 2005 07:09:46 -0700 | From | Dan Kegel <> | Subject | CONFIG_ALPHA_GENERIC problem with gcc-4.1 |
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I've been doing builds of linux-2.6.11 as a sanity check for new versions of gcc, and a problem just popped up in arch/alpha/Makefile (see http://gcc.gnu.org/ml/gcc/2005-07/msg00397.html) I think I can work around this myself by using CONFIG_ALPHA_EV6 instead of CONFIG_ALPHA_GENERIC, but here's my analysis of the problem; maybe the alpha kernel maintainer can take it from here?
Take the following with a grain of salt; I don't know much about alpha or gcc, I'm just doing a little QA.
arch/alpha/Makefile says: 36 # If GENERIC, make sure to turn off any instruction set extensions that 37 # the host compiler might have on by default. Given that EV4 and EV5 38 # have the same instruction set, prefer EV5 because an EV5 schedule is 39 # more likely to keep an EV4 processor busy than vice-versa. 40 ifeq ($(CONFIG_ALPHA_GENERIC),y) 41 mcpu := ev5 42 mcpu_done := y 43 endif ... 84 # For TSUNAMI, we must have the assembler not emulate our instructions. 85 # The same is true for IRONGATE, POLARIS, PYXIS. 86 # BWX is most important, but we don't really want any emulation ever. 87 CFLAGS += $(cflags-y) -Wa,-mev6
Thus when you pick CONFIG_ALPHA_GENERIC, gcc is invoked with the contradictory options -mcpu=ev5 -Wa,-mev6
This probably means that even on ev5, some ev6 instructions are used. In particular, see include/asm-alpha/compiler.h:
#if defined(__alpha_bwx__) #define __kernel_ldbu(mem) (mem) #define __kernel_ldwu(mem) (mem) #define __kernel_stb(val,mem) ((mem) = (val)) #define __kernel_stw(val,mem) ((mem) = (val)) #else #define __kernel_ldbu(mem) \ ({ unsigned char __kir; \ __asm__("ldbu %0,%1" : "=r"(__kir) : "m"(mem)); \ __kir; })
That inline assembly is fine on ev5, but only if the assembler is emulating the ldbu instruction with a macro -- exactly the kind of thing arch/alpha/Makefile is trying to inhibit when it says -Wa,-mev6.
This is an issue now because building the kernel with CONFIG_ALPHA_GENERIC fails on the current gcc-4.1 snapshot with
> {standard input}:496: Error: macro requires $at register while noat in effect > make[1]: *** [arch/alpha/kernel/core_cia.o] Error 1
and it looks like a kernel problem, not a gcc problem: don't try to use ev6 instructions on ev5 or earlier processors.
That probably means conditionalizing that -Wa,ev6 properly, but if that's hard, maybe it means dropping support for ev4 and ev5 processors, and mapping CONFIG_ALPHA_GENERIC to ev6. I wouldn't know... - Dan
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