Re: [Crash-utility] crash fails to build with gcc-4.5

When trying to build crash with gcc-4.5 on x86-64 you get: unwind_x86_32_64.c:50:2: error: initializer element is not constant unwind_x86_32_64.c:50:2: error: (near initialization for 'reg_info[7].offs') unwind_x86_32_64.c:50:2: error: initializer element is not constant unwind_x86_32_64.c:50:2: error: (near initialization for 'reg_info[8].offs') unwind_x86_32_64.c:50:2: error: initializer element is not constant ... When you start to dig into this you quickly end up playing with lots of really fun macros from unwind_x86_64.h. Eventually, you end up playing with this one: #define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1) If you pull this macro out and play with it by itself it seems to work fine with both gcc-4.5 and gcc < 4.5. It is only when it is used in combinations with the other macro expression that gcc-4.5 fails to evaluate it and I have no clue why. When looking at the BUILD_BUG_ON_ZERO macro upstream in include/linux/kernel.h we can see it has been replaced with this version: #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) It turns out that gcc-4.5 is perfectly happy with the updated version! This was done in commit: 8c87df457cb58fe75b9b893007917cf8095660a0 BUILD_BUG_ON(): fix it and a couple of bogus uses of it gcc permitting variable length arrays makes the current construct used for BUILD_BUG_ON() useless, as that doesn't produce any diagnostic if the controlling expression isn't really constant. Instead, this patch makes it so that a bit field gets used here. Consequently, those uses where the condition isn't really constant now also need fixing. Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even if the expression is compile time constant (__builtin_constant_p() yields true), the array is still deemed of variable length by gcc, and hence the whole expression doesn't have the intended effect. It looks like this could end up being a potential bug in gcc. I'll file a bug with gcc and try to provide them with a simplified test case. However, since this macro changed upstream and acts as a workaround for the issue I would propose making the update in crash as well. Troy

--- diff --git a/unwind_x86_64.h b/unwind_x86_64.h index a79c2d5..52fcf7a 100644 --- a/unwind_x86_64.h +++ b/unwind_x86_64.h @@ -61,7 +61,7 @@ extern void free_unwind_table(void); #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER) #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0])) #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) -#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1) +#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f)) #define get_unaligned(ptr) (*(ptr)) //#define __get_user(x,ptr) __get_user_nocheck((x),(ptr),sizeof(*(ptr))) -- Crash-utility mailing list Crash-utility(a)redhat.com https://www.redhat.com/mailman/listinfo/crash-utility